Table of contents

X-ray powder diffraction from solid deuterium was first observed under high pressure at SPring-8. At pressures up to 62 GPa and room temperature, three diffraction lines (100, 002, 101) of the hcp lattice were observed. The derived cell volume and the c/a ratio were consistent with single-crystal data. At 83 K and 94 GPa, three diffraction lines were also obtained and assigned to the hcp lattice.

We have studied the vibrational modes and their frequencies in both atomic and molecular phases of dense hydrogen to find the stable structures and evaluated the zero-point energies (ZPEs) and the effect on molecular dissociation. The most probable structure in the atomic phase is Cs IV whose vibrational modes have real frequencies over the whole Brillouin zone. And the structure in the molecular phase is very close to Cmca, whose vibrational modes with imaginary frequencies work as guides to the stable structure. Our estimates of the ZPE are very close to those of Kagan et al (Kagan Yu, Pushkarev V V and Kholas A 1977 Sov. Phys.–JETP46 511). Adding the ZPE to the static energy, we estimated its effect on the pressure of the molecular dissociation. The reduction of the dissociation pressure due to the inclusion of the ZPE becomes over 100 GPa.

Powder x-ray diffraction measurements on iodanil (C₆O₂I₄) have been carried out at pressures up to 39 GPa at room temperature with a diamond-anvil cell under the best hydrostatic conditions using helium as the pressure-transmitting medium. The diffraction patterns up to 23.3 GPa were fitted with a space group P 2₁ /c. New peaks appeared above 26.8 GPa and their intensities increased with increasing pressure while the original ones observed for the low-pressure phase were gradually depressed. This phase transition was accompanied with a mixed state of low- and high-pressure phases over the wide pressure range between 26.8 and at least 39 GPa.

Pressure dependence of several molecular vibration modes in Cs₂TCNQ₃ has been observed through infrared absorption measurements. The behaviour of the C–CN and C–H stretching modes suggests that a phase transition takes place at around 3.6 GPa. The π^*-electrons, which are localized on TCNQ⁻ molecules in the low-pressure phase, are delocalized significantly in the high-pressure phase. However, the radical-like and neutral-like molecules still coexist in the high-pressure phase indicating that the electrons are not completely delocalized. The electron–molecular-vibration coupled mode disappears in the high-pressure phase in coincidence with the disappearance of the inter-radical charge-transfer band S₁, proving the strong coupling between them.

The structure and lattice compression of the β-O₂ phase have been studied by synchrotron x-ray diffraction experiments at 277 and 300 K. The pressure dependence of the lattice constants, a and c, obtained from a powder indicated anisotropic compressibility. a is almost three times as compressible as c. This large linear compressibility of a may be attributed to the antiferromagnetic interaction in the basal plane. Single-crystal analyses at 6.1 GPa and 300 K suggested that the oxygen atom is disordered with a threefold axis of symmetry.

We have examined the metallization mechanism and possibility of molecular dissociation in iodanil (IA) and hexa-iodobenzene (HIB) under pressure by using first-principles calculations. We found that the metallization in IA is caused by the band overlap in the molecular phase and dissociation does not follow the metallization. In HIB, on the other hand, the band overlap mechanism is found to be less probable, which implies that a structural transformation will occur before the metallization. Both mechanisms are completely different from that for I₂ diatomic molecular crystal, and suggest essential roles of C₆ rings and O atoms.

Lithium is known as a 'simple metal' and the lightest alkaline metal in the periodic table. At ambient pressure lithium forms a body-centred-cubic structure and the conduction electrons are considered to be almost free from interaction with the atomic core. However, Neaton and Ashcroft (Neaton J B and Ashcroft N W 1999 Nature400 141) predicted that dense lithium at around 100 GPa will be found to transform to a low-symmetry phase and show a semi-metallic behaviour, in their calculation. Recently Hanfland et al(Hanfland M, Syassen K, Christensen N E and Novikov D L 2000 Nature408 174) reported the experimental behaviour of the existence of new high-pressure phase of lithium above 40 GPa which tends towards symmetry-breaking transitions. Here we report electrical resistance measurements on lithium performed at pressures up to 35 GPa at the temperature of 80 K.

The dielectric properties of C₆₀ have been measured as functions of temperature and hydrostatic pressure in the ranges 80–370 K and 0–0.8 GPa. The results show sharp anomalies at the rotational transition above 260 K and large relaxation peaks associated with the rotational 'glass transition'. From the measured frequencies of the loss peaks we calculate the energy barrier for molecular jumping between the 'pentagon' and 'hexagon' molecular orientations. The energy barrier increases by 13% GPa⁻¹.

We have theoretically studied the properties of solid iodine, bromine and chlorine under pressure, by employing the full-potential linear muffin-tin orbital method within the local density approximation (LDA). Furthermore, in this paper we study bromine by the use of the generalized gradient approximation (GGA) and compare the results with those obtained using LDA. We examine the pressure dependence of the frequencies of Raman-active A_g modes using the frozen-phonon method. We also examine the scaling rules and find that they hold for these band-theoretical results.

Acoustic velocities and adiabatic elastic constants of structure I of methane hydrate (MH) have been determined as a function of pressure up to 0.6 GPa at 23°C by the high-pressure Brillouin spectroscopy developed for a single molecular crystal. The pressure dependence of the acoustic velocities of MH is very similar to that of ice-Ih except for the longitudinal acoustic (LA) velocity. The value of the LA velocity along the ⟨100⟩ direction of MH at 0.02 GPa is 3.63 km s⁻¹ which is about 7% lower than the average of the LA velocities in the ice-Ih phase at −35.5°C and atmospheric pressure.

Results from recent high-pressure experiments in the field of fullerenes are briefly reviewed. In particular, new results on one-, two- and three-dimensional polymerized C₆₀ and C₇₀ are discussed. Results discussed include the first synthesis of a well defined, one-dimensional polymer based on C₇₀, transformations from two-dimensional (2D) to three-dimensional phases in C₆₀, and doping of 2D C₆₀ polymers.

Decaborane, a molecular boron hydride, was compressed to 131 GPa at room temperature to explore possible non-molecular phases in this system and their physical properties. Decaborane changed its colour from transparent yellow to orange/red above 50 GPa and then to black above 100 GPa, suggesting some transformations. Raman scattering and infrared (IR) absorption spectroscopy reveal significant structural changes. Above 100 GPa, B–B skeletal, B–H and B–H–B Raman/IR peaks gradually disappeared, which implies a transformation into a non-molecular phase in which conventional borane-type bonding is lost. The optical band gap of the material at 100 GPa was estimated to be about 1.0 eV.

X-ray powder diffraction experiments were performed at pressures up to 14 GPa and at room temperature. The orthorhombic lattice was retained at pressures up to 10 GPa. Above 10 GPa, however, a broad glassy background developed. The lattice constants, a, b, c, and the orientation angle at 2.72 GPa are 5.845, 5.220, 8.296 Å and 41.3°, respectively. The orientation angle at high pressure is smaller than that of the low-temperature phase.

We explore the effect of pressure on the fluorescence spectra of the intramolecular electron transfer compound N-(1-pyrenylmethyl), N-methyl-4-methoxyaniline (Py–Am) and its model version, with poly(methyl methacrylate) blended in, at high pressure up to 7 GPa. The emission properties of Py–Am and pyrene show distinct difference with the increase of pressure. This difference indicates the strength of the charge transfer interaction resulting from the adjusting of the conformation of Py–Am with increase of pressure. The relationship between the electronic state of the molecule and pressure is discussed.

A search for superconductivity of magnetic elemental metals is performed. A successful discovery of the onset of superconductivity is reported in the case of iron under pressure. By electrical resistance measurement, a maximum value of the superconducting transition temperature T_c of 2 K and the upper critical magnetic field H_c of 0.2 T are observed under pressure of 20 GPa where iron is in the crystallographic hcp phase and non-magnetic. Further confirmation of the superconducting transition of hcp iron was obtained by the detection of the diamagnetic signal due to the Meissner effect in accordance with the results of the electrical resistance measurements.

A dual-fluorescence emitting behaviour of coumarin 153 powder has been detected at high pressure while at ambient pressure the dye exhibits only single-band emission. Because of the strong electron-withdrawal group at site 7, these two fluorescent peaks can be ascribed to local excited state emission and charge transfer state emission, respectively.

Angle-dispersive x-ray diffraction measurements using CuGeO₃ (I) and CuGeO₃ (III) as the starting materials were carried out to 81 and 31 GPa, respectively, at room temperature. Data for phase (I) show that phase transitions occur at ∼7, ∼14, and ∼22 GPa, respectively, corresponding to (I) → (II), (II) → (II'), and (II') → (VI) transitions, as reported previously. The tetragonal phase (VI) was found to be stable up to 81 GPa, the highest pressure determined in this study. The volume changes at the transition pressures are estimated to be of ∼5%, ∼0%, and ∼14% for (I) → (II), (II) → (II'), and (II') → (VI) transitions, respectively. Data from measurements where phase (III) was the starting material show that phase (III) first changes to phase (IV) at ∼7 GPa and then to (IV') at 13.5 GPa, and finally to phase (V) at ∼18 GPa, with volume changes of 1.5%, 0%, and 20%, respectively, at the transition pressure. The volume change of 20% at 18 GPa is consistent with the pyroxene–perovskite transition.

A moissanite anvil cell (MAC) has been used in research at high-pressure recently. In this paper, we describe its application in x-ray diffraction studies with laser heating. A high temperature of 3700 K has been achieved in a MAC; the lattice constants of graphite were determined by means of in situ x-ray diffraction at room temperature and 3700 K, and the results are in good agreement with reference data.

C₆₀ has been studied by means of time-resolved x-ray diffraction measurements using synchrotron radiation. Diffraction patterns were recorded at intervals of 1–10 min for samples under high pressure (12.5 and 14.3 GPa) and high temperature (up to 800°C) for, at the longest, 3 h. Time, pressure, and temperature dependences of the C₆₀ structure are presented and the relevance to the hardness of materials derived from C₆₀ is discussed.

An in situ energy dispersive x-ray diffraction study on nanocrystalline ZnS was carried out under high pressure up to 30.8 GPa by using a diamond anvil cell. The phase transition from the wurtzite to the zinc-blende structure occurred at 11.5 GPa, and another obvious transition to a new phase with rock-salt structure also appeared at 16.0 GPa—which was higher than the value for the bulk material. The bulk modulus and the pressure derivative of nanocrystalline ZnS were derived by fitting the Birch–Murnaghan equation. The resulting modulus was higher than that of the corresponding bulk material, indicating that the nanomaterial has higher hardness than the bulk material.

The specimen used in this study was a single crystal of phase-A Mg_6.99Si_1.99H_6.06O₁₄ synthesized using a multi-anvil apparatus at conditions of 1000°C and 10 GPa. Sets of x-ray diffraction intensities were measured with a single crystal of 60 × 50 × 30 μ m using synchrotron radiation up to 9.4 GPa. The unit-cell parameters observed gave K₀ = 105 GPa (assuming that K' = 4). With increasing pressure, significant decreases of O–O distances for hydrogen bonds were observed.

Since first opening its doors to public research in 1997, SPring-8 has seen the accomplishment of many important studies in a wide variety of fields through its stable operation and cutting edge technology. High-pressure experiments have been carried out on a number of beamlines using a diamond anvil cell or a multi-anvil press. Here, we review the multi-anvil presses installed on the SPring-8 beamlines and a few research projects currently utilizing this technology. The significant difference in post-spinel boundary between multi-anvil experiments and diamond anvil studies will also be discussed.

The present status of high-pressure research at Beijing Synchrotron Radiation Facility is reported. A ten-poles wiggler beamline provides a white beam for investigating samples using a diamond anvil cell. In situ energy-dispersive diffraction is used to determine the pressure-induced phase transitions and equations of state. High pressure can be stably applied by a stepper-motorized loading system with a strain sensor. Some megabar experiments have been carried out without damage on diamonds. Improved beam collimation reduces the background and eliminates gasket scatter. Some research and future developments are also presented.

NiAl₂O₄ is known to transform from a normal spinel, (Ni²⁺)[Al ₂³⁺ ]O ₄, to an inverse spinel, (Al³⁺)[Ni²⁺Al³⁺]O₄, and vice versa. In this process, the larger Ni²⁺ ions which occupy the tetrahedral (8a) sites in normal spinel move to the octahedral (16d) sites in inverse spinel while half of the smaller Al³⁺ ions move in the opposite direction. The extent of this move is measured by the disorder or inversion parameter, I (the fraction of tetrahedral sites occupied by the Al ions; I = 0 for normal spinel and I = 1 for inverse spinel). Previous studies suggest that the lattice constant of spinel can decrease as the disorder parameter increases to better accommodate the Ni ions. In situ neutron diffraction studies performed by us indicate that this process is also occurring during the reduction of NiAl₂O₄ to Ni and Al₂O₃. It is possible that the compressive residual stresses generated during reduction play a role in the structural evolution of NiAl₂O₄.

To systematically investigate the effect of pressure on the structure of NiAl₂O₄, x-ray diffraction studies at the X17 beamline of the National Synchrotron Light Source were performed. The pressure (up to 35 GPa) was applied via a diamond anvil cell and the experiments were conducted using a polychromatic x-ray beam. By comparing the relative intensities of certain spinel reflections that are sensitive to cationic disorder, a trend toward inverse spinel as a function of pressure was observed. The results are presented in comparison to previous studies on this material.

The structure of liquid GaSb has been investigated up to 10.1 GPa by means of energy-dispersive x-ray diffraction using a synchrotron radiation source and a multi-anvil apparatus. With increasing pressure up to about 5 GPa, the second and third peaks in the pair distribution functions, g(r), shift remarkably toward smaller r-values, while the first peak shifts toward larger r-values. This shows that the structure of liquid GaSb does not contract uniformly. Corresponding to the shift of the first peak, the coordination number increases. These results show that the local structure in liquid GaSb changes into a more highly coordinated state under pressure.

Using a large-volume high-pressure apparatus, Li₂O–4GeO₂ glass and pure GeO₂ gel have been compressed to 14 GPa at room temperature and their local structural changes have been investigated by an in situ XAFS (x-ray absorption fine-structure) method. On compression of Li₂O–4GeO₂ glass, the Ge–O distance gradually becomes short below 7 GPa, showing the conventional compression of the GeO₄ tetrahedron. Abrupt increase in the Ge–O distance occurs between 8 and 10 GPa, which corresponds to the coordination number (CN) changing from 4 to 6. The CN change is completed at 10 GPa. On decompression, the reverse transition occurs gradually below 10 GPa. In contrast to the case for Li₂O–4GeO₂ glass, the Ge–O distance in GeO₂ gel gradually increases over a pressure range from 2 to 12 GPa, indicating that continuous change in CN occurs. The Ge–O distance at 12 GPa is shorter than that of Li–4GeO₂ indicating that the change in CN is not completed even at this pressure. On complete release of pressure, the Ge–O distance reverts to that of the starting gel.

For the realization of a practical pressure scale based on equation of state (EOS) data for selected calibrants, a detailed modelling of the thermal contributions is required. While 'parametric EOS' forms with their temperature-dependent parameters V₀ (T), K₀ (T), and K'₀ (T) are useful for limited ranges in pressure and temperature, the separate modelling of the zero-temperature and thermal contributions is more appropriate especially for wide temperature ranges under high pressures (Holzapfel W B, Hartwig M and Sievers W 2001 J. Phys. Chem. Ref. Data30). The remaining uncertainties due to explicit anharmonic contributions beyond the implicit contributions of the quasiharmonic approximation are therefore discussed here in more detail.

The melting behaviour, structure, and density of molten indium at high pressures have been studied in an externally heated diamond anvil cell (DAC) using x-ray diffraction/scattering measurements. Melting at high pressure was identified by the appearance of diffuse scattering from the melt. Analysis of the diffuse scattering shows that at 710(3) K the coordination number at the nearest neighbour increases from 10.1(4) at 1.0 GPa to 12.1(5) at 6.3 GPa. A method for measuring the density of amorphous materials is introduced for DAC studies and the first result on molten indium is presented.

A collimation system was designed to reduce the scattering background in the energy-dispersive x-ray diffraction (EDXD) experiments at the high-pressure station of Beijing Synchrotron Radiation Facility. The results showed that the collimation system could improve the signal-to-noise ratio substantially. Some better data were obtained which were very difficult to access before in EDXD experiments. The system provides a useful technique for experimentation at pressure higher than 100 GPa.

Single-crystal structure analysis of SiO₂ stishovite, (rutile type, P4₂/mnmz = 2) was carried out using the newly devised diamond anvil cell. The electron-density distribution was investigated at high pressures up to 50 GPa using synchrotron radiation at SPring-8 and a laboratory x-ray source generator of Ag Kα rotating anode generator. Using large diamond crystal windows instead of beryllium for the cell has several advantages for single-crystal diffraction study supplying the large Q-value.

We have performed a high-pressure synchrotron x-ray diffraction study of a metallic amorphous state in SnI₄ induced by pressure. The Faber–Ziman structure factor S(Q) was obtained from diffraction intensities measured at pressures between 25 and 65 GPa. The first peak in S(Q) is relatively intense and sharp and the second peak is overlapped with the third one. Such features are also found in S(Q) for pure metallic glasses of Ni and Fe at 1 atm but not in molecular liquids. The obtained reduced radial distribution function g(r) shows no evidence for the presence of the SnI₄ molecules in the metallic amorphous state.

The high-pressure phases of TiO₂ have been investigated theoretically on the basis of first-principles density functional theory. Both the equation of states of the low-pressure phase and the structural phase transitions (the rutile-to-α-PbO₂-type and α-PbO₂-to-baddeleyite transitions) were successfully explained in agreement with previous experiments. The calculation suggests the possibility that the high-pressure phase next to the baddeleyite phase does not have the brookite structure, which has been observed in ZrO₂ and HfO₂. Furthermore, the stability of the low-pressure phases in TiO₂ was discussed on the basis of the atomic electronic structure.

High-pressure powder x-ray diffraction experiments have been performed on Zn with a He-pressure medium at low temperature. When the sample was compressed in the He medium at low temperature, large nonhydrostaticity developed, yielding erroneous lattice parameters. On the other hand, when the pressure was changed at high temperatures, good hydrostaticity was maintained. No anomaly in the volume dependence of the c/a axial ratio has been found.

Powder x-ray diffraction experiments on a low-Z element, Be, were performed at pressures up to 171 GPa using the synchrotron radiation source at SPring-8. All profiles were assigned to the hcp structure of the ambient phase, and the theoretically predicted hcp–bcc phase transition was not observed. The previously proposed hcp–hexagonal phase transition around 14.5 GPa was also not confirmed. The equation of state of hcp-Be was determined by a least mean squares fitting to the Birch–Murnaghan equation from these diffraction data. Calculated values of K₀ and K₀ ' were 97.2 ± 2.5 GPa and 3.61 ± 0.07, respectively.

It was found by the x-ray diffraction experiment under hydrostatic pressure that the carbon nanotubes are compressed easily with a high volume compressibility of 0.024 GPa⁻¹. The single-walled carbon nanotubes are polygonized when they form bundles of hexagonal close-packed structure and the inter-tubular gap is smaller than the equilibrium spacing of graphite. Under high pressure, further polygonization occurs to accommodate the extra amount of volume reduction. The ratio of the short and the long diagonals in the hexagonalized cross section is found to have changed from 0.991 at zero pressure to 0.982 at 1.5 GPa pressure, when the Bragg reflection from the nanotube lattice diminished. Accompanying polygonization, a discontinuous change in electrical resistivity was observed at 1.5 GPa pressure, suggesting a phase transition had occurred.

In situ high-pressure energy-dispersive x-ray diffraction measurements on α-LiIO₃ have been performed by using a diamond anvil cell device with synchrotron radiation up to 75 GPa at room temperature. No new phase was found. The second Birch–Murnaghan equation of state is fitted with B₀ = 55 ± 3 GPa for the zero-pressure bulk modulus, B₀ ' = 2.9 ± 0.4 for its pressure derivative.

Structural phase transitions of titanium (Ti) have been investigated at pressures up to 220 GPa at room temperature using a monochromatic synchrotron x-ray diffraction technique. At 140 GPa, the hexagonal phase (ω-Ti) was transformed into an orthorhombic phase (δ-Ti) with a distorted bcc structure via an intermediate phase (γ-Ti), which has recently been proposed. The δ-Ti and γ-Ti phases had a unique zigzag-chain-like structure, which resulted from an orthorhombic distortion.

Structural phase transitions in ReO₃ were studied under high pressure with the angle-dispersive x-ray diffraction method using synchrotron radiation. The pressure range of 8–18 GPa was clarified to be the coexistence region of the cubic phase and rhombohedral I phase. In addition, the apparent change of diffraction pattern observed above 38 GPa defined the unit cell of the rhombohedral II phase.

X-ray diffraction measurements under pressure have been performed on seven different compositions in the U_xLa_1−xS system (x = 0, 0.08, 0.40, 0.50, 0.60, 0.80, 1). All the compounds have the same structure (NaCl type) at ambient pressure, but show different behaviours under pressure. A transformation into the CsCl-type structure is only observed for x ≤ 0.60. For x = 0.80 and 1, the high-pressure phase has yet to be determined. We also observe a difference in bulk modulus (for x ≤ 0.50, B₀ ≤ 90 GPa whereas for x ≥ 0.60, B₀ ∼ 100 GPa) and in the transition pressure (∼30 GPa for low-uranium-content compounds and from 45 to 80 GPa for high concentrations).

High-pressure synchrotron x-ray powder diffraction patterns were collected using ID09 of ESRF (Grenoble, France) for a powder sample of PbTiO₃, placed in a diamond anvil cell. The patterns were collected at room temperature using nitrogen (up to 37 GPa) and methanol–ethanol solution (up to 7 GPa) as pressure-transmitting media. The bulk moduli were calculated for the first time using the Vinet equation of state and they were compared to those of isostructural compounds. The trend of the spontaneous polarization as a function of pressure confirms that the ferroelectric–paraelectric phase transition at 11.2 GPa possesses a second-order character.

We present accurate x-ray diffraction data at high pressures for AuIn_2,AuGa₂ and AuAl₂, obtained using a diamond anvil cell with the ELETTRA synchrotron source. The resulting P–V data obtained from the d-values were used to get the universal equation of state (UEOS), which is compared with theoretical estimates. Deviation from linearity is evident in the UEOS curves of AuIn₂ and AuGa₂, thus verifying that some of the observed anomalies in these systems below 5 GPa are due to electronic topological transitions.

A structure of a high-pressure phase of tellurium (Te-III) has been re-examined by an x-ray diffraction method. This phase appears at a pressure between 7 and 27 GPa, where a second-order phase transition to Te-IV occurs. The newly obtained crystal lattice of Te-III at 8 GPa is monoclinic with a = 8.4682(14) Å, b = 4.7424(8) Å, c = 3.9595(7) Å, β = 88.112(11)°. The space group is C2/m, with six atoms in the unit cell, two in positions 2a at (0, 0, 0) and four in positions 4i at (x, 0, z) with x = 0.324(11), z = 0.675(3).

The pressure behaviour of a series of transition metal borides has been studied both experimentally and by means of ab initio calculations. X-ray diffraction patterns measured up to ∼50 GPa for VB₂ and ZrB₂ show no obvious phase transition. Bulk moduli of 322 and 317 GPa, respectively, were obtained using a Murnaghan equation of state. Hartree–Fock LCCO (linear combination of crystal orbitals) calculations performed for TiB₂ have allowed its compression behaviour to be studied. The bulk modulus obtained (292 GPa) and the proposed important contribution of the interlayer interaction to the elastic behaviour under high pressure are consistent with the experimental results for the other borides.

Substituting B for Co in the strong permanent magnet materials ACo₅ (A = Ln or U) substantially modifies the magnetic properties as a function of the B/Co ratio. Preliminary neutron diffraction studies on TbCo₃B₂ show site-dependent magnetic moments for both the Tb and the Co atoms. The changing interatomic distances and magnetic properties with changing B/Co ratio lead to a rich magnetic phase diagram as a function of pressure. It is our intention to study some of these materials using high-pressure Mössbauer spectroscopy in the future. As a preliminary step it is essential to study the high-pressure crystallographic phase diagram of these materials. This work shows preliminary high-pressure crystallographic results on UCo₃B₂ to 42 GPa, indicating a phase transition around 6 GPa.

The superconductivity and the lattice properties of a sintered MgB₂ material have been investigated under high pressure up to 10 GPa. The transition temperature was found to decrease linearly with increasing hydrostatic pressure at a rate of 1.03 K GPa⁻¹, which can be explained with the classical Bardeen–Cooper–Shrieffer theory based on an electron–phonon coupling mechanism. The crystal lattice exhibits an anisotropic compressibility characterized by a larger compressibility along the c-direction than the a/b-directions. The anisotropy is attributed to a weaker inter-plane bonding along the c-axis in comparison with a stronger intra-plane bonding perpendicular to the c-axis. The bulk modulus of the measured material was deduced to be 172 GPa.

In this paper, we investigate the pressure-induced structural transition of α-Ga₂O₃ powder by means of energy-dispersive x-ray diffraction (EDXD) measurements. The EDXD results show that, with increase of pressure, a new pressure-induced phase appears. The new tetragonal structure (β-Ga₂O₃) can remain stable over the pressure range (≤38 GPa) under study.

The Brillouin and Raman scattering spectra of fluid and solid hydrogen at high pressures (2–14 GPa) and temperatures (293–520 K) were measured to investigate the intermolecular interaction in the fluid and solid states. A Benedict type of equation of state was determined in P ≤ 15 GPa, T ≤ 550 K for fluid hydrogen with an average deviation of 1.0% from existing experimental data. We examined three types of intermolecular potential, and found that the Hemley–Silvera–Goldman potential gives superior fits to experimental data over a wide temperature range above 5 GPa. It was also found that the effect of intrinsic mode anharmonicity becomes significant at high temperatures for solid hydrogen.

We developed a new high-field ESR system under pressure. A clamped-type pressure cell with sapphire pistons is used to transmit the electromagnetic wave. The first measurements on the spin–Peierls system CuGeO₃ under pressure and our development of the new standard of pressure for high-field ESR are presented.

Synchrotron x-ray diffraction (XRD) and infrared (IR) absorption spectra of hydrous and 'anhydrous' forms of phase X were measured to 30 GPa at room temperature. Three OH stretching modes were found in the hydrous phase, and surprisingly one sharp OH mode was observed in the previously characterized anhydrous phase. All OH stretching modes soften and broaden with increasing pressure and become very weak above ∼20 GPa. XRD indicates that the crystal structure remains stable up to 30 GPa. Combining IR absorption and XRD results, the behaviour is attributed to pressure-induced distortion of the Si₂O₇ groups and disorder of the hydrogen atoms. The bulk moduli of the hydrous and 'anhydrous' phases are in the region of 74 GPa.

Ultraviolet–visible absorption spectra of solid hydrogen sulphide (H₂S) were measured at various pressures from 0.3 to 29 GPa. The absorption edge observed around 4.8 eV at 0.3 GPa indicated a red-shift with increasing pressure, and positioned below 3 eV at 29 GPa. On the basis of the spectra obtained, the energy gap was determined as a function of pressure. The transition to phase IV at 11 GPa was found to lead to a small jump in its pressure dependence and to yield an Urbach tail in the absorption edge.

The pressure-induced phase transformation of solid CF₄ at room temperature was studied with x-ray and Raman scattering experiments. The space group of phase III was determined as P 2₁ /c. The pressure dependences of the intramolecular vibrational modes suggest phase transitions from phase III to IV at 8.9 GPa and from phase IV to V at 13.6 GPa.

Recent developments in high-pressure in situ Brillouin spectroscopy of a simple molecular system are reviewed by demonstrating experimental and analytical methods for the study of acoustic velocities in any direction, adiabatic elastic constants, and elastic anisotropy. Detailed applications to solid argon (Ar) are presented, at pressures up to 70 GPa in a diamond anvil cell, using recently developed approaches that combine the method of in situ Brillouin spectroscopy, for a single crystal of Ar up to 4 GPa, and the envelope method applied to both longitudinal acoustic and transverse acoustic modes, for recrystallized Ar between 4 and 70 GPa.

The fluorescence properties of two nitrogen-containing poly-(phenylene vinylene)-related copolymers were investigated under high pressure. Because of the large difference in interchain interaction between these two copolymers, the emission states are different from each other. Under the perturbation introduced by compression, the emission intensity exhibits different trends of change.

Raman spectra of ice in aqueous LiCl solution (LiCl·12H₂O) have been measured as a function of pressure (0.1–700 MPa) at liquid nitrogen temperature (−196°C). It is found that the ice phase in LiCl·12H₂O transforms to an amorphous phase at ∼520 MPa, as in the case of pressure-induced amorphization of ice I_h to a high-density amorphous ice. We have also observed the spectral changes of the amorphous phase as a function of temperature (−196°C up to room temperature) under high pressure. We show spectral evidence for the existence of a transition from the relaxed amorphous phase to the supercooled liquid at high pressures and low temperatures.

X-ray powder diffraction and Raman scattering experiments for solid methane were carried out at pressures up to 37 GPa and room temperature. The diffraction pattern of phase B at 16.9 GPa is assigned to a cubic lattice with lattice constant of 7.914 Å (B. At the transition from phase B to the HP phase, the pressure–volume curve shows an anomaly without the structural change.

The pressure dependence of the elastic properties of sII-type Ar hydrate has been determined up to 0.70 GPa at room temperature by using a high-pressure Brillouin scattering method. At about 0.10 GPa, ratios of the elastic constants to the density of C₁₁ /ρ = 10.16, C₁₂ /ρ = 6.30, C₄₄ /ρ = 2.32 × 10⁶ m² s^-2 are obtained. Comparing with sI-type CH₄ hydrate, the acoustic velocities for the longitudinal acoustic and transverse acoustic modes of Ar hydrate (sII) were found to be about 15 and 25% lower, respectively.

The specific heat of CePd_2.02Ge_1.98 has been measured with an ac calorimetric technique up to 22 GPa for temperatures in the range 0.3 K ≤T ≤10 K. A thermocouple allowed the temperature oscillations to be read when an ac heating current was sent through the sample. The inverse of the thermovoltage V_ac recorded at low temperature exhibits a pronounced anomaly as a function of pressure. It is shown that 1/V_ac extrapolated to zero temperature is a measure of the Sommerfeld coefficient γ.

High-pressure study has played an important role in the investigation of conventional superconductors. Since the discovery of cuprate superconductors, high-pressure study has become even more important, especially as regards high-pressure synthesis and the effect of pressure. In this report, the new materials Ca-doped Pr-123, (Fe, Cu)-1212, and MgB₂—a very new and interesting system synthesized under high pressure with good quality—will be discussed.

Chemical inner pressure has been thought to explain the high T_c of Ca-doped Pr-123. As another possibility, the replacement of the physical pressure effect by a chemical effect will be discussed.

The nearly single phase (R_0.4Pr_0.6)_0.5Ca_0.5Ba₂Cu₃O_7−δ (R = La, Pr, Nd, Sm, Eu, Gd and Y) superconducting compounds should be prepared at high temperature and high pressure. The critical temperature of these compounds is over 100 K which is much higher than that of the traditional R-123 superconductors which do not have Ca doping on the rare-earth site. Our results shows that Pr behaves in the same way as other rare-earths in the 123-phase compounds due to the doping with Ca ions.

Layered tetragonal BaCo_0.9Ni_0.1S₂ contains high-spin Co(II) in octahedral sites and is an insulator. Hole doping by introducing sulfur vacancies results in a polaronic conductivity and produces a transition to a monoclinic metallic phase with a nearly isotropic conductivity. Pressure P ≥1.5 kbar suppresses the monoclinic phase and at P ≥10 kbar the high temperature tetragonal phase is transferred to an isostructural metallic phase containing low-spin Co(II). The single-crystal resistivity along the c-axis and in the a–b plane of high-pressure, tetragonal BaCo_0.9Ni_0.1S_1.87 is shown to be highly anisotropic and to resemble that of the overdoped superconductive system La_{2 −x}Sr_xCuO₄.

We report our recent developments of experimental systems for measuring the de Haas–van Alphen (dHvA) effect under hydrostatic and uniaxial pressures. The dHvA effect of CeB₆ has been studied under both hydrostatic and uniaxial pressures and the effects of the pressures on the electronic structure are discussed.

The magnetic and magnetoresistance properties of La_0.825Sr_0.175MnO₃ powder compact prepared by the mechanical alloying method and high temperature–high pressure treatment have been investigated. Analysis by means of x-ray diffraction and scanning electron microscopy show that the La_0.825Sr_0.175MnO₃ powder has an average grain size of 100 nm and has been partly non-crystallized by mechanical alloying. Compared with the crystal and general nanogranular samples of the same material, the powder compact is considerably different in both magnetic and magnetoresistance properties.

The perovskite oxides CaFeO₃ and La_1/3Sr_2/3FeO₃ have been investigated by high-pressure ⁵⁷Fe Mössbauer spectroscopy. The critical temperatures of the charge disproportionation (CD) and the magnetic order (MO) have been determined as a function of pressure. In CaFeO₃ the CD (2Fe⁴⁺ → Fe³⁺ + Fe⁵⁺) occurs at an almost constant temperature of 290 K in the pressure range of 0–17 GPa. Above 20 GPa, the CD is suppressed. The MO temperature of 125 K at an ambient pressure rises to 300 K at 34 GPa. In La_1/3Sr_2/3FeO₃ the CD (3Fe^11/3+ → 2Fe³⁺ + Fe⁵⁺) and the MO occur at the same temperature up to 21 GPa, which decreases from 207 to 165 K with increasing pressure. Above 25 GPa, however, the MO temperature rises above 400 K.

We present the results of highly sensitive magnetic measurements in a diamond-anvil cell using a SQUID vibrating coil magnetometer, which were made to investigate the possibility of increasing its sensitivity by improving the system. The Néel temperature T_N of PrSn₃ (T_N = 8.4 K at ambient pressure) was measured with good signal-to-noise ratio up to about 10 GPa using a tungsten gasket, where the magnitude of the anomaly in the temperature dependence of the susceptibility, assigned as T_N, was as small as 10⁻³ emu cm⁻³. It appeared that elimination of unnecessary motion of the lead wire of the detection coil was needed to increase the signal-to-noise ratio.

The variations of the superconducting transition temperature T_c and the metal–insulator (MI) transition temperature T_MI were investigated as a function of pressure in the superconducting Cu_{1 −x}Zn_xIr₂S₄ (0.3 ≤ x ≤ 0.5) system. The experiment was performed by measuring the temperature dependence of resistance under the pressures up to 1.5 GPa. It is shown that the external pressure destroys the superconductivity, and gives rise to the MI transitions. The result is discussed in terms of the stabilization of the insulating phase at high pressures and the phase separation associated with the charge segregation. It is proposed that the BCS Cooper pairs compete with the proposed bipolarons under certain pressures.

The temperature dependence of the electrical resistance is measured for zinc–gallium alloys under various pressures. The appearance of a new intermediate phase is suggested from results of the measurements around 3 GPa. An isobaric phase diagram with the intermediate phase is proposed.

We present a detailed pressure study of the electrical resistivity ρ(T) and the specific heat C(T) of the non-Fermi-liquid (NFL) compound YbRh₂Si₂ and of ρ(T) for a single crystal in which 5 at.% of Si is replaced by isoelectronic Ge. The magnetic phase diagram is deduced up to p ∼ = 2.5 GPa. A comparison of the effects of the volume change introduced by doping and/or by hydrostatic pressure will be given. We show that the NFL behaviour observed in ρ(T) as well as the magnetic phase diagram are not influenced by the disorder introduced by alloying.

High pressure can strongly influence the electronic properties of a metal, by modifying either the shape or the topology of its Fermi surface (FS). In the case of low-dimensional anisotropic superconductors, typified by high-T_c cuprates, some quasi-2D organic salts and heavy-fermion compounds, we show that the occurrence of an electronic topological transition is generic to a non-monotonic pressure dependence of the critical temperature T_c. On the other hand, a change in shape of the FS can be correlated with the steady increase of T_c as a function of the ratio between next-nearest-and nearest-neighbour hopping, as is observed in the high-T_c cuprates.

The crystal structure and the magnetic properties of TlNiO₃ have been characterized and compared with those of TNiO₃ (T = rare earth and Y). The electronic structure of Ni(III) has been investigated by Mössbauer spectroscopy. Through these analyses, the effects of the A-site ion on structural distortion, magnetic behaviour and electronic structure of Ni(III) in a perovskite have been discussed.

A new hybrid ruthenate–cuprate superconductor RuSr₂GdCu₂O₈ has been synthesized. This compound exhibits superconductivity and magnetic ordering around 40 K ( = T_c) and 130 K ( = T_m). The effect of pressure on the electrical resistance has been measured up to 2.1 GPa in order to make clear the interplay between the magnetism and the superconductivity. It is found that both T_c and T_m increase with pressure with rates of dT_c/dP = 1.9 K GPa⁻¹ and dT_m/dP = 5.7 K GPa⁻¹, which indicates that the superconductivity does not compete with the magnetic ordering. An x-ray diffraction experiment under pressures up to 13 GPa has been carried out in order to clarify the relation between the electronic properties and the lattice compression.

Magnetization measurements have been carried out for disordered Fe₇₂Pt₂₈, Fe₆₆Pd₃₄, and Fe₆₈Pd₃₂ Invar alloys under high pressure using a technique combining a pressure-clamp-type Drickamer cell and a pulse magnet. In Fe₇₂Pt₂₈ at room temperature, the magnetization decreased rapidly with increasing pressure up to 2.5 GPa, but above 2.5 GPa the rate of decrease became small and remained at a small value up to 5.6 GPa. In Fe–Pd Invar alloys at room temperature, the magnetization decreased linearly with increasing pressure. But, at 4.2 K, the change of magnetization with pressure was small in Fe₆₆Pd₃₄, which means that Fe₆₆Pd₃₄ behaves as a strong ferromagnet.

We have measured the pressure effect on the superconducting transition temperature T_c of black phosphorus up to 160 GPa using a superconducting quantum interference device vibrating coil magnetometer. It was found that T_c had a maximum value of about 9.5 K at about 32 GPa, began decreasing with pressure and reached about 4.3 K at about 100 GPa.

A polycrystalline sample of RuSr₂GdCu₂O₈ exhibits the superconducting transition at about 40 K and ferromagnetic order at about 130 K. We have investigated the crystal structure of this sample by means of a synchrotron radiation x-ray powder experiment and analysed the data by the Rietveld method. The results indicated the Cu sites to contain 5% Ru. The pressure dependences of the superconducting and ferromagnetic transition temperatures of this sample were measured. The superconducting transition temperature scarcely varies with pressure. The ferromagnetic transition temperature increases with increasing pressure at lower pressure. However, the increase of the ferromagnetic transition temperature became saturated above 1.6 GPa. The magnitude of the susceptibility decreases with increasing pressure below the ferromagnetic transition temperature.

The charge-density-wave (CDW) transition temperature, T_CDW, of ZrTe₃ is found to increase for pressures up to 0.6 GPa, while the superconducting transition temperature, T_c, decreases with increasing pressure. According to a band calculation, it is found that the pressure-induced enhancement of the CDW and suppression of the superconductivity are not simply explained by the effect of nesting of the Fermi surface, suggesting the possibility of a new relation for the competition between the CDW and superconductivity.

Polycrystalline MgB₂ with a sharp superconducting transition at 39 K was directly synthesized from the elements using high pressure. The sample showed high critical current densities. The electron energy-loss spectrum of B shows the peaks of σ and antibonding bonds, in good agreement with the hole-doping theoretical calculations. The pressure dependences of the sound velocities were measured up to 0.5 GPa, and subsequently elastic moduli, the Debye temperature and the specific heat were calculated. The isothermal Murnaghan equation of state of MgB₂ is established. Also, the pressure coefficient of the phonon frequency and the volume dependence of the electron–phonon coupling constant are calculated.

Electrical resistivity measurements in a magnetic field are carried out on UGe₂ which exhibits pressure-induced superconductivity. The superconductivity is observed from 1.06 to 1.44 GPa. In the temperature and field dependences of the resistivity at P > P_C where the ferromagnetic ordering disappears, it is observed that the application of an external field along the a-axis increases the coefficient A of the Fermi-liquid behaviour (∝ AT²) abruptly—corresponding to the metamagnetic transition. The characteristic enhancement of H_C2 is reconfirmed for H ∥ a-axis. The upper critical field of H_C2 is anisotropic: H_C2 (T) exhibits positive curvature for H ∥ b-axis and H ∥ c-axis.

It had been reported that for potassium chloride (KCl) the B1–B2 phase transition (PT) occurs under shock and static compressions, but the measured transition points showed large scatter. In this study, Hugoniot measurement experiments were performed on KCl single crystals by the inclined-mirror method combined with use of a powder gun. The anisotropic Hugoniot elastic limits and PT points were observed. The PT points along the ⟨100⟩, ⟨110⟩ and ⟨111⟩ axis directions were determined as 2.5, 2.2 and 2.1 GPa, respectively. The anisotropic transition was reasonably explained in terms of the displacement mechanism along the ⟨111⟩ axis direction.

We present the results from laser driven shock wave experiments for equation of state (EOS) studies of gold metal. An Nd:YAG laser chain (2 J, 1.06 μm wavelength, 200 ps pulse FWHM) is used to generate shocks in planar Al foils and Al + Au layered targets. The EOS of gold in the pressure range of 9–13 Mbar is obtained using the impedance matching technique. The numerical simulations performed using the one-dimensional radiation hydrodynamic code support the experimental results. The present experimental data show remarkable agreement with the existing standard EOS models and with other experimental data obtained independently using laser driven shock wave experiments.

Low amplitude shock waves (from 1 to 300 bar) have been generated in gold layers deposited on a quartz substrate, by laser pulses at an incident fluence from 0.4 to 4.0 J cm⁻². The quartz was used as a pressure gauge for recording the induced shock profile. At a fluence <1.4 J cm⁻², the shock pressure does not exceed 10 bar and the shock front is followed by a tension peak typical of an absorption in solid state. An analytical model of the compression–tension process has been developed, accounting for shock pressure and shock profile evolution as a function of irradiation conditions and material properties. From this model a mechanical interpretation is given to previous observations of spalling of the irradiated target surface.

Progress made in recent years on three topics that have been investigated at the Laboratory for Shock Wave and Detonation Physics Research are presented in this report. (1) A new equation of state (EOS) has been derived which can be used from a standard state to predict state variable change along an isobaric path. Good agreements between calculations for some representative metals using this new EOS and experiments have been found, covering a wide range from hundreds of MPa to hundreds of GPa and from ambient temperature to tens of thousands of GPa. (2) An empirical relation of Y/G = constant (Y is yield strength, G is shear modulus) at HT–HP has been reinvestigated and confirmed by shock wave experiment. 93W alloy was chosen as a model material. The advantage of this relation is that it is beneficial to formulate a kind of simplified constitutive equation for metallic solids under shock loading, and thus to faithfully describe the behaviours of shocked solids through hydrodynamic simulations. (3) An attempt at microstructure characterization for a failure wave in shocked glass has been carried out for the first time. Analyses on both the fractal dimension of the cracks' propagating path and the degree of damage in the failed region qualitatively revealed that ZF1 glass has a much less damaged structure than K9 glass at nearly the same loading stress. Based on the above analysis, we conjecture inhomogeneous immiscible phases, more in K9 than in ZF1, distributed in the glass body of the intrinsic factor, exhibiting as numerous locally strained spots due to the shock induced different compressibilities between the matrix and the immiscible phases. When the surface cracks, activated by the shearing action of one-dimensional strain loading, propagate and arrive at the strained spot boundaries, new cracks would be generated, accompanied by crack turning and branching, and thus cause glass body fracturing and fragmenting. In other words, the more numerous the strained spots are, the more severely damaged the structure of the shocked glass.

Experimental investigation of lithium compressed by multiple shock waves up to a pressure 210 GPa demonstrates an abnormal dependence on electric resistivity. As against normal behaviour of a metal, the resistivity monotonically increases in the pressure range 30–150 GPa from typical metallic values at ambient conditions by more than 15 times, returning to metallic values at pressures higher then 160–210 GPa. The obtained results demonstrate the anomalous resistivity of lithium both in solid and liquid states. This effect is explained by theoretical calculations done with the use of the linear muffin-tin orbital method.

A pump and probe technique is used for time-resolved measurements of the microstructure of condensed matter under laser shock compression. Two types of experiment (picosecond x-ray diffraction and nanosecond Raman spectroscopy) are performed. The picosecond time-resolved x-ray diffraction results for laser-shocked Si(111) give the time evolution of the strain profiles in 60 ps intervals. Nanosecond time-resolved Raman spectroscopy for laser-shocked poly-tetrafluoroethylene shows transient bond scission of the polymer chain.

Isentropic compression experiments that utilize intense magnetic fields to compress samples have been designed, developed and performed. The technique has been shown to work to pressures of more than 1 Mbar on Sandia National Laboratory's Z pulsed power machine. We are extending the technique to use high-explosive pulsed power.

Fine-grained bulk alloys with no crack in the 70:30 mol% Fe–Co system were prepared by means of shock compression of water-atomized powder and mechanical alloying (MA) treated ones. The grain size of the water-atomized bulk body was smaller (≤50μ m) than that of the molten bulk body (about 100μ m). The grain size decreased greatly with the MA treatment time, and ones for 21 h were estimated to be about 15 nm from the x-ray diffraction patterns. The coercivity value of the water-atomized bulk body was much larger than that of the molten bulk body. The coercivity value of the MA-treated bulk body increased with the MA treatment time, and then decreased, despite the very small grain size, probably due to the effect of ferromagnetic exchange interaction.

A heat conduction model for three parallel layers of dissimilar materials is proposed to describe the heat flow through the sample/μ m-size high-temperature layer/window after passage of a strong shock front. This model provides a possible approach to shock temperature measurements for metals using a disc sample. Using this model we derived a shock temperature or melting temperature of meteoritic iron based on the observed interfacial temperature by optical radiometry techniques. The data sets determined are in agreement with those measured using a film sample of stainless steel analogous to meteoritic iron in composition.

In this paper, the dynamic process of ejection from a metal surface groove under a shock wave is investigated by molecular dynamics simulation combined with a hybrid tight-binding-like potential. By taking 'snapshots' and analysing the pressure, we classify reflection rarefaction waves and second-uploading compression waves propagating in the material, and find one negative-pressure region and one high-pressure region induced by two wave series. The velocities of both the ejected atom and the free surface of the groove increase with the angle of the groove. When the half-angle of the groove is more than 60°, there is no ejected body, and this result is consistent with experiment.

When applying a laser shock to a substrate with a coating in order to test the adhesion strength of the interface, traction can be generated not only at the interface, but also within the materials. The effects of a possible rupture of these materials prior to the debonding is analysed by shock wave propagation mechanisms and experimentally evidenced for plasma sprayed coatings of alumina on an aluminium substrate. An estimate of the bond strength and the spall strength of the coating is obtained by numerical simulation.

Recently, we have found, by means of a shock wave experiment, that an empirical relation Y/G ≈ 1.9 × 10⁻² (Y is the yield strength and G is the shear modulus) is applicable for describing the strength effect for shocked 93W (93% W with 7% Fe–Ni–Co as binder) in the pressure range up to 150 GPa. This represents an extension of existing knowledge of the empirical approximation Y/G ≈ constant for potassium obtained at liquid-N₂ temperature and in the pressure range below 0.55 GPa. This approximation is advantageous in allowing one to simply and conveniently construct the constitutive equation for shocked metals.

The electrical conductivity of shock-compressed iron was measured up to 208 GPa by using an improved sample assembly in which the iron sample is encapsulated in a single-crystal sapphire cell. High-pressure shock compressions were generated by plate impact with a two-stage light-gas gun. The measured conductivity of iron varies from 1.45 × 10⁴ Ω ⁻¹ cm⁻¹ at 101 GPa and 2010 K, to 7.65 × 10³ Ω⁻¹ cm⁻¹ at 208 GPa and 5220 K. After analysing these data together with those reported previously, we found that the Bloch–Grüneisen expression is valid for ε-iron in the pressure and temperature range up to 208 GPa and 5220 K.

A thermodynamic equation of state is derived which is appropriate for investigating the thermodynamic variations along isobaric paths to predict compression behaviours of porous materials. This equation-of-state model is tested on porous iron, copper, lead and tungsten with different initial densities. The calculated Hugoniots are in good agreement with the corresponding experimental data published previously. This shows that this model can satisfactorily predict the Hugoniots of porous materials with wide porosity and pressure ranges.

The behaviour of elastic moduli of substances is analysed in the megabar pressure range. A new effect—inversion of the shear moduli and mechanical properties upon compression—is predicted for various classes of substances. The melting-curve data for different materials confirm the predicted phenomenon. The materials traditionally considered the softest, such as rare-gas solids and molecular substances, may become the hardest in the megabar range. This should be taken into account in developing experimental high-pressure techniques.

First-principles calculations are performed for the lattice dynamics and electron–phonon interaction of the body-centred-cubic (bcc) phase of solid vanadium.

A remarkable phonon anomaly is found, i.e. frequencies of the transverse mode around a quarter of the Γ–H line show softening with increasing pressure and become imaginary at pressures higher than ∼130 GPa.

The superconducting transition temperatures T_c of bcc vanadium estimated as a function of pressure increases at first linearly with pressure, and then the rate of increase of T_c is abated around 80 GPa.

This calculated pressure dependence of T_c shows qualitatively the same behaviour as the experimental result.

We studied crystal structures of potassium niobate (KNbO₃) at high pressure by means of first-principles self-consistent total-energy calculations within the local density approximation using the full-potential linear muffin-tin orbital method. For the first time, we have calculated the atomic equilibrium volume, bulk modulus, total energy, and transition pressures for KNbO₃, covering the full pressure range for which the above-mentioned experiments have been done. Two pressure-induced transitions are derived theoretically, namely one from orthorhombic (I) to cubic structure at around 13.2 GPa and a second from cubic to orthorhombic (II) structure at a pressure of 39.7 GPa. This fully confirms the recent experiments by Kobayashi et al(Kobayashi Y, Endo S, Ashida T, Ming L C and Kikegawa T 2000 Phys. Rev.61 5819)

X-ray diffraction experiments carried out on alkali metals under high pressure have provided new insight into pressure-induced structural transformations. New structures have been identified, and some of these have surprising similarities, such as low coordination numbers. The present ab initio calculations of electronic and structural properties provide theoretical support for the analysis of these experiments, and may also serve to predict new properties, such as superconductivity, of the materials when exposed to high pressures.

We have performed first-principles electronic structure calculations for high-pressured phases of selenium by using the full-potential linearized augmented-plane-wave method. We present electronic structures and the optimized atomic positions in the unit cell for the third and fourth phases. From the positions obtained, the space group of the fourth phase was determined as P 2₁ /m, which is almost consistent with the experimental results.

Structural stability is investigated for the CaCl₂ structure of SiO₂ by using a first-principles linear response theory. Softening of modes is found in the direction of the Γ–X (100) axis in k-space at 810 GPa. The results of the present work strongly support the assertion that the P2₁/n structure is very reasonable suggestion for the post-CaCl₂ structure.

The recently developed classical mean-field potential (MFP) approach was employed to calculate the Helmholtz free energy for periclase (MgO), the Hugoniot equation of state for beryllium (Be) at pressures up to 2000 GPa, and the families of isentropic curves for tungsten (W). The excellent agreement between the theory and the experiment further demonstrated the high accuracy and applicability of the MFP approach.

In an effort to achieve a comprehensive understanding of the structure of dense H₂, we have performed path-integral Monte Carlo simulations for three combinations of pressures and temperatures corresponding to three phases of solid hydrogen. Our results suggest three kinds of distribution of molecules: orientationally disordered hexagonal close packed (hcp), orientationally ordered hcp with Pa3-type local orientation order and orientationally ordered orthorhombic structure of Cmca symmetry, for the three phases.

By calculating the single-crystal elastic constants of W and Mo by first-principles methods the pressure strengthening of the yield stress was obtained. This was used to calculate the behaviour of the hydrogen sample hole size in a tungsten gasket. It was found that drastic changes occur depending on r_s⁰ /r_t where r_s⁰ is the initial sample radius and r_t is the diamond tip radius. It is seen why, with r_s⁰ /r_t = 0.9, the Carnegie group cannot and have not exceeded pressures of 230 GPa and why, with r_s⁰ /r_t = 0.5, the Cornell group has been able to reach 342 GPa.

We observe an anisotropy of the propagation velocities of longitudinal and transverse ultrasonic waves, as well as of the hardness, for disordered graphite-like samples obtained from the C₆₀ fullerite, which is heated to different temperatures under a pressure of 7.5 GPa. The anisotropy of the elastic properties and the hardness is connected to the additional pressure component that occurs in the quasi-hydrostatic experimental conditions. The elastic characteristics of the samples are determined. We propose a model description relating the observed properties of superhard sp² carbon to its possible structural features and to the mechanism of its formation.

A comprehensive numerical simulation study of laser-driven shock wave propagation in planar aluminium foils, 20– 50μ m, is performed using the one-dimensional radiation hydrodynamic code MULTI. The effect of the spatial mesh size on the shock velocity and peak shock pressure is found to be significant and the optimum mesh size is obtained. Shock velocities and maximum pressure are calculated through simulations for two sets of laser intensities used in an experiment (Fu S, Gu Y, Wu J and Wang S 1995 Phys. Plasmas2 3461). By using different sets of laser absorption coefficient values we determine the appropriate values reproducing the experimental results. We demonstrate that the simulations can be used as an effective tool for benchmark calculation of laser absorption coefficients.

The ω (hexagonal) to β (bcc) transformation in Zr and Hf occurs at 30 and 71 GPa under static pressures. This transition has not been found in Ti up to 87 GPa. On the basis of full-potential linearized augmented plane wave calculations aided with thermal and entropy correction we predict an ω → β transition in Ti at around 102 GPa along the 300 K isotherm. In addition to this, we calculate the ω → β transitions in Zr and Hf at around 27 and 65 GPa respectively, which are in excellent agreement with the experimental values. The ω → β transition pressures, 102, 27 and 65 GPa for Ti, Zr and Hf respectively, do not follow the trend implied by the principle of corresponding states. We have analysed the causes for this anomalous trend. We observe that the ω → β transition depends on how the increased d-population due to s–d transfer under pressure is distributed in the different d-substates. For example, at ambient conditions, the bcc phase is unstable and the difference between the average charges in the bcc stabilizing d-t_2g state and the destabilizing d-e_g state are 0.008, 0.082 and 0.013 for Ti, Zr and Hf respectively. Compression increases this difference and stabilizes the bcc structure when it becomes about 0.1. The charge transfer needed for stabilizing the β structure is highest for Ti followed by Hf and then Zr, in line with the trend in transition pressures.

We report electronic structure calculations on zinc at various compressions. Our calculations show that an electronic topological transition (ETT) associated with the anomaly in the axial ratio (c/a) at high pressures is sensitive to k sampling used in the calculations, exchange–correlation potential and temperature. We show that K-point ETT does not contribute to the anomaly, and thus disagree with the model of contributions from many ETTs. Hence only the L-point ETT is significant. We suggest that, even though temperature smearing of the Fermi function reduces the signatures of ETT, it does not wipe out the anomaly at room temperature.

Pressure-induced behaviours of 2p energy levels of lithium in solid H₂ are investigated by using the path integral Monte Carlo method with a constant-pressure ensemble (NPT). For the doped solid where a Li atom replaces one H₂ molecule, the splittings between the three degenerate energy levels of the Li atomic 2p excited state are equal. For the doped solid where a Li atom replaces two H₂ molecules, a novel different split of the three 2p energy levels is observed. An increase in pressure results in a significant shift of the individual 2p energy levels of the Li atom. The centroid of the 2p energy levels takes on a fascinating pressure dependence in which, with increasing pressure, the centroid of the 2p energy levels shifts towards higher energy below a turning pressure (P_t), arrives at the highest energy at P_t, then shifts towards lower energy above P_t.

Ohmic electrodes in the form of n-type (Si-doped) cubic boron nitride (c-BN) bulk crystals were fabricated by utilizing a covering technique, depositing Ti(10 nm)/Mo/(20 nm)/Pt–Au(200 nm) ohmic contact metal on both the sides of the c-BN substrate. The size of the specimen electrode was 100 × 100 μ m² on one side and 300 × 300 μ m² on the other side. Measurements on the specimen were made using a specially made device. Linear current–voltage characteristics were obtained. It is considered that the contact between the Ti-and Si-doped c-BN was ohmic.

Iron carbide without any graphite was studied under high pressure and high temperature (HPHT); diamond layers were obtained both on diamond and on cubic boron nitride seeds at 5.5 GPa and 1700–1750 K. The results showed that transition-metal carbide was the main intermediate in the course of the transformation from graphite to diamond under HPHT.

A determination of the phase constituents of ceramic coatings produced on Al–Cu–Mg alloy by microarc discharge in alkaline solution was performed using x-ray diffraction. The profiles of the hardness, H, and elastic modulus, E, across the ceramic coating were determined by means of nanoindentation. In addition, a study of the influence of microarc oxidation coatings on the tensile properties of the aluminium alloy was also carried out. The results show that the H-and E-profiles are similar, and both of them exhibit a maximum value at the same depth of coating. The distribution of the α-Al₂O₃ phase content determines the H- and E-profiles of the coatings. The tensile properties of 2024 aluminium alloy show less change after the alloy has undergone microarc discharge surface treatment.

The results are reported of high-pressure–high-temperature (HPHT) treatment experiments on natural diamonds of different origins and with different impurity contents. The diamonds are annealed in a temperature range up to 2000^oC at stabilizing pressures up to 7 GPa. The evolution is studied of different defects in the diamond crystal lattice. The influence of substitutional nitrogen atoms, plastic deformation and the combination of these is discussed. Diamonds are characterized at room and liquid nitrogen temperature using UV–visible spectrophotometry, Fourier transform infrared spectrophotometry and photoluminescence spectrometry. The economic implications of diamond HPHT treatments are discussed.

The catalytic effects of copper-based alloys in diamond growth have been investigated. A single crystal of diamond has been obtained by the temperature gradient method (TGM), using Cu–Mn–Co and Cu–Co alloys as catalysts. It was found that the melted Cu–Mn–Co and Cu–Co alloys show low viscosity. The eutectic temperatures of these two alloys with graphite were between 1130 and 1150°C, and the temperature of the transition to diamond was over 1300°C at 5.5 GPa. High-quality diamond could not be obtained in Cu–Co alloy by the TGM. Our results suggest that adding Cu to a catalyst cannot decrease the reaction temperature for diamond growth.

On the basis of phase equilibria, the lowest temperatures, T_min, above which at high pressures cubic boron nitride crystallization from melt solution is allowable in terms of thermodynamics have been found for a number of systems that include boron nitride.

Diamond films have been grown on polished Si substrates seeded with nanocrystalline diamond powder colloid using hot-filament chemical vapour deposition. Instead of using the conventional gaseous carbon source, a carbonized W filament was used as the carbon source. The only feeding gas was hydrogen. Compared with those produced by traditional methods, the polycrystalline diamond grown by this new method has smaller grain size. The growth mechanism is also discussed.

Polycrystalline diamond films have been patterned on a polished Si substrate by means of selective seeding via hot-filament chemical vapour deposition. In addition to the process of selective seeding, the CH₄/H₂ concentration and the sizes of the patterns have effects on the selectivity. The mechanism of selective growth of diamond is also discussed in this paper.

In this paper, we review recent developments relating to cubic boron nitride (cBN) abrasive grains and sintered cutting tools. The demand for high-speed machining and the ecological benefits of using ferrous materials have led to developments in the area of heavy-duty dry cutting and grinding processes in recent years. Optimization of the process of manufacturing cBN materials is an important issue, both fundamentally and as regards applications. We review recent developments in cBN applications and discuss the challenges arising from new processes encountered in basic cBN study at high pressure and high temperature.

High-resolution electron microscopy (HREM) studies of the microstructure and specific defects in hexagonal boron nitride (h-BN) precursors and cubic boron nitride (c-BN) crystals made under high-pressure high-temperature conditions revealed the presence of half-nanotubes at the edges of the h-BN particles. Their sp³ bonding tendency could strongly influence the nucleation rates of c-BN. The atomic resolution at extended dislocations was insufficient to allow us to determine the stacking fault energy in the c-BN crystals. Its mean value of 191 ± 15 mJ m⁻² is of the same order of magnitude as that of diamond. High-frequency (94 GHz) electron paramagnetic resonance studies on c-BN single crystals have produced new data on the D1 centres associated with the boron species. Ion-beam-induced luminescence measurements have indicated that c-BN is a very interesting luminescent material, which is characterized by four luminescence bands and exhibits a better resistance to ionizing radiation than CVD diamond.

Boron-doped p-type diamond thin film was grown heteroepitaxially on silicon-doped n-type cubic boron nitride (c-BN) bulk crystal by the conventional hot-filament chemical vapour deposition method. A diamond thin-film/c-BN heterojunction p–n diode was fabricated by the covering technique for the first time. The rectification ratio of the diode reached five orders. The threshold value is 1 V; the reverse bias voltage is 6 V. The results indicate that the device is of great importance.

Despite great technological importance and many investigations, a material with a measured hardness comparable to that of diamond or cubic boron nitride has yet to be identified. Our combined theoretical and experimental investigations led to the discovery of a new polymorph of titanium dioxide, where titanium is ninefold coordinated to oxygen in the cotunnite (PbCl₂) structure. Hardness measurements on this phase, synthesized at pressures above 60 GPa and temperatures above 1000 K, reveal that this material is the hardest oxide yet discovered. Furthermore, it is one of the least compressible (with a measured bulk modulus of 431 GPa) and hardest (with a microhardness of 38 GPa) polycrystalline materials studied so far.

Photoluminescence measurements have been conducted for a CdTe/Cd_1-xMn_x Te (x = 0.4) single-quantum-well structure at low temperatures under pressures to 0.49 GPa and magnetic fields to 60 T. At the ambient pressure, a new emission was induced by the application of a magnetic field. The emission has been assigned to exciton emission from the barrier layer, which is suppressed below 9 T due to the energy transfer from the exciton to local d electrons. At 0.49 GPa, the emission recovered at 44 T. In the field region where the energy transfer occurs, an anomalous red-shift of the exciton energy was observed clearly for the case of the ambient pressure. The alloy potential fluctuation effect and the magnetopolaron effect are examined as candidates for the mechanism to cause this phenomenon.

We have investigated for the first time the high-pressure optical absorption of β-FeSi₂ thin films (90 nm in thickness) prepared from Si/Fe multilayers on Si(001) with a template and SiO₂ capping. The pressure coefficient for the direct band gap of β-FeSi₂ is determined as 15.9 meV GPa⁻¹. This small coefficient is due to the negative deformation potential of the valence band maximum, and the large bulk modulus of β-FeSi₂.

Pb–Sn–Te bulk nanocrystalline (NC) materials are prepared successfully by quenching melts under high pressure. The mean particle size is about 100 nm and the crystal structure is NaCl type. The mechanism of formation of the bulk NC alloy is explained: there is an increasing of the nucleation rate and a decrease in the growth rate of nuclei with increase of pressure during the solidification processes. The thermoelectric properties of Pb–Sn–Te bulk NC alloy are enhanced. This method is promising for producing thermoelectric materials with improved high-energy conversion efficiency.

Owing to its unusual bonding and vast variety of unique crystal structures, boron is one of the most fascinating elements in the periodic table. Here we report the large-scale synthesis of well-ordered boron nanowires and their structural stability at high pressure. Boron nanowires with uniform diameter and length grown vertically on silicon substrates were synthesized by radio-frequency magnetron sputtering with a target of pure boron using argon as the sputtering atmosphere without involvement of templates and catalysts. Detailed characterization by high-resolution transmission electron microscopy and electron diffraction indicates that the boron nanowires are amorphous. Structural stability of the boron nanowires at room temperature has been investigated by means of in situ high-pressure energy-dispersive x-ray powder diffraction using synchrotron radiation in a diamond anvil cell. No crystallization was observed up to a pressure of 103.5 GPa, suggesting that the amorphous structure of boron nanowires is stable under high pressure at ambient temperature.

By using aluminium nanopowder prepared by wire electrical explosion, pure monophase NiAl compound with fine crystallites (≤10μm) and good densification (98% of the theoretical green density) was successfully fabricated by means of self-propagating high-temperature synthesis (SHS) under a high pressure of 50 MPa. Investigation shows that, due to the physical and chemical characteristics of the nanoparticles, the SHS reaction mode and mechanism are distinct from those when using conventional coarse-grained reactants. The SHS reaction process depends on the thermal conditions related to pressure and can occur at a dramatically low temperature of 308^oC, which cannot be expected in conventional SHS reaction. With increasing pressure, the SHS explosive ignition temperature (T_ig) of forming NiAl decreases due to thermal and kinetic effects.

High pressures in solid-state chemistry can play two important roles: (i) in decreasing the inter-atomic distances in existing materials, such a modification inducing electronic or structural phenomena; (ii) in the synthesis of new materials through the densification effect, the stabilization of the precursors, the compressing of the corresponding atoms, or improvement of the reactivity.

Second-harmonic-generation (SHG) measurements on ZnSe at high pressure, up to 7 GPa, have been reported. The zinc-blende–rock-salt transition pressure has been determined at room temperature from the SHG in ZnSe using a femtosecond laser. The pressure required to induce transformation from a zinc-blende to a rock-salt structure decreases from 11.5 to 1.07 GPa in a femtosecond laser field. SHG can be used to monitor structural changes under pressure of some materials with nonlinear optical properties.

High-pressure in situ energy-dispersive x-ray diffraction experiments on GaN nanocrystals with 50 nm diameter have been carried out using a synchrotron x-ray source and a diamond-anvil cell up to about 79 GPa at room temperature. A pressure-induced first-order structural phase transition from the wurtzite-type structure to the rock-salt-type structure starts at about 48.8 GPa. The rock-salt-type phase persists to the highest pressure in our experimental range.

Pressures up to a few 100 GPa and temperatures as high as a few 1000 K have been achieved with high dynamic pressures using a two-stage light-gas gun. Results are reviewed for molecular fluids, metallic hydrogen, solids, implications for planetary interiors, and structures and properties of materials recovered intact from high dynamic pressures.

The results obtained with the use of pressure-grown GaN single-crystalline substrates allow us to draw the following conclusions important for the construction of In-free UV light emitting diodes and lasers and InGaN-based high-power blue lasers. (1) The application of pressure-grown GaN single-crystalline substrates allows us to grow near-dislocation-free layer structures by both metal– organic chemical vapour deposition and molecular beam epitaxy. (2) The elimination of dislocations leads to highly efficient UV emission from GaN and GaN/AlGaN quantum wells, which is impossible for strongly dislocated structures grown on sapphire. (3) At high excitations (e.g. in lasers), dislocations are also effective non-radiative recombination centres in InGaN-containing structures, so the elimination of these defects is crucial for better performance of blue lasers. In this paper, the optical and structural properties of the near-dislocation-free GaN-based structures leading to the above conclusions are discussed.

We have studied the high-pressure and high-temperature behaviour of α-PbO₂-type TiO₂–SnO₂ (5 mol%) nanocomposite up to 62.3 GPa and 1700 K in a laser-heated diamond-anvil cell by means of synchrotron energy-dispersive x-ray diffraction. We found that it transforms to the baddeleyite phase at 19.4 GPa at room temperature. This phase was stable up to about 40 GPa. At 62.3 GPa and 1700 K, the diffraction pattern showed that there exists another nonquenchable phase. We discussed the mechanisms for these high-pressure transformations in α-PbO₂-type TiO₂–SnO₂ (5 mol%) nanocomposite.

Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 bulk metallic glass (BMG) was annealed at 573 K under a high pressure of 3 GPa. The effects of the structural relaxations of BMG on its mechanical and acoustic properties are investigated by using x-ray diffraction, ultrasonic study, density measurements, compression testing, as well as microhardness. It is found that high-pressure relaxation results in a microstructural transformation in the BMG. A sample relaxed under high pressure exhibits 3% plastic strain compared with the crystallized form, and the compression strength achieved, 1800 MPa, is improved by nearly 20% over that for the as-prepared sample; moreover, a BMG with relaxed structure exhibits markedly different acoustic properties.

Measurements of the temperature dependence of the AC susceptibility were made for Fe–Ni Invar mechanical alloys under hydrostatic pressures up to 1.5 GPa. The Curie temperatures decreased linearly with pressure. The rate of decrease became larger for specimens annealed at higher temperatures. The temperature of annealing after ball milling has been directly related to the extent of the chemical concentration fluctuation, and the extent becomes smaller for specimens annealed at higher temperature. This tendency can be explained by assuming a Gaussian distribution function.

Solvothermal synthesis appears to be an interesting route for preparing nitrides such as gallium nitride and aluminium nitride, using ammonia as solvent. A nitriding additive is used to perform the reaction and, in the case of gallium nitride, is encapsulated by melt gallium. The syntheses are performed in the temperature range 400–800°C and in the pressure range 100–200 MPa. The synthesized powders are characterized by x-ray diffraction and scanning electron microscopy. Finely divided gallium nitride GaN and aluminium nitride AlN, both with wurtzite-type structure, can be obtained by this route.

We have studied the pristine C₆₀ and its molecular complexes with the organic donors bis(ethylenedithio) tetrathiafulvalene (BEDT-TTF or ET) and tetramethyltetraselenafulvalene (TMTSF) by means of ESR and Raman spectroscopy at high pressure. The important changes in the ESR signal of C₆₀ were observed under axial pressure combined with shear deformation. It is shown that the treatment at a anisotropic pressure of 4 GPa results in a reduction in the symmetry of the C₆₀ molecule and the formation of radicals. Treatment of the molecular complex of (ET)₂·C₆₀ at a pressure of ∼4.5 GPa and a temperature of 150°C leads to the formation of C₆₀ dimers. The Raman spectra of the molecular complex C₆₀·TMTSF·2(CS₂) were measured in situ at ambient temperature and pressures up to 9.5 GPa. The pressure behaviour of the Raman peaks reveals singularity at 5.0 ± 0.5 GPa related to the softening and splitting of some of the phonon modes. The residual softening of the A_g (2) mode is the same as in the case of KC₆₀, which may be an indication that the transition has a charge-transfer character, resulting in the formation of the C₆₀⁻¹ anionic state charge-transfer complex.

Energy-dispersive x-ray diffraction measurements have been carried out in the perovskite La_0.5Ca_0.5MnO₃ and bismuth-doped La_0.25Bi_0.25Ca_0.5MnO₃ under hydrostatic pressure in a diamond cell. On the substitution of La³⁺ ion with Bi³⁺ ion, a shoulder peak appears in the observed main peak of La_0.25Bi_0.25Ca_0.5MnO₃ at 43.9 GPa, but not in that of La_0.5Ca_0.5MnO₃ with the pressure up to 45.9 GPa. This phenomenon can be explained by a number of discrete clusters that are simultaneously present in the sample, due to the pressure enhanced interactions between charge, orbital and coupling with the lattice distortion coming from the unique 6s² lone-pair characteristics of Bi³⁺.

Gallium nitride, aluminum nitride and indium nitride are basic materials for blue optoelectronic devices. The essential part of the technology of these devices is annealing at high temperatures. Thermodynamic properties of the Ga–N system and their consequences to application of high nitrogen pressure for the annealing of GaN based materials are summarized.

The diffusion of Zn, Mg and Au in high dislocation density heteroepitaxial GaN/Al₂O₃ layers will be compared with the diffusion in dislocation-free GaN single crystals and homoepitaxial layers. It will be shown that high dislocation density can drastically change the diffusion rates, which strongly affects the performance of nitride devices.

Inter-diffusion of Al, Ga and In in AlGaN/GaN and InGaN/GaN quantum well (QW) structures will be also considered. It will be shown that in contrast to stability of metal contacts, which is strongly influenced by dislocations, the inter-diffusion of group III atoms in QW structures is not affected strongly by the presence of high dislocation density. This is related to the different rate controlling slow process in these two diffusion processes. This feature of interdiffusion processes explains the success of heteroepitaxial techniques in the technology of nitride based light emitting diodes.

We show that when we use highly dispersed oxides called fumed silica, a pressure-induced structural transition occurs at lower pressures (2–8 GPa) than would normally be expected for bulk a-SiO₂ (over 10 GPa). Furthermore, this transition finally results in a transparent monolith at 6–8 GPa, accompanied by densification, even at room temperature. We suggest that this novel polymorphic modification of a-SiO₂ results from the highly reactive nature surface strained Si–O bonds that are formed particularly in the compressed fumed silica samples.

Energy-dispersive x-ray diffraction studies are carried out on the distorted perovskite La_0.5−xBi_xCa_0.5MnO₃ (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25) under high pressure at room temperature. The unusual expansion of the 202–040 d-spacing under pressure is observed, and the change of the Mn–O bond angle brings about the disappearance of the basal-plane Q₂ distortion mode. With doping content increasing, a shoulder peak appears in the observed main peak of La_0.25Bi_0.25Ca_0.5MnO₃ at 43.9 GPa. The pressure-enhanced interactions between charge, orbital, and coupling with the lattice distortion are discussed.

An ideal n = 3 member of the group of Ruddlesden–Popper (RP) phases Ca₄Mn₃O₁₀ was obtained by solid-state reaction under high pressure. This phase has been studied by transmission electron microscopy, convergent beam electron microscopy, high-resolution transmission electron microscopy, and parallel electron energy-loss spectroscopy. The lattice parameters are derived as a = b = 0.37 nm and c = 2.69 nm with the space group I4/mmm, which is the space group of the ideal RP phase.

We have measured the temperature and magnetic field dependence of the resistance for bulk samples of single-walled nanotubes treated in different ways. Hydrostatic pressure-treated samples and as-grown samples show a two-dimensional variable-range hopping behaviour while samples treated in non-hydrostatic pressure or in acid for a long time show a three-dimensional transport behaviour. We suggest that the observed two-dimensional behaviour arises from a network structure with a fractal dimension lower than three.

In situ interferometric observation of the process of CO₂ dissolving into PMMA (polymethyl methacrylate) was carried out at high pressure and room temperature, in view of the interest to polymer foam technology. By analysing interferograms, the phase shift of a laser beam transmitted though the sample (the phase is related to the dissolved gas content and polymer thickness) was obtained as a function of time and pressure. At constant pressure, the phase shift initially increased over time and reached a maximum value, and then decreased asymptotically to a constant value as equilibrium was approached. In addition, the equilibrium value of the phase shift increased with increase in pressure for pressures lower than the glass transition pressure P_g, and decreased at pressures higher than P_g. Thus, the optical properties exhibited during the process of the gas dissolving were drastically different for the glassy state and the rubbery state of the polymer.

The pressure dependence of the Curie temperatures of the Fe₂P-type intermetallic compounds MnRhP, MnRhAs and MnRhAs_0.6P_0.4 was investigated up to 8 GPa. For MnRhAs both the Curie temperature T_C and the temperature T_t of the transition from AF_I to the canted state increased with pressure. However, T_t decreased abruptly at 5 GPa where a pressure-induced ferromagnetic transition was observed. In contrast, the rate of increase of T_C increased substantially at this pressure. The large positive pressure dependence of T_C in these compounds is found to be strongly related to the value of the lattice parameter c.

The ferroelectric transition temperature T_c of (NH₂CH₂COOH)₃·H₂SO₄ (TGS), which is a typical order–disorder-type ferroelectric, was determined by dielectric constant and Raman scattering measurements under high pressure. T_c increased, passed through a maximum and then decreased slightly with increasing pressure, and then abruptly dropped at about 2.5 GPa, where a transition to a new high-pressure phase was confirmed to exist. A tentative p–T phase diagram was proposed for TGS.

The physical properties relating to 4f electrons in cerium phosphide, especially the temperature dependence and the isomorphous transition that occurs at around 10 GPa, were studied by means of x-ray powder diffraction and charge density distribution maps derived by the maximum-entropy method. The compressibility of CeP was exactly determined using a helium pressure medium and the anomaly that indicated the isomorphous transition was observed in the compressibility. We also discuss the anisotropic charge density distribution of Ce ions and its temperature dependence.

A new intermetallic compound, Mn₃Ge, has been synthesized by direct reaction of elemental components at 6.2 GPa and 1000°C for 30 min using a belt-type high-pressure apparatus. The compound crystallizes into a cubic structure with the space group Pm3m, namely the L1₂-type (Cu₃Au-type) structure. The structure was refined by Rietveld analysis of the powder x-ray diffraction data and the lattice constant was determined as a = 0.380 19(3) nm. The compound shows metallic conductivity and ferromagnetism with a Curie temperature of 400 K.

A hexagonal hydride of ZrCr₂H_x with a hydrogen concentration of x = 5.75 was synthesized using high hydrogen pressure (5 kbar) at 100°C for 72 h. The synthesized hydride is well characterized as regards structure and/or stability, as compared with cubic or hexagonal ZrCr₂ hydrides that are formed under conventional hydrogenation conditions.

An amorphous Fe–N alloy was prepared by ball milling a mixture of Fe and h-BN. Its crystallization processes induced by mechanical milling (MM) and annealing under normal and high pressure were studied. The crystallization product of the amorphous Fe–N alloy induced by MM and annealing at temperatures between 690 and 800 K under pressures of 3–4 GPa is ε-Fe_xN, while the thermal crystallization product under normal pressure is γ'-Fe₄N. The difference between the crystallization products produced by mechanical and thermal crystallization is attributed to the effects of local pressure and local temperature produced by ball collisions.

The two-leg ladder SrCu₂O₃ belongs to the homologous series Sr_{n −1}Cu_nO_2n−1 and was first synthesized by Hiroi et al (Hiroi Z, Azuma M, Takano M and Bando Y 1991 J. Solid State Chem.95 230). Due to the massive reaction of the sample material with the crucible (Au or Pt), only short reaction times up to 30 min have been used typically. We suggest an improved composite crucible with a single-crystalline MgO inset in a sealed Pt capsule and demonstrate its capability for single-crystal growth of SrCu₂O₃. Experiments with a typical duration of 15 h were conducted at pressures between 3 and 6 GPa at temperatures between 950 and 1500°C. Slow cooling of the melt effected directed growth of the first SrCu₂O₃ milimetre-sized single crystals.

Using a diamond anvil cell,⁵⁷Fe Mössbauer measurements of the high-pressure phase of iron, ε-Fe at 20 GPa, have been performed at 4.5 K under external magnetic fields up to 7 T. The magnitudes of the hyperfine magnetic fields depend linearly on the external magnetic fields, H_ext. This implies that there is no induced hyperfine field due to the local magnetic moment and ε-Fe under 20 GPa at 4.5 K is determined as a Pauli paramagnet.

The pressure dependence of first-order magnetic Raman peak of NiO single crystal was studied up to 20 GPa at room temperature. At ambient pressure, an unknown peak is also observed at nearly the same position as the one-magnon one and their separation becomes remarkable with increasing pressure. Pressure coefficients of the one-magnon peak and the other peak are obtained as 0.4 and 1.5 cm⁻¹ GPa⁻¹, respectively. The next-nearest-neighbour antiferromagnetic exchange constant J₂ is obtained as a function of the lattice constant.

The decomposition behaviour of CuO is studied at high temperature and high pressure. Experimental pressure and temperature determine the result. In the region of higher temperature and pressure (≥5.5 GPa, ≥1400 °C), the product is just copper. In the region of lower temperature and pressure (< 5.0 GPa, < 1100 °C), CuO does not decompose. Between the two regions, the product is a mixture of Cu and Cu₂O or a mixture of Cu₂O and CuO.

Superhard MgB₂ bulk material with a golden metallic shine was synthesized by high-pressure sintering for 8 h at 5.5 GPa and different temperatures. Appropriate pressure and temperature conditions for synthesizing polycrystalline MgB₂ with high hardness were investigated. The samples were characterized by means of atomic force microscopy and x-ray diffraction. The Vickers hardness, bulk density, and electrical resistivity were measured at room temperature.

Lead telluride (PbTe) with rock-salt structure was successfully obtained by a high-pressure and high-temperature (HPHT) method. The orientation of the PbTe samples varies with pressure increase. The results—a decrease in the Seebeck coefficient, resistivity and thermal conductivity of PbTe with pressure but an increase in the thermoelectric power figure σS²—indicate that the figure of merit Z of PbTe samples can be improved several times over by using HPHT.

The ternary inter-metallic compound, UFe₅Al₇, crystallize in a tetragonal ThMn₁₂ type structure. In the as-cast samples a residual phase of FeAl (∼2% wt) was identified in the grain boundaries. The amount of the residual cubic phase of FeAl was determined by Rietveld analysis and reduced by the annealing process.

UFe₅Al₇ maintains the tetragonal symmetry as a function of pressure, while FeAl keeps the cubic structure as was determined by the Rietveld analysis. The volume–pressure curve calculated from the x-ray analysis is V/V₀ = 0.87 for UFe₅Al₇ at 26.0 GPa.

Crystallized polyethylene terephthalate (PET) samples were obtained at high pressures of 200–400 MPa at a temperature of 603 K, and another group of the samples were made at pressures of 250–350 MPa and different temperatures with a fixed supercooling. The samples were investigated by means of differential scanning calorimetry and scanning electron microscopy. Characterization results suggested that high pressure could increase the crystallization rate and promote the thickening process of PET lamellar crystals.

C₃N₄H₄ was treated at 6.0 GPa and 1500 °C for 2.5 min. Powder x-ray measurement shows that the sample is decomposed and a hexagonal graphite phase forms. Transmission electron microscopy studies show that small amounts of diamond and amorphous carbon phase coexist with the graphite phase. Parallel electron energy-loss spectroscopy analysis was also carried out for these phases.

Using nanoparticles of ZrO₂ (disordered structure) prepared by the method of precipitation as starting materials, ZrO₂ nanosolids have been synthesized under different pressures and at different temperatures. The x-ray diffraction results show that the crystallization temperature of the nanoparticles and the temperature at which the structural cubic–monoclinic transformation occurs are obviously reduced for the nanosolids synthesized under high pressure. X-ray photoelectron spectroscopy and EPR measurements indicate that there are some Zr³⁺ ions in nanosolids. With increasing pressure, the number of Zr³⁺ ions in the nanosolids essentially does not vary, and yet the content of oxygen ions with unsaturated bonds and dangling bonds in the interfacial region gradually decreases. The effect of synthesis pressure on the structure and interface states is mainly due to the decrease of the interatomic distance and the increasing interdiffusion of atoms in the interface phase.

The single crystals of six kinds of metal nitride halide, β- M NX (M = Zr, Hf; X = Cl, Br, I), were grown in sealed Au (or Pt) tubes by the reaction of M N or M NX powders with NH₄X as fluxes under high-temperature and high-pressure conditions such as 3–5 GPa at 900–1200^oC. The x-ray structure analysis revealed that all six kinds of compound crystallize in a rhombohedral space group Rm, Z = 6. β-ZrNCl, β-ZrNBr, and β-HfNCl are isotypic with SmSI, and the others isostructural with YOF.

The paper reviews the results of experimental studies and thermodynamical modelling of metastable T–P diagrams of initially amorphous GaSb–Ge and Zn–Sb alloys which provide a new insight into the problem of pressure-induced amorphization.

Several rare-earth silicates have been synthesized at 10 GPa and 1600–1700 °C: a La orthosilicate (La₄Si₃O₁₂) with a defect Ba₃(PO₄)₂-type, a new structure type (K) for Nd and Gd disilicates (Nd₂Si₂O₇ and Gd₂Si₂O₇) with a diorthosilicate structure, and a new structure type (L) for Dy disilicate (Dy₂Si₂O₇) with a structure containing linear triple tetrahedral groups [Si₃O₁₀], but having one in six atoms distributed with 50% occupancy over two tetrahedral positions.

The temperature dependences of the dc conductivity and thermopower of the bulk amorphous alloy Al₃₂Ge₆₈ were investigated at 6–420 K and at 80–370 K, respectively. The samples were prepared by solid-state amorphization of a quenched crystalline high-pressure phase while heating from 77 to 400 K at ambient pressure. Amorphous Al₃₂Ge₆₈ was found to be a p-type semiconductor with an unusual combination of transport properties. The behaviour of the properties was described semi-quantitatively in terms of a modified Mott–Davis model assuming that the Fermi level lies inside the valence band tail.

Since their discovery in 1965, various compositions of clathrate phases of silicon have been investigated and have revealed a direct correlation between the doping element and their properties. The recent development of a new synthesis technique using high-pressure and high-temperature (HPHT) conditions allows the synthesis of peculiar clathrate compositions which can show fascinating properties, such as Ba₈Si₄₆ which is a sp³ silicon-based structure with superconducting characteristics. This work reports the synthesis of the first binary silicon clathrate doped with an electronegative element and prepared using HPHT: I₈Si_{46 −x}I_x. Some chemical and structural results are also presented.

We report the results of direct synthesis of aluminium nitride (AlN) under high nitrogen pressure up to 1 GPa and temperatures up to 2000 K. At pressure from 10 to 650 MPa we observe the combustion synthesis of AlN. As the result of the combustion process one can obtain the AlN sintered powder or AlN/Al metal matrix composites. For N₂ pressure higher than 650 MPa the crystal growth of AlN from the solution of atomic nitrogen in aluminium is possible. Both needle-like and bulk AlN single crystals, up to 1 cm and 1 mm, respectively, have been obtained.

Structural changes in bulk metallic Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 glass subjected to heat treatments under high pressure were investigated by means of synchrotron radiation x-ray diffraction (SRXRD). In situ SRXRD measurements showed that the crystallization process of this material is comprised of two stages. Subsequent heating at 10 GPa converts the sample from the amorphous (Am) phase into a metastable fcc phase and then leads to the fcc phase changing back to the Am phase, indicating that there is a kind of 'reversible' phase transition phenomenon occurring in the alloy. Such phenomenon is explained on the basis of free energy considerations.

The effect of pressure on the low-temperature states of the newly discovered clathrate Ba₆Ge₂₅ is investigated by means of measurements of the electrical resistivity. At ambient pressure, Ba₆Ge₂₅ undergoes a two-step structural phase transition between 230 and 180 K from metallic behaviour to a high-resistivity state characterized by a mean free path of about 3 Å. Interestingly, a Bardeen–Cooper–Schrieffer-like (BCS-like) superconducting transition occurs at T_C ≈ 0.24 K from the resulting 'bad metal'. With increasing pressure, the structural phase transition is depressed but T_C increases drastically. T_C reaches a maximum value of 3.85 K at the critical pressure p_C ≈ 2.8 GPa, where the structural distortion is completely suppressed and the system exhibits metallic behaviour. Higher pressures lead to a slight decrease of T_C.

Raman results for different single-walled carbon nanotube bundles doped with Br₂ were studied both at ambient pressure and under high pressure up to 6 GPa. Our study indicates that bromine resides in the interstitial channel of nanotube bundles as a form of polymer.

New deuterides of Laves phases: ErFe₂D₅, YFe₂D₅, ZrFe₂D_3.5 and ZrCo₂D₂, have been obtained by using of gaseous deuterium at high pressure. A new orthorhombic structure was found for ErFe₂D₅ and YFe₂D₅, while ZrFe₂D_3.5 and ZrCo₂D₂ were formed with a large expansion of the initial C15 cubic lattice. Formation of hydrides with high hydrogen concentration substantially changes the magnetic properties of ErFe₂ and YFe₂ but has no significant influence on the magnetization of ZrFe₂. The possibility of the formation of new deuterides (hydrides) in ZrCr₂ and YMn₂ has also been confirmed.

Using nanoparticles of CeO₂ and ZrO₂ prepared by a chemical precipitation method as starting materials, single-phase cubic Ce_0.5Zr_0.5O₂ solid solution (c-Ce_0.5Zr_0.5O₂) has been synthesized under 3.1 GPa at 1073 K for the first time. The EPR and XPS measurements show that there exists no Ce³⁺ in it and that the Ce⁴⁺ has not been reduced to Ce³⁺ after annealing. The transport mechanism is ionic for the c-Ce_0.5Zr_0.5O₂. The bulk conductivity is the same as that of CeO₂, but smaller than that of Y₂O₃-stabilized ZrO₂.

In order to prepare bulk C₃N₄, high-pressure pyrolysis of melamine (C₃N₆H₆) at different temperatures was carried out. The products were characterized by C, N, H element analysis, Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, and x-ray diffractometry. The results of the analysis reveal that graphitic phase C₃N₄ has been synthesized. It provides a novel route to synthesis of the theoretical superhard cubic C₃N₄ and other C₃N₄ phases from organic compounds by a high-pressure and high-temperature method.

The temperature dependences of the dielectric constants of the hydrogen-bond ferroelectrics KH₂PO₄ (KDP) and KD₂PO₄ (DKDP) were measured under high hydrostatic pressure. Their ferroelectric transition temperatures T_c monotonically decreased with increasing pressure and the ferroelectric state vanished at p_c: 1.7 GPa for KDP and 6.1 GPa for DKDP. On the other hand, the Curie constant remained finite at p_c, which indicates that the ferroelectric phase transition at high pressure is of displacive type. At pressures around p_c, quantum paraelectricity was observed in KDP and DKDP.

The ionic conductivity of Sc₂(WO₄)₃ at 400 °C shows a normal decrease with increase in pressure up to 2.9 GPa but then increases anomalously at pressures up to 4.3 GPa. Synchrotron in situ x-ray diffraction results show that Sc₂(WO₄)₃ undergoes pressure-induced amorphization at pressures coincident with the reversal in conductivity behaviour. The loss of crystal structure at high pressure may be associated with the property of negative thermal expansion in Sc₂(WO₄)₃.

The Ni–Cr–Al Russian alloy was prepared. Its magnetic and mechanical properties were better than those of MP35N alloy. We fabricated the a piston–cylinder-type hybrid high-pressure cell using the Ni–Cr–Al alloy. It has been found that the maximum working pressure can be repeatedly raised to 3.5 GPa at T = 2 K without any difficulties.

Nanocrystalline iron sulphide (FeS) coated with polyvinyl alcohol, with particle size ranging from several to several tens of nanometres, has been prepared by the chemical precipitation synthesis method. The phase transition of FeS has been investigated by using in situ high-pressure diffraction with synchrotron radiation at pressures up to 42.5 GPa. Most of the diffraction lines are broadened and weakened. At the pressure of 11.8 GPa, a new phase transition was observed. However, only eleven x-ray reflections were recorded under high pressure; the crystal structure is unknown.

We report in situ high-pressure studies up to 1.0 GPa on MgB₂ superconductor which had been synthesized at high pressure. The as-prepared sample is of high quality as regards having a sharp superconducting transition (T_c) at 39 K. The in situ high-pressure measurements were carried out using a Be–Cu piston–cylinder-type instrument with a mixed oil as the pressure-transmitting medium, which provides a quasi-hydrostatic pressure environment at low temperature. The superconducting transitions were measured using the electrical conductance method. It is found that T_c increases with pressure in the initial pressure range, leading to a parabolic-like T_c–P evolution.

Bulk samples of superhard phases of fullerites were prepared by high-pressure (15 GPa) and high-temperature treatment of pure pristine C₆₀ and C₇₀ for the first time. We applied the acoustic microscopy technique to measure the longitudinal and shear sound velocities and determine the elastic constants of the fullerite specimens. Acoustic microscopy images reflecting the microstructure of the fullerite specimens were obtained.

Using the pulse echo overlap method and the pulse superposition method, the acoustic velocities V_l, V_s and the attenuations α_l, α_s of Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5 bulk metallic glass have been measured below 2 GPa with 10 and 20 MHz carrier frequencies, respectively. The results reveal that V_l and V_s increase linearly with increasing pressure (P); the slope of the V_l – P curve decreases with increasing frequency, while that of the V_s – P curve increases with increasing frequency. This means that there is a slight dispersion of sound under pressure. The attenuation increases obviously with the carrier frequency from 10 to 20 MHz. The effects of frequency on the velocity and attenuation under high pressure are discussed.

Dual implantation of erbium and silicon into thermally grown SiO₂ film on Si was performed using a metal vapour vacuum arc ion source; this was followed by rapid thermal annealing at 500–1100°C for 20 s. 1.54 μ m photoluminescence (PL) was observed at 77 K. Rutherford backscattering spectrometry shows that Er ions mainly distribute within 80 nm of the surface. The highest Er peak concentration obtained exceeded 5 × 10²¹ cm⁻³, which is the highest Er concentration reported in Si-based materials. Transmission electron microscopy demonstrates that nanocrystalline silicon (nc-Si) embeds in SiO₂ matrices. The Er^{3 +} excitation energy is obtained from the electron–hole pairs nonradiatively combining at defects and impurities in SiO₂ matrices or at the interface of the nc-Si/SiO₂ (or c-Si/SiO₂), and energy transferring to Er^{3 +} ions results in 1.54 μ m light emission. The dependence on the Si-ion dose and annealing temperature of the PL intensity has also been investigated.

Single-crystal growth of various transition metal oxides exhibiting fascinating physical properties was performed, based on the powder x-ray diffraction studies at high pressures of several GPa. Results on a S = 1/2 one-dimensional alternating exchange antiferromagnet (VO)₂P₂O₇, an oxychloride high-T_c superconductor Ca_2−xNa_xCu₂Cl₂, and CaFeO₃ with unusual Fe⁴⁺ ions are reported.

High P, T measurements of viscosity and density of Fe–S liquids are reported. Viscosity was measured using Stokes method and synchrotron radiographic techniques for real-time imaging of a falling/rising composite sphere in Fe–S liquids. For P up to 4 GPa and T up to 1523 K, measured viscosities are of the order of 10⁻²–10⁻³ Pa s. Density of Fe–10 wt% S liquids was measured using the sink/float method with composite spheres. The high P density data indicate a density increase of 29% between 1 atm and 15 GPa at 1923 K.

The diamond formation process in aqueous fluid catalyst under high-pressure and high-temperature conditions has been observed for the first time. Quench experiments and in situ x-ray diffraction experiments using synchrotron radiation have been performed upon a mixture of brucite (Mg(OH)₂) and graphite as the starting material. It was confirmed that brucite decomposed into periclase and H₂O at 3.6 GPa and 1050°C while its complete melting occurred at 6.2 GPa and 1150°C, indicating that the solubility of MgO in H₂O greatly increases with increasing pressure. The conversion of carbon from its graphite to its diamond form in aqueous fluid was observed at 7.7 GPa and 1835°C.

The transfer function of a piezoelectric transducer, buffer rod and sample assembly is used to measure the sound velocity of solid materials. From the recorded transfer function, pulse echo patterns at frequencies of the passband of the input signal are reproduced after convoluting with monochromatic RF input signals. The time delay is obtained by performing pulse echo overlap and phase comparison measurements on reproduced signals. Results for a single crystal of MgO along the [100] direction from this study are in good agreement with previous measurements but have the advantage of offline data analysis and fast data acquisition.

The viscosities of albite (NaAlSi₃O₈) melt under high pressures have been measured using an x-ray radiography falling sphere method with synchrotron radiation. This method has enabled us to determine the precise sinking velocity directly. Recent experiments of albite melt showed the presence of a viscosity minimum around 5 GPa (Poe et al1997 Science276 1245, Mori et al2000 Earth Planet. Sci. Lett.175 87). We present the results for albite melt up to 5.2 GPa at 1600 and 1700°C. The viscosity minimum is clearly observed to be around 4.5 GPa, and it might be explained not by the change of the compression mechanism in albite melt but by change of the phase itself.

Magnesiowüstite (Mg_0.1Fe_0.9)O was studied at ambient temperature under both hydrostatic and quasihydrostatic pressures to 35 GPa. The elastic behaviour, equation of state, transition pressure, and the structure before and after the B1–rhombohedral transition were determined. K₀-and K₀ '-values for the B1 phase were calculated to be 155 ± 10 and 3.6 ± 0.8 GPa respectively under hydrostatic conditions. The second-order B1–rhombohedral transition occurred at 20 GPa. Quasihydrostatic conditions were used to determine the stress/strain in different crystallographic directions to elucidate the transition mechanism which is triggered by the softening of the c₄₄ in the B1 phase.

Compressional wave velocities (V_p) were measured in situ in serpentinized olivine-bearing pyroxenite at 1.0 GPa from room temperature to 960°C in a multi-anvil pressure apparatus. The velocity showed a remarkable decrease from temperature more than 500^oC which was from 6.2 to 5.0 km s⁻¹. The decline in velocity is attributed to the changes of rock fabric, including the following aspects: composition, size and shape of grain, crack, grain boundary and dislocation changes resulting from dehydration at high temperature. The fine-granular aggregates of minerals formed through dehydration were chemically analysed, and they were found to be composed of forsterite and enstatite. Dehydration of serpentine has a significant effect on the low-velocity zone and earthquake generation.

P-wave velocities (V_P) and electrical conductivities (σ) in serpentinite, collected from Ailaoshan orogenic belt, Yunnan province, China, were measured at 2.0–3.5 GPa and high temperature with the pulse transmission–reflection combined method and impedance spectroscopy by the means of a multi-anvil pressure apparatus, the YJ-3000 press. V_P decreased and σ increased markedly at temperature higher than about 560°C. It is argued that the onset of the dehydration of serpentine is the main cause for V_P decreasing and σ increasing. Combining the present experimental data with the results of previous experiments, we were led to speculate that the formation of the low-velocity layer between the subducted slab and mantle wedge is closely related to the dehydration of serpentine.

Corundum megacrysts in alkali basalts are an important source of gems. However, the genesis of corundum megacrysts is still a controversial problem. Up to now, no experimental evidence has been acquired for the crystallization of corundum from basalt under high temperature and high pressure. With alkali basalt as the starting material in this experiment, the transformation of basalt into eclogite at the pressure of 3.5 GPa and the temperature of 1400°C was realized. In the experimental product, corundum has been discovered embraced in garnet.

The phase transitions and equation of state of cobalt oxide (CoO) have been studied at ambient temperature under non-hydrostatic and hydrostatic high pressures, with the diamond-anvil cell technique. We discovered that under hydrostatic pressure conditions the CoO structure transformed from the low-density rhombohedral to a high-density rhombohedral phase at 90 ± 1 GPa with the molecular volume decreasing by about 2.7%. This phase transition with volume discontinuity may be related to magnetic collapse, which was predicted by theory. The pressure of the phase transition of CoO from face-centred cubic B1 to rhombohedral structure was 43 ± 2 GPa. The decompression spectrum data of CoO under non-hydrostatic pressure showed that these phase transitions are reversible.

We have set up an electrical conductivity measurement system under high-pressure and high-temperature conditions with a multi-anvil high-pressure apparatus based on an AC complex impedance method. With this system, we have successfully measured the electrical conductivity of San Carlos olivine under pressure up to 5 GPa. We explain the physical meaning of the activation enthalpies and activation volumes and give values for these data, which will be a great help for understanding the conduction mechanism for olivine.

A new combined transmission–reflection method is presented for measuring elastic velocities of rocks and minerals at elevated temperature and pressure, which resolves the problems of gradients of temperature and pressure existing in the original sample assembly with a pyrophyllite cube. At temperature up to 900°C and pressure up to 4 GPa, single-crystal quartz and serpentine were used as the samples tested. By the use of this new technique, more precise and reasonable data on elastic properties of rocks and minerals at elevated temperature and pressure can be achieved.

We report quantitatively accurate structure-factor and radial-distribution-function measurements of liquid water in a diamond-anvil cell (DAC) using x-ray diffraction. During the analysis of our diffraction data, we found it possible (and necessary) to also determine the density. Thus, we believe we present the first-ever diffraction-based determination of a liquid structure factor and equation of state in a DAC experiment.

The dynamic process and kinetics of gas hydrate formation and dissociation have been studied by means of Raman spectroscopy. The formation and dissociation of methane and carbon dioxide hydrates as well as the phenomenon of interchange of the components were observed as functions of time, temperature and pressure conditions. From the direct observation, it was found that the reformation of carbon dioxide hydrate occurred at the surface and inside the sample of methane hydrate as well as producing ice crystals.

The equations of state of hydrogen and deuterium are studied using the ideal mixing model. An effective potential with many-body effects has been taken into account. The Helmholtz free energy of a molecule is calculated by employing pure fluid perturbation theory, and the expression for the free energy of an atom is modelled following a similar formula in liquid metal perturbation theory. The Helmholtz free energy for a system with dissociation is given by combining the molecular free energy and atomic free energy. The equation of state obtained this way is in good agreement with experiment.

High pressures of hydrogen up to 3.0 GPa and temperatures up to 373 K were used as a pretreatment to introduce structural changes in the bulk and on the surface of Cu–Zr amorphous alloys which then were examined by means of x-ray diffraction and microscopy. The hydrogenative pretreatment of high hydrogen fugacity followed by annealing at 623 K, aimed at causing desorption of hydrogen, and an eventual exposure of the samples to air at room temperature to oxidize Zr, resulted in a distinct increase of catalytic activity in the dehydrogenation of 2-propanol. A tentative mechanism to account for the enhancement of the catalytic activity induced by the above combined pretreatment is discussed.

Monochromatic synchrotron x-ray diffraction data collected at CHESS and ESRF at varying temperatures and pressures were used to investigate the crystal structures of phases with the composition Ni₃S₂. At low pressures Ni₃S₂ has the rhombohedral heazlewoodite structure (Ni₃S₂ I), but transforms to two new structures at higher pressures and temperatures. Ni₃S₂ III is orthorhombic (space group Cmcm, a = 3.118 Å, b = 10.862 Å, c = 6.730 Å) and contains Ni coordinated by five S atoms in a square pyramid. The structure of Ni₃S₂ III is described in this report along with an analysis of electronic structures of nickel sulphides.

We use pressure as an empirical parameter to investigate quantum tunnelling reactions. Pressure modifies both the thickness and the barrier height of the potential, leading to enhancement of the tunnelling rate of many orders of magnitude. We use a high-spin–low-spin relaxation of a spin-crossover system to illustrate the phenomenon. Emphasis is placed on an ongoing project. A pair of anthracene molecules, perfectly oriented in a dianthracene lattice, undergoes photodimerization via tunnelling at low temperature. We report results for the perdeuterated system. These observations suggest the tunnelling of a formally very heavy particle.

Pressure effects on the thermal rate process from trans-1 to 2 shown in scheme 1 were studied in ethanol and highly viscous 2-methylpentane-2, 4-diol. In the former, the reaction was moderately accelerated as expected from the transition state theory (TST). In the latter, the TST became invalid at high pressures and low temperatures and a so-called dynamic solvent effect was observed. The results clearly demonstrated that the medium and the chemical coordinates have to be treated independently.

The degradation of polyethylene terephthalate (PET) in supercritical methanol was investigated with the aim of developing a process for chemical recycling of waste plastics. A batch reactor was used at temperatures of 573–623 K under an estimated pressure of 20 MPa for a reaction time of 2–120 min. PET was decomposed to its monomers, dimethyl terephthalate and ethylene glycol, by methanolysis in supercritical methanol. The reaction products were analysed using size-exclusion chromatography, gas chromatography–mass spectrometry, and reversed-phase liquid chromatography. The molecular weight distribution of the products was obtained as a function of reaction time. The yields of monomer components of the decomposition products including by-products were measured. Continuous kinetics analysis was performed on the experimental data.

The solubility of carbon in water at the pressure of 7.7 GPa and at temperatures of 1000–2900 K is investigated by means of statistical mechanical theory, and it is found that the excess saturation concentration of carbon in hot water is very sensitive to the temperature. The peak of excess saturation concentration is located at 2450 K, according to calculations using the code CHEQ, where the driving force for graphite-to-diamond transformation reaches the maximum. The conclusions are consistent with those from recent diamond synthesis experiments.

The fluorescence spectra of 4-dimethylamino(benzonitrile) (DMABN) have been measured for a mixture of supercritical CO₂ and methanol at high pressures up to 15.5 MPa at 318 K. It is found that the addition of a small amount of methanol can stabilize the intramolecular charge transfer (ICT) state of DMABN in the supercritical fluid mixture. The spectral shift of the ICT band has been analysed on the basis of Onsager reaction field theory; a significant increase of the local composition of methanol around the DMABN molecule in the ICT state is demonstrated.

Three high-pressure structures of methane hydrate, a hexagonal structure (str. A) and two orthorhombic structures (str. B and str. C), were found by in situ x-ray diffractometry and Raman spectroscopy. The well-known structure I (str. I) decomposed into str. A and fluid at 0.8 GPa. Str. A transformed into str. B at 1.6 GPa, and str. B further transformed into str. C at 2.1 GPa which survived above 7.8 GPa. The fluid solidified as ice VI at 1.4 GPa, and the ice VI transformed to ice VII at 2.1 GPa. The bulk moduli, K₀, for str. I, str. A, and str. C were calculated to be 7.4, 9.8, and 25.0 GPa, respectively.

We measured the specific volumes of water at temperatures from 253 to 298 K and at pressures from 200 to 380 MPa. The specific volume data obtained were correlated with an empirical Tait equation as a function of temperature and pressure. In addition to our experimental data, some calculated data from the 'IAPWS-95' equation of state (released by the International Association for the Properties of Water and Steam) were used to enable extrapolation up to 373 K. Compressibility and expansivity were calculated by differentiating this equation.