Table of contents

The systematics of Mott transitions in low-Z and alkali fluids H, N, O, Rb, and Cs are discussed. Such transitions have only been obtained in elemental, monatomic, and disordered systems by using high pressures and temperatures. By finding elements which undergo this transition, it is possible to test Mott's ideas in systems which are relatively simple. For fluid H, N, and O, 100 GPa pressures and  K temperatures are required and were achieved by dynamic shock compression. For fluid Rb and Cs, 0.01 GPa and  K are required and were achieved statically by Hensel et al. Despite the fact that these two groups of elements at ambient conditions are so different chemically and that pressures required to observe this transition differ by a factor of 10⁴ for the two groups, the metallic conductivities are essentially the same for all five and the density dependences of their semiconductivities are determined systematically by the radial extents of the electronic charge-density distributions of the various atoms. The latter observation provides a physical understanding for the Herzfeld criterion of metallization. It is the tuning of both high pressure and temperature which permits these two apparently different classes of elements at ambient conditions to become quite similar in nature at extreme conditions.

High pressure has emerged as an important tool to tackle several problems in medicine and biotechnology. Misfolded proteins, aggregates and amyloids have been studied, which point toward the understanding of the protein misfolding diseases. High hydrostatic pressure (HHP) has also been used to dissociate non-amyloid aggregates and inclusion bodies. The diverse range of diseases that result from protein misfolding has made this theme an important research focus for pharmaceutical and biotech companies. The use of high pressure promises to contribute to identifying the mechanisms behind these defects and creating therapies against these diseases. High pressure has also been used to study viruses and other infectious agents for the purpose of sterilization and in the development of vaccines. Using pressure, we have detected the presence of a ribonucleoprotein intermediate, where the coat protein is partially unfolded but bound to RNA. These intermediates are potential targets for antiviral compounds. The ability of pressure to inactivate viruses, prions and bacteria has been evaluated with a view toward the applications of vaccine development and virus sterilization. Recent studies demonstrate that pressure causes virus inactivation while preserving the immunogenic properties. There is increasing evidence that a high-pressure cycle traps a virus in the 'fusion intermediate state', not infectious but highly immunogenic.

In 1927 P W Bridgman proposed that the otherwise stable structure of atoms could be broken down in the condensed state of matter by mutual encroachment of neighbouring atoms, this being brought about by pressure. Bridgman's principle of destructibility on systematic densification applies equally to molecular systems and from the modern perspective the aftermath of Bridgman's insight is to be sought in structural transitions, in the ensuing but intertwined re-arrangement of electronic states, and generally in the appearance of quite new states of order, both classical and quantal. This pathway is developed in a brief retrospective with examples drawn from hydrogen and light elements, as well as specific hydrogen-rich systems which, as with hydrogen itself, may also offer the prospect of high-temperature superconductivity.

The high-T_c cuprate superconductors are characterized by a quasi-two-dimensional layered structure where most of the physics relevant for high-T_c superconductivity is believed to take place. In such compounds, the unusual dependence of the critical temperature T_c on external pressure results from the combination of the nonmonotonic dependence of T_c on hole doping or hole-doping distribution among inequivalent layers, and from an 'intrinsic' contribution. After reviewing our work on the interplay among T_c, hole content, and pressure in the bilayered and multilayered cuprate superconductors, we will discuss how the proximity to an electronic topological transition (ETT) may give a microscopic justification of the 'intrinsic' pressure dependence of T_c in the cuprates. An ETT takes place when some external agent, such as doping, hydrostatic pressure, or anisotropic strain, modifies the topology of the Fermi surface of an electronic system. As a function of the critical parameter z, measuring the distance of the chemical potential from the ETT, we recover a nonmonotonic behaviour of the superconducting gap at T = 0, regardless of the pairing symmetry of the order parameter. This is in agreement with the trend observed for T_c as a function of pressure and other material specific quantities in several high-T_c cuprates and other low dimensional superconductors. In the case of epitaxially strained cuprate thin films, we argue that an ETT can be driven by a strain-induced modification of the in-plane band structure, at constant hole content, at variance with a doping-induced ETT, as is usually assumed. We also find that an increase of the in-plane anisotropy enhances the effect of fluctuations above T_c on the normal-state transport properties, which is a fingerprint of quantum criticality at T = 0.

It has been shown recently (Holzapfel et al 2001 J. Phys. Chem. Ref. Data 30 515–29) that equations of state (EOS) data for simple metals like Cu, Ag and Au can be described most accurately on the base of a modified Mie–Grüneisen model including intrinsic anharmonicities. In contrast to commonly applied parametric EOS forms, which use only temperature dependent parameters, V (T),K(T) and , in the same parametric form, the present approach describes the thermal pressure more accurately and allows therefore to extrapolate more safely into wider ranges of pressure and temperature. Starting from the phonon density of state and a separate modelling of zero temperature and thermal contributions including quasiharmonic and intrinsically anharmonic contributions, the low energy part of the phonon density of states is represented by a modified pseudo-Debye model, which needs only one additional Einstein frequency for the high energy part. This optimised model is successfully applied to describe not only the EOS of Cu, Ag and Au, but also of other simple solids like NaCl and MgO, which are of special interest for pressure measurements in wide ranges of pressure and temperature.

A number of melting curves of various materials have recently been measured experimentally and calculated theoretically, but the agreement between different groups is not always good. We discuss here some of the problems which may arise in both experiments and theory. We also report the melting curves of Fe and Al calculated recently using quantum mechanics techniques, based on density functional theory with generalized gradient approximations. For Al our results are in very good agreement with both low pressure diamond-anvil-cell experiments (Boehler and Ross 1997 Earth Planet. Sci. Lett.153 223, Hänström and Lazor 2000 J. Alloys Compounds305 209) and high pressure shock wave experiments (Shaner et al 1984 High Pressure in Science and Technology ed Homan et al (Amsterdam: North-Holland) p 137). For Fe our results agree with the shock wave experiments of Brown and McQueen (1986 J. Geophys. Res.91 7485) and Nguyen and Holmes (2000 AIP Shock Compression of Condensed Matter505 81) and the recent diamond-anvil-cell experiments of Shen et al (1998 Geophys. Res. Lett.25 373). Our results are at variance with the recent calculations of Laio et al (2000 Science287 1027) and, to a lesser extent, with the calculations of Belonoshko et al (2000 Phys. Rev. Lett.84 3638). The reasons for these disagreements are discussed.

By means of electronic structure calculations, we have studied phase transitions in YLiF₄ using the Vienna ab initio simulation package (VASP) within the generalized gradient approximation (GGA) and the local density approximation (LDA). The scheelite to fergusonite phase transition takes place at 9.3 GPa with a 0.5% volume collapse. By comparing the total energy difference for several possible post-fergusonite structures, BaWO₄(II) type, LaTaO₄ type, BaMnF₄ type and wolframite type, we propose that the wolframite-like structure is the most likely new phase for the second phase transformation.

X-ray diffraction has been measured for liquid group 14 elements at high pressures using synchrotron radiation. Static structure factors S(Q) and pair distribution functions g(r) show different pressure dependences for liquid silicon, liquid germanium and liquid tin. In liquid silicon, a significant change in the local structure occurs between 8 and 14 GPa. In liquid germanium, an anisotropic contraction of local structure occurs continuously with increasing pressure. In liquid tin, the structure contracts almost uniformly. These pressure dependences are discussed in relation to the pressure-induced phase transitions in crystalline phases and the changes in bonding between atoms.

The structure of liquid InAs has been investigated at pressures up to 13 GPa by means of synchrotron x-ray diffraction. The results show a change of the contraction behaviour. Below 9 GPa, liquid InAs contracts almost uniformly, which is inferred from there being no remarkable pressure-induced change in the shapes of the structure factor, S(Q), and the pair distribution function, g(r), and in the coordination number. Above 9 GPa, the liquid contracts nonuniformly, which is inferred from the gradual change in the shapes of S(Q) and g(r) and the continuous increase of the coordination number from 6 to 7. The analysis of g(r) shows that liquid InAs consists of β-Sn-like and bcc-like local structures, which do not appear in crystalline InAs, throughout the whole pressure region up to 13 GPa. The fraction of the high-pressure form (bcc-like local structure) increases above 9 GPa, while it is almost constant below 9 GPa. These findings suggest that liquid InAs transforms from the state consisting of a relatively stable local structure into a transient state in which the local structure continuously changes with pressure.

Recent studies of the response of metals and alloys to shock-wave loading at elevated temperatures are summarized. Shock-wave tests have been carried out for metal single crystals, polycrystalline metals of different purity, and for alloys. High resistance to sub-microsecond tensile fracture of single crystals is maintained when melting should start. This is treated as evidence of a superheated solid state reached under dynamic tension. In polycrystalline metals, melting starts earlier at grain boundaries; this is known as the pre-melting phenomenon. As a result, their tensile strength drops to zero on approaching the melting curve. Anomalous growth of the dynamic yield stress was observed for low-strength metals, whereas the yield stress of high-strength alloys decreases with temperature. The different behaviour of metals and alloys is treated in terms of the relationship between the phonon drag of the motion of dislocations and the drag forces created by obstacles.

In situ x-ray diffraction studies of graphite–diamond transitions with various solvent catalysts under high pressures and high temperatures at the Photon Factory and SPring-8 are reviewed. By combining synchrotron radiation and a large-volume multi-anvil high-pressure apparatus, real-time observations have been successful in the diamond formation process with the help of various catalysts, such as transition metals, carbonates and aqueous fluids. The experimental procedures and the technical details are described. The diffraction data with various catalysts are shown and the problems and limitations of this method are discussed.

Variable temperature (at 9.4 and 14.1 T) and pressure (at 9.4 T) ¹⁷O NMR relaxation and chemical shift experiments have been performed on DMSO solutions of [Gd(DMSO)₈]³⁺. It has been found that DMSO exchange is more than two orders of magnitude slower than water exchange (k_ex²⁹⁸ = 6.2 × 10⁶ s⁻¹ for [Gd(DMSO)₈]³⁺/8.3 × 10⁸ s⁻¹ for [Gd(H₂O)₈]³⁺). Comparison of activation volumes (ΔV^‡ = +8.2 cm³ s⁻¹ for [Gd(DMSO)₈]³⁺/−3.3 cm³ s⁻¹ for [Gd(H₂O)₈]³⁺) shows that activation modes are different for solvent exchange on [Gd(DMSO)₈]³⁺ and [Gd(H₂O)₈]³⁺: DMSO exchange follows a dissociative activation mode in contrast to H₂O exchange which follows an associative activation mode. This change in activation mode is explained by the increase in molecular volume from H₂O to DMSO.

Most materials expand when heated. However, many exceptions are now known. Recently, interest in this has been revived with the discovery of isotropic negative thermal expansion (NTE) in ZrW₂O₈. From equation of state considerations, one can relate NTE to negative Grüneisen parameters (thermal or electronic). Under pressure, these lead to equation of state anomalies with the pressure derivative of the bulk modules being small or negative. Many of these materials undergo pressure-induced amorphization. This in some of them can be understood on the steric constraint model. It is also argued that NTE in most materials may be understood from the fact that these materials have two degenerate or nearly degenerate energy states. On increase of temperature, the material then samples the lower volume state, leading to NTE.

The high-pressure elastic properties of single-crystal andradite garnet Ca₃Fe₂³⁺Si₃O₁₂ were determined by Brillouin scattering to 11 GPa. The pressure dependence of the elastic stiffness constants and aggregate bulk and shear moduli were obtained by inversion of the data to finite Eulerian strain equations. The inversion yields C₁₁ = 286.7 ± 0.6 GPa, C₁₂ = 88.6 ± 0.6 GPa, C₄₄ = 83.8 ± 0.3 GPa, K_0S = 154.5 ± 0.6 GPa, G_0S = 89.7 ± 0.4 GPa, , and . Both individual and aggregate elastic moduli define nearly linear modulus–pressure trends. The elastic anisotropy of andradite garnets increases weakly in magnitude with compression. Previous studies of the high-pressure elasticity of andradite garnet are highly discrepant, with reported pressure derivatives of the bulk modulus varying by 46% and pressure derivatives of the shear modulus varying by 253%. We are able to provide plausible explanations for these discrepancies. In particular, differences between previous x-ray diffraction data and a static compression curve constructed from our Brillouin data can be attributed to the effects of non-hydrostatic stresses on the x-ray data.

We studied the refolding and aggregation of pressure unfolded proteins. Horseradish peroxidase was found to be very stable and no partially folded intermediates were populated during the refolding. However, the removal of the haem group or the Ca²⁺ ions or reduction of the disulfide bridge destabilized the protein, resulting in a significant amount of aggregation prone intermediate conformation. Substitution of the haem for fluorescent porphyrin however did not influence the refolding of the protein.

The critical step in the pathogenesis of transmissible spongiform encephalopathies (TSEs) appears to be a conformational transition of a normal prion protein (PrP^C) into a misfolded isoform (PrP^Sc). To gain insight into the structural conversion of the prion protein we have exploited the use of high hydrostatic pressure combined with various spectroscopic techniques.

In vitro transitions of the recombinant PrP to a scrapie-like form have never resulted in an infectious structure. It is our hypothesis that the acquisition of the disease-causing conformation depends on folding pathways which are difficult to attain. We attempt to favour, via specific reaction conditions at high pressure, alternative routes of misfolding leading to a stable infectious amyloidogenic conformer. Our results have demonstrated the potential of high pressure to reveal various prion structural changes, which are inaccessible by conventional methods. Especially, we have characterized a pressure-induced conformer in which the normal α-helical structure is changed into a highly aggregated β-sheet conformation showing markedly increased resistance to proteolysis (key markers of potential infectious agents).

Our work may have important implications, not only for ultimately proving the protein-only hypothesis and for understanding the basic mechanism of the disease, but also for developing preventative and therapeutic measures.

The effect of pressurization at different pressures (from 200 to 1000 MPa, at 200 MPa intervals, t_const. = 15 min) and periods of time (from 15 to 35 min, at 10 min intervals, p_const. = 800 MPa) on the changes of proteins and nitrogen compounds of skimmed milk was studied.

The pressurization caused an increase in the amount of soluble casein and denaturation of whey proteins. The level of nonprotein nitrogen compounds and proteoso-peptone nitrogen compounds increased as a result of the high-pressure treatment. These changes increased with an increase in pressure and exposure time. High-pressure treatment considerably affected the changes in the conformation of milk proteins, which was reflected in the changes in the content of proteins sedimenting and an increase in their degree of hydration.

An overview of several new developments in diamond anvil cell techniques is presented. The novel techniques developed in our laboratory address fundamental properties of compressed materials, e.g. vibrational, electronic and spin excitations. We focus on the techniques made possible by third-generation synchrotron x-ray sources and by the development of the magnetic susceptibility probes in diamond anvil cells. A brief list of topics is given below:

1. Application of nuclear resonant inelastic x-ray scattering technique to the measurements of the phonon density of states and elastic properties of iron-containing materials at megabar pressures.

2. Magnetic collapse and high-spin to low-spin transitions probed by the x-ray K β emission process in transition metal compounds.

3. Superconductivity in multi-megabar pressure range studied by the magnetic susceptibility technique.

Selected applications of these techniques will be covered, including elastic properties of compressed hcp-Fe, insulator–metal transition in FeO and superconductivity in Li.

We report on temperature dependent magnetic susceptibility and Mössbauer effect studies of the influence of hydrostatic pressure (up to 1.2 GPa) on dynamic electronic structure phenomena in 3d transition metal coordination compounds. The systems under investigation are mononuclear spin crossover compounds of iron (II) and chromium (II), dinuclear complexes of iron (II) exhibiting coexistence of intramolecular antiferromagnetic coupling and thermal spin crossover, 1D, 2D and 3D polynuclear spin crossover complexes of iron (II), a valence tautomeric system of cobalt (II) showing a thermal transition from a high spin [Co^II (semiquinone)] to a low spin [Co^II (catecholate)] species on lowering the temperature and a photomagnetically active Prussian blue type system with temperature- and pressure induced electron transfer. It is demonstrated that pressure effect studies can be very helpful in elucidating the mechanisms and cooperative interactions of solid state compounds with electronic bistability.

Magnetism in iron plays a central role in understanding the physical properties of its polymorphs, including the close-packed high pressure phases. We explore the rich and complex magnetic structures of these phases in two ways. We use a first-principles based, magnetic tight-binding total energy model to study non-collinear magnetic structures, and an all-electron method to study the collinear state in hcp iron that we predict in the hcp iron stability range. For the non-collinear study we compute the magnetization energy and moments for various non-collinear ordered spin configurations. For fcc iron we find non-collinear structures with a wavevector (0,0,q) with q close to 0.5 to be energetically stable, in agreement with previous first-principles calculations. In the high pressure stability field of hcp iron we find a stable collinear antiferromagnetic structure (afmII), previously predicted with an all-electron method. We further investigate the afmII structure, computing physical properties from first principles that support the notion of antiferromagnetic correlations in hcp iron. We show that a recently observed anomalous splitting in Raman spectra of hcp iron under compression can be quantitatively explained by spin–phonon interactions. To address the absence of Mössbauer splitting in experiments on hcp iron we have also calculated the hyperfine field of afmII iron and find it to be so small that the predicted splitting would be smaller than the resolution limit of experiments.

We report further resistive measurements under pressure of samples of iron from multiple sources. The normal state resistivity of ε-iron varied as ρ₀+AT^5/3 at low temperature over the entire superconducting pressure domain. The superconductivity could be destroyed by mechanical work, and was restored by annealing, demonstrating sensitivity to the residual resistivity ρ₀. There is a strong correlation between the ρ₀ and A coefficients and the superconducting critical temperature T_c. The critical field B_c2 corresponding to the maximum T_c was found to be 0.73 T, implying a coherence length of about 20 nm, i.e. slightly shorter than the electronic mean free path. Within the partial resistive transition there was a significant current dependence, with V (I) = a(I−I₀)+bI², and , possibly indicating flux-flow resistivity, even in the absence of an externally applied magnetic field.

We have observed a reversible high-spin to low-spin transition around 11 kbar at room temperature for the polymeric spin crossover complex Fe(pyridine)₂[Ni(CN)₄] using high pressure Raman spectroscopy. The pressure dependence of vibrational frequencies in the two spin-states has been studied up to 50 kbar. The results suggest that the vibrational entropy change associated with the spin crossover is independent of pressure in this pressure range.

Recent progress in the application of impulsive stimulated light scattering to opaque materials under high pressure is reviewed. Measured elastic constants and sound velocities of polycrystalline hcp ε-iron to 115 GPa are presented.

At ambient pressure the solubility of magnesium in iron is very limited (of the order of 0.025 at.% in the liquid state), and vice versa. In a series of experiments using the piston–cylinder and multi-anvil apparatus at pressures up to 20 GPa and temperatures up to 2000 °C, we found an increase in the solid solubility of magnesium in iron up to 4 at.% Mg. The solution of magnesium in the bcc-structured Fe-based alloy increases the lattice parameter by approximately 2.4% with respect to that of pure iron.

Some recent improvements made to the high-pressure single-crystal x-ray diffraction (XRD) experiments in use in our laboratory are reviewed. In particular a set-up to perform high-pressure (0–3.0 GPa)–low-temperature (300–9 K) investigations as well as a protocol for high-pressure XRD data collection using a CCD detector are shown. Examples of studies are also presented.

Hot compressed water in the sub- and supercritical state exhibits exciting physical and chemical properties, which can be varied continuously from gas-like to liquid-like behaviour. Correspondingly, the solvent properties can change from non-polar behaviour as present, for example, in organic solvents to highly ionic characteristics like in salt melts. This opens up several promising opportunities for separation processes and chemical reactions. Under supercritical conditions, substantial amounts of gases and organic substances can homogeneously be mixed with water, which then can be separated by adjusting the subcritical conditions by forming additional phases. This can beneficially be combined with chemical reactions occurring in the homogeneous state leading to integrated processes, which are more effective and competitive. Three approaches to the technical application of hot compressed water are presented to show and discuss the technology, potential, technical hurdles and future research demand in this area of research and development.

• In supercritical water oxidation (SCWO) water is used as a medium in which organic pollutants are completely degraded under the addition of oxygen, which is completely miscible with water under the process conditions of up to 650 °C and pressures around 25 MPa. Thus, high space–time yields in compact reactor designs can be realized.

• Hydrogen is produced from biomass by hydrothermal gasification. Here, in an excess of water, the reaction at temperatures up to 700 °C and pressures around 30 MPa directly leads to valuable hydrogen instead of synthetic gas, as in conventional gasification processes, or methane at subcritical conditions in water. After reaction, pressurized hydrogen is obtained and can easily be enriched due to the different partition coefficients of hydrogen and carbon dioxide between the aqueous and gas phase.

• Even homogeneous catalysis is possible in supercritical water. This has been demonstrated with the cobalt-catalysed cyclotrimerization of acetylenes to form benzene derivatives or hydroformylation to produce aldehydes from olefins. There, only the addition of CO is necessary, the H₂ required being formed by the equilibrium of the water–gas-shift reaction. After a homogeneous reaction in the supercritical state, the reaction mixture can be separated at subcritical conditions.

In support of the chemical and technical developments and to principally understand the experimental findings fundamental aspects have to be investigated as well. Intensive studies have been devoted to chemical kinetics including the modelling with elementary reaction steps, e.g. to separate ionic and radical reaction pathways. Depending on the reaction conditions, ionic or radical reaction pathways can be favoured or suppressed, allowing for control selectivity. Furthermore, corrosion of relevant reactor materials has been investigated.

The structural, electronic, and spectroscopic properties of a high-pressure phase of methane hydrate (MH-III) are studied by first-principles electronic structure calculations. A detailed analysis of the atomic positions suggests that ionization of hydrogen-bonded water molecules occurs around 40 GPa and centring or symmetrization of hydrogen bonds occurs around 70 GPa. These pressures are much lower compared with ionization around 55 GPa and centring around 100 GPa in pure ice. The transition may be observed with low-temperature IR/Raman spectroscopy of OH stretching modes or neutron diffraction.

We report new data on the melting curve of H₂O in the range 20–90 GPa and 1000–2400 K obtained in a laser-heated diamond anvil cell. We found a marked discrepancy between our present results and previous work which covered the pressure range up to 38 GPa. The melting curve shows a discontinuous change in slope at about 43 GPa, indicating a first-order phase transition most likely to the ice X phase. We could find no evidence for a strong dissociation of H₂O up to 200 K above the melting temperatures.

Co-solvents were studied to determine if the change in the cohesive energy density (CED) generates an effective solvation pressure equivalent to the application of an external hydrostatic pressure. Raman modes of chloroform under hydrostatic pressure with co-solvents (chloroform–ethanol, chloroform–acetone) and in the vapour phase were recorded. In some cases the Raman frequency shifts indicate that the solvation pressure behaves as a true hydrostatic pressure. The pressure-induced gelation of starch grains was studied in aqueous media. A higher co-solvent concentration is postulated to put the grains under effective negative pressure, and indeed an increase in the external pressure needed for gelation was seen after the introduction of solvents. The quantitative agreement between the change of solvation pressure and hydrostatic pressure is very good over a wide range of solvent concentration.

The present paper is devoted to studying the influence of water vapour pressure on the α-Al₂O₃ phase nucleation process and on the size distribution and doping level of the synthesized fine crystalline corundum. The effect of preliminary compaction of precursors and pressure variation on the corundum synthesis process in water vapour was also investigated. The products of synthesis were studied by various physical and chemical methods. It was found that the rise of water vapour pressure during synthesis leads to diminution of the concentration of nucleation centres and promotes increase of the corundum crystal size. The higher the pressure the more perfect the corundum crystals that are formed, having lower concentration of defects.

Two different experimental set-ups have been installed at ID22 beamline (ESRF) for synchrotron x-ray fluorescence (SXRF) analyses in diamond anvil cells (DACs). The set-ups differ from each other by the incident energy, the focusing device and the collection geometry. Analyses of standard aqueous solutions containing well-known concentrations of Sr and Rb have been performed in both experimental configurations and the detection limits (DLs) determined. The most important factors contributing to both the fluorescence intensity yield and the background have been discussed in order to determine the experimental configuration providing the lowest detection limits. The limiting factor for the analyses at very low concentrations is the background arising from the Compton and Rayleigh scattering in the diamond anvils, which have a major contribution in the collection geometry imposed by the design of the DAC. We show that slight improvements of this design enable the acquisition of the fluorescence at 150°, permitting SXRF analyses of aqueous solutions to be performed at the 40 ppm level in a standard DAC working to 10 GPa and 600 °C, at least.

Solvation dynamics of a fluorescent probe in micellar environment has been studied as a function of pressure using picosecond time-dependent fluorescence spectroscopy. Steady-state and time-dependent fluorescence spectra of coumarin 153 (C153) solubilized in two kinds of aqueous surfactant micellar solutions, i.e. neutral micelle; triton-X 100 (TX100) and ionic micelle; sodium dodecylsulfate (SDS) have been measured at high pressures. Both steady-state and time-dependent spectra exhibit opposite pressure dependence. For the steady-state spectra, with increasing pressure, the peak maximum shifts toward blue in the TX100 medium, while it shifts toward red in the SDS medium. The solvation time decreases for TX100, while it increases for SDS. The results suggest the different hydration structure surrounding micelles.

This paper describes a solid phase transformation of aluminium hydroxide into oxyhydroxide (boehmite), and then into well formed fine crystals of corundum (α-Al₂O₃) in the medium of a supercritical water fluid at a temperature of 450 °C and a pressure of 30 MPa. Using electron microscopy the development and processing of corundum crystal formation from incipient crystals to crystal growth and their perfecting is traced.

We present results of a study on the phase equilibria and pressure–volume–temperature relations for water and ice VII using an optical system designed for Raman spectroscopy and pressure–temperature measurements. The study shows that the strontium borate sensor represents an important tool for high-pressure–high-temperature manometry for temperatures below 600 K. In the pressure–temperature ranges 0–5 GPa and 240–600 K we detected phase transformations between four phases of H₂O as documented by Raman spectra, pressure–temperature scans, and visual observations. Analysis of the interference fringes and comparison of the experimental data on thermal pressure with the published equations of state (EOSs) show that the heating/cooling cycles were carried out under quasi-isochoric conditions. The experimental results are discussed/analysed on the basis of different EOSs for water and ice.

We study the nitrogen-rich part of the phase diagram Hf–N, Zr–N and Ti–N, employing first-principle calculations for an assessment of energy and enthalpy as a function of pressure. At zero pressure the novel cubic Th₃P₄-type structures are metastable modifications of M₃N₄ (M = Hf,Zr). The lowest energy configuration of both compounds is an orthorhombic Zr₃N₄-type. This orthorhombic structure will transform into the Th₃P₄-type at 9 and 6 GPa, for Hf₃N₄ and Zr₃N₄, respectively. The lowest energy configuration of Ti₃N₄ is a CaTi₂O₄-type structure. It will first transform into the orthorhombic Zr₃N₄-type at 3.8 GPa, then further transform into the cubic Th₃P₄-type at 15 GPa. The spinel type is metastable throughout the phase diagram for all three systems.

The phase boundary between mononitrides MN and the M₃N₄-phases is accessed as a function of pressure. We include the entropy of gaseous nitrogen from tabulated data to estimate the free enthalpy ΔG of the nitride phases. The orthorhombic modification of Hf₃N₄ turns out to be thermodynamically stable with respect to a decomposition into the mononitrides and nitrogen for temperatures up to about 1000 °C. The stability of Zr₃N₄ is in question; within the estimated error no final conclusion can be drawn. Ti₃N₄, on the other hand, will only be metastable. At higher pressures, however, the free energy of nitrogen is substantially reduced and the 3:4 compositions become more stable. We reproduce the experimental requirements (18 GPa and 2800 K) for the synthesis of the novel Hf₃N₄. At 2800 K the pressures needed to synthesize cubic phases of Zr₃N₄ and Ti₃N₄ are estimated to be 40 and 100 GPa, respectively.

To understand the principles of pressure effects on biochemical processes, it is helpful to consider the effects of changing volume at molecular levels. Non-covalent interactions constitute the main target for the modulation of biomolecular characteristics through pressure, mainly because they stabilize nearly all the biochemical compounds. On the other hand, the two parameters pressure and temperature must be considered in close relationship since their effects on equilibrium or kinetics—where the role of weak interactions is predominant—are antagonistic in molecular terms. An interpretation of reaction and activation volumes is usually given in terms of intrinsic and solvent contributions. Most covalent bonds participating in the protein primary structure are pressure insensitive, at least up to 1000–1500 MPa. Thus, in the absence of covalent bond formation or breaking, the largest contributions are expected to arise from hydration changes that accompany non-covalent interactions. These structural aspects will be discussed through significant model systems and biochemical examples.

Raman spectroscopic measurements of the tetrahedrally coordinated crystal MnSiO₃ (rhodonite) in an argon pressure medium show that it becomes amorphous above 33 GPa. This observation consolidates our findings and explanation for the global structural trends exhibited by the extended chain silicate family (: Mg, Ca, Mn, Fe; B: Si) upon compression. In particular, crystals of this family are made of two types of building blocks coined P and C. Those crystals comprised solely of P blocks transform to dense higher coordinated crystalline phases; those comprised of P and C blocks, such as MnSiO₃ rhodonite, become amorphous; whereas those comprised solely of C blocks show both crystalline and amorphous regions upon compression. The reason that this medium range order length scale (building block scale) classification is correlated with the type of transitions taking place upon compression is due to the instability of C blocks and C–P interfaces with respect to P blocks and P–P interfaces at high pressures.

We present the results of a theoretical study of the response of molecular CO₂-I and CO₂-III, and polymeric CO₂-V polymorphs to hydrostatic pressure. Total energy calculations and geometry optimizations have been performed under the local density functional approximation combining a pseudopotential and planewave scheme as implemented in the VASP code. Using the atoms in molecules theory, the network of inter- and intra-molecular chemical bonds of the different phases are rigorously characterized in terms of the values of the electron density and the Laplacian at the bond critical points. The chemical graph of a hypothetical orthorhombic structure displays bonding features that are associated with a precursor geometry of polymeric carbon four-fold coordinated phases. In addition, the bulk compressibility is decomposed into atomic and molecular contributions with the aim of providing a better understanding of the reasons that explain the emergence of low compressible polymorphs at high pressures.

In recent years, the application of pressure on biological systems has gained increasing interest. Pressure-induced destabilization of electrostatic and hydrophobic interactions is currently exploited to study conformational protein stability and macromolecular assemblies of proteins. Due to links between severe human pathologies and ordered protein oligomerization into aggregates, which have become apparent, a better knowledge of the molecular and structural determinants that ensure the packing efficiency and stability of such complexes has taken on special importance.

Here, we report the effect of pressure on the property of human ataxin-3 of aggregation. The results indicate the importance of its polyglutamine chain length in the stability and the tendency of the protein to form spheroids. Partial unfolding of the protein leading to solvent exposure of hydrophobic domains appears to be a prerequisite in the aggregation process of ataxin-3.

Well-shaped single crystals of NdNiO₃ perovskite have been grown for the first time under high oxygen pressure conditions, in a belt-type press at 4 GPa. The reaction took place in sealed platinum capsules, in the presence of KClO₃ as the oxidizing agent. It seems that the choice of hydroxides of the cations involved as departure reagents is crucial for the success of the crystal growth, via water vapour transport reactions. The perovskite has been characterized by x-ray diffraction, scanning microscopy and differential scanning calorimetry measurements, suitable to identify the metal–insulator transition. The transition temperature of the as-grown crystals, of 191.2 K in the heating run, is significantly lower than that reported for powder NdNiO₃ materials. This suggests a better metallicity of the present crystals, probably due to an improved oxygen stoichiometry associated with the much stronger oxidizing conditions utilized for its preparation.

High-pressure/high-temperature conditions were used to synthesize the new rare earth oxoborates Ln₄B₆O₁₅ (Ln = Dy, Ho; 8.0 GPa, 1000 °C) and α-Ln₂B₄O₉ (Ln = Eu, Gd, Tb, Dy; 10.0 GPa, 1150 °C) in a Walker-type multianvil apparatus. In addition to their new compositions, which were unattainable under normal pressure conditions, the structures exhibit the new structural motif of edge sharing BO₄ tetrahedra leading to a structural extension of oxoborate chemistry.

The author has been exploring the synthesis of inorganic materials in organic solvents at temperatures (200–300 °C) higher than their boiling points (solvothermal reaction), and has developed various reaction methods for the synthesis of ultrafine particles of metal oxides. In this paper, the reactions of aluminium compounds (aluminium hydroxide (Al(OH)₃; gibbsite), aluminium alkoxides, and aluminium salts) in various organic solvents (alcohols, glycols, aminoalcohols, and inert organic solvents) are first reviewed, and reaction mechanisms and effects of the starting materials and solvents on the products are discussed. Then, the specificity of the use of glycols, especially 1,4-butanediol (glycothermal reaction), is clarified, and glycothermal synthesis of crystalline mixed oxides such as yttrium aluminium garnet is described. Finally, the use of the solvothermally prepared products as the catalyst materials is described.

The microwave-assisted polyol process was investigated for the synthesis of several nanophase metals under different conditions in the temperature range of 100 to 200 °C. Nanophase Ag metal particle synthesis using ethylene glycol (EG) was carried out at 100 and 150 °C in the presence of polyvinyl pyrrolidone (PVP), dodecyl amine (DDA), oleic acid (OA) or trioctylphosphine (TOP) to determine whether particle size and shape can be controlled. The Ag particle size was changed with different capping agents. The use of PVP led to well defined Ag nanoparticles in the range of about 30–50 nm. Morphologically well defined nanophase metals of Fe, Co and Ni were produced in the presence of both PVP and DDA, while the use of just PVP or PVP and TOP did not yield such particles. The results presented here show that a variety of well defined nanophase metals can be produced by using the microwave–polyol process and optimizing the metal capping agents.

Single crystals of Fe₂O₃ (haematite), Be₃Al₂Si₆O₁₈ (beryl) have been grown on seeds from hydrothermal solutions. Optical microscopy and AFM have been used for the study of the growth morphology of atomically smooth faces of single crystals as well as of the surfaces growing in accordance with the regeneration growth mechanism. Profiles of the crystallization front have been obtained for different directions of crystallization. Their analysis has shown that the growth morphology may be numerically characterized using the elements of fractal theory. The calculated values of the fractal dimensions are different for layer-by-layer growth and regeneration growth. For beryl single crystals, fractal analysis of the growth surface has been carried out and numerical fractal characteristics have been obtained using the area–perimeter method.

Lignin was selectively converted into single chemical species in water–p-cresol mixtures at 673 K. Complete depression of char formation was realized in a mixture of 1.8 g of water and 2.5 g of p-cresol. The frame structure of lignin was efficiently decomposed within a reaction time of 4 min. The species obtained had a molecular weight of 214 (M⁺) assigned by gas chromatography–mass spectroscopy and was identified as hydroxylphenyl-(hydroxyltolyl)-methane (HPHTM) by ¹H and ¹³C nuclear magnetic resonance. Its yield approached the maximum of 80% C at  min of reaction time. HPHTM was presumably produced by the addition of p-cresol at the most active C α position of the hydroxyphenylpropane derivative that was formed by the hydrolysis of lignin.

The exchange of SO₄²⁻ ions with CO₃²⁻ ions in natural strontium sulfate (celestite) single crystals was investigated under alkaline hydrothermal conditions. The hydrothermal treatments were conducted using a stainless steel Teflon-lined vessel. Experiments were performed at different temperatures from 150 to 250 °C for various reaction times between 1 and 96 h with a molar ratio of CO₃²⁻/SO₄²⁻ = 1, 5 and 10. Structural characterization of partially and completely converted strontianite (SrCO₃) crystals was conducted by XRD, FTIR and SEM. X-ray diffraction results showed that under hydrothermal conditions the exchange of CO₃²⁻ ions in celestite was completed at 250 °C in a Na₂CO₃ solution with a molar ratio CO₃²⁻/SO₄²⁻ = 10 for 96 h, and celestite changed to strontianite. The morphology of the converted strontianite crystals depicted that the conversion proceeds without severe changes of the original shape and dimension of the crystals. A peculiar texture, however, consisting of elongated channels and tiny holes distributed randomly normal to the (001) cleavage plane, was formed in order to compensate the dimensional change of the lattice parameters associated with the conversion of celestite to strontianite. Details regarding the conversion mechanism are discussed in the present work.

Polycrystalline calcium titanate films up to 300 nm in thickness were grown on titanium and Ti6Al4V substrates by the hydrothermal–electrochemical technique in an autoclave at 200 °C. The films examined by x-ray diffraction exhibited reflections from orthorhombic calcium titanate on both substrates. The analysis by x-ray photoelectron spectroscopy showed calcium, titanium and oxygen at binding energies consistent with calcium titanate. The films grown on Ti6Al4V exhibited a small aluminium content but no vanadium. All films showed magnesium incorporation equivalent to a few per cent of the calcium content, which can be useful for the eventual use of the coating for biocompatible applications.

The hydrothermal–electrochemical growth of ATiO₃ (A = Ba, Sr) dielectric thin films on Ti substrates in A(OH)₂ solutions is reviewed. The film thickness increased monotonically with increase in the quantity of electricity passed through the Ti substrate, and could be controlled coulometrically by this factor. The current efficiency for the film growth was in the range from about 0.6% to 3.0%. (Ba,Sr)TiO₃ solid-solution films could be grown with control of the Ba/Sr composition. ATiO₃ thin films could also be grown uniformly on porous Ti bodies using cyclic electrolysis. A tracer experiment revealed that the ATiO₃ film grew at the film/substrate interface by transport of both A-site and oxygen atoms from the solution to the interface. The films grown were composed of a thin surface layer having an isotropic polycrystalline structure and an inner layer having a columnar structure. Nanopores were found at the boundaries of columnar grains constituting the inner layer. The observed pores are considered to act as short-circuit paths for mass transport from the film surface to the film/substrate interface. The resistivity of the BaTiO₃ thin film was as high as 10¹² Ω cm in the voltage range up to 2 V. The breakdown field was higher than 30 MV m⁻¹. The dielectric constant and dielectric loss were about 340–350 and 7%–10%, respectively, at 1 kHz and 0.1 V_rms. The polarization–voltage hysteresis loops were those of a paraelectric phase.

The effect of varying hydrothermal hot-pressing (HHP) parameters on the expansion of waste glass powder was investigated by conventional heat treatment. Glass ceramic porous materials were prepared by hydrothermal hot pressing under standard conditions at 200 °C, for 2 h at a constant uniaxial pressure of 20 MPa, while varying experimental variables such as glass particle size, water content, reaction interval, temperature and heating rate. SEM investigation showed the presence of a new glass phase, which incorporated water in its structure. The degree of reactivity attainable between glass particles and water seems to control the expansion process during heating of HHP glass compacts. It was found that the expansion process is independent of experimental parameters such as reaction time, temperature and heating rate, but does depend on the particle size and water content. During the heat treatment, the glass foaming process was preceded by decomposition of the new glass phase in the HHP compacts. A minimum apparent density of 0.40 g cm⁻³ was obtained on specimens prepared with low water content (5 wt%) and medium particle size (39–45 µm). X-ray diffraction patterns of the expanded glasses revealed the formation of SiO₂ (α-cristobalite and quartz) and CaSiO₃ (wollastonite).

During these last fifty years, high pressure science and technology have been the object of tremendous development in a variety of scientific areas, involving physics, geosciences, chemistry and biosciences. Pressure, as a basic thermodynamic variable, is commonly found in nature: from the centre of the Earth to the giant planets. Consequently the use of high pressures can be considered as an important tool for improving scientific knowledge and developing new processes.

It was my pleasure to welcome to Bordeaux, during one week, more than 400 scientists representing approximately 30 different countries, as participants in the International Conference on High Pressure Science and Technology, joining the 19th AIRAPT and 41st EHPRG Meetings.

I deeply thank all the institutions that supported this conference, as well as all the participants for the high scientific level of all their contributions.

I would like to dedicate the proceedings to all the scientists who developed high pressure research in the past, and in particular I wish to thank Jacques Basset and Michel Contré for their scientific advice at the beginning of my research activity on high pressure.