Table of contents

For the geometrically frustrated spinel system CdV₂O₄ with V³⁺ (S = 1) that undergoes structural and magnetic transitions at T_c1 = 97 K, and may exhibit a transition to the antiferromagnetic state at T_c2 = 35 K, the crystal structures at 299 and 85 K have been determined with space groups Fdm and I4₁ /amd, respectively, by means of x-ray four-circle diffraction. At 85 K, the VO₆ octahedron is distorted due to the Jahn–Teller effect with contraction of the V–O bond along the tetragonal c axis, and the network of V ions is achieved by the linkage of a distorted V₄-tetrahedron block with two kinds of V–V bonds. On the basis of these structural properties, magnetic susceptibilities at temperatures between T_c1 and T_c2 are explained in terms of the tetragonally distorted pseudotetramer model.

It was observed that a ferroelectric domain boundary (DB) is always accompanied by an antiferromagnetic DB in hexagonal YMnO₃, by means of interference effects of the second-harmonic signal. The clamping of these two order parameters at the ferroelectric DB is shown theoretically to originate from Dzyaloshinski–Moriya interaction. This interaction favouring a right angle between the neighbouring spins is found to be operative within the DB and to reverse the direction of the spins across the ferroelectric DB.

The discovery of ordered but aperiodic quasicrystal structures has required the development of new diffraction methods to determine their structures. The purpose of this review is to highlight the challenges posed by the aperiodicity of quasicrystals to the study of their surface structures by low-energy electron diffraction (LEED). This paper describes how the LEED technique has been thus far adapted to study several quasiperiodic surface structures, and summarizes the results of those studies. It also presents new spot-profile-analysis LEED results from the fivefold surface of Al–Pd–Mn. An x-ray diffraction experiment and a He-atom diffraction experiment, both on Al–Pd–Mn, are also described for comparison.

π-conjugated compounds have been intensively studied for their use in 'plastic electronics', an alternative to inorganic semiconductor devices. In particular, research in the use of π-conjugated compounds as the active layer in electroluminescent devices has advanced rapidly: potential applications include e.g. flat, flexible display panels. In organic electroluminescent devices the annihilation of positive and negative charge carriers to the ground state proceeds via two fundamental steps: (i) formation of a neutral exciton, either spin singlet or spin triplet, and (ii) the radiative or non-radiative decay of the exciton. In most π-conjugated materials only the spin singlet exciton is emissive, and this is why the spin statistics of step (i) determines the maximum achievable electroluminescent efficiency. A number of recent studies indicate that exciton formation is spin dependent and that more singlets form in organic electroluminescent devices than what is expected from spin degeneracy. We review recent experimental and (to a lesser degree) theoretical results. In particular we focus on our own recent work, where we have measured the ratio, r = σ_S /σ_T, of the spin-dependent formation cross section, σ, of singlet (S) and triplet (T) excitons in π-conjugated oligomer and polymer films, using a spectroscopic/magnetic resonance technique. The experimental results reveal a distinct difference between exciton formation in molecular devices compared to polymer devices.

In situ neutron diffraction experiments have been carried out to study the kinetics of the transformation of high-density amorphous (HDA) water into its low-density amorphous state at temperatures 87 K ≤ T ≤ 110 K. It is found that three different stages are comprised in this transformation, namely an annealing process of the high-density matrix followed by a first-order-like transition into a low-density state, which can be further annealed at higher temperatures T ≤ 127 K. The annealing kinetics of the HDA state follows the logarithm of time as found in other systems showing polyamorphism. According to the theory of transformation by nucleation and growth the apparent first-order transition follows an Avrami–Kolmogorov behaviour. An energy barrier ΔE ≈ 33 k Jmol⁻¹ is estimated from the temperature dependence of this transition.

Square L × L (L = 24–128) Ising lattices with nearest neighbour ferromagnetic exchange are considered using free boundary conditions at which boundary magnetic fields ± h are applied, i.e., at the two boundary rows ending at the lower left corner a field +h acts, while at the two boundary rows ending at the upper right corner a field −h acts. For temperatures T less than the critical temperature T_c of the bulk, this boundary condition leads to the formation of two domains with opposite orientations of the magnetization direction, separated by an interface which for T larger than the filling transition temperature T_f (h) runs from the upper left corner to the lower right corner, while for T < T_f (h) this interface is localized either close to the lower left corner or close to the upper right corner. Numerous theoretical predictions for the critical behaviour of this 'corner wetting' or 'wedge filling' transition are tested by Monte Carlo simulations. In particular, it is shown that for T = T_f (h) the magnetization profile m(z) in the z-direction normal to the interface is simply linear and the interfacial width scales as w ∝ L, while for T > T_f (h) it scales as w ∝√ L. The distribution P (ℓ) of the interface position ℓ (measured along the z-direction from the corners) decays exponentially for T < T_f (h) from either corner, is essentially flat for T = T_f (h) and is a Gaussian centred at the middle of the diagonal for T > T_f (h). Furthermore, the Monte Carlo data are compatible with ⟨ℓ⟩ ∝ (T_f (h) − T)⁻¹ and a finite size scaling of the total magnetization according to M(L, T) = {(1 − T/T_f (h))^ν_⊥L} with ν_⊥ = 1. Unlike the findings for critical wetting in the thin film geometry of the Ising model, the Monte Carlo results for corner wetting are in very good agreement with the theoretical predictions.

With the example of the capillary condensation of Lennard-Jones fluid in nanopores ranging from 1 to 10 nm, we show that the non-local density functional theory (NLDFT) with properly chosen parameters of intermolecular interactions bridges the scale gap from molecular simulations to macroscopic thermodynamics. On the one hand, NLDFT correctly approximates the results of Monte Carlo simulations (shift of vapour–liquid equilibrium, spinodals, density profiles, adsorption isotherms) for pores wider than about 2 nm. On the other hand, NLDFT smoothly merges (above 7–10 nm) with the Derjaguin–Broekhoff–de Boer equations which represent augmented Laplace–Kelvin equations of capillary condensation and desorption.

The scattering processes between the quasiparticles in the spin-up superfluid and the quasiparticles in the spin-down normal fluid are added to the other relevant scattering processes in the Boltzmann collision terms. The Boltzmann equation has been solved exactly for temperatures just below T_c1. The shear viscosity component of the A₁-phase drops as C₁ (1 − T /T_c1)^1/2. The numerical factor C₁ is in fairly good agreement with the experiments.

Globular protein solutions of lysozyme in water and added salt are modelled according to the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, in order to determine their fluid–fluid and fluid–solid coexistence lines. Calculations are based on both computer simulations and theoretical approaches. Our results indicate that, when the potential parameters are obtained by fitting physical properties directly deducible from either static or dynamic light scattering data, the fluid–fluid phase coexistence predictions agree quite well with the experiments. Our description of the solid phase allows only a qualitative reproduction of the experimental solubility boundaries. The overall accuracy of our predictions is discussed in view of the well known limitations of the DLVO representation of protein solutions.

Melt-spun Al alloys containing 2.7–10 at.% La were studied by means of x-ray diffraction, transmission electron microscopy, electrical resistivity, thermal and microhardness measurements. Fully amorphous structures were only obtained for thin AlLa_7.7 and AlLa₁₀ ribbons and exhibit concentration fluctuations. Phase separation into two metastable bcc structures is observed; they appear successively and are finally replaced by the equilibrium phases α (fcc)Al and (orthorhombic)Al₁₁La₃. The transformation kinetics is analysed in terms of a general Johnson–Mehl–Avrami–Kolmogorov model and of local kinetic parameters. The Avrami exponent monitors changes in the crystallization behaviour; nucleation of the stable phases occurs after partial dissolution of the metastable phases. The low values of the activation energy reflect the high instability of these glasses.

Optical absorption bands can be used as fingerprints of defects and their charge states in insulators and semiconductors. On the basis of the photochromicity usually shown by such materials, a method is introduced by which the optical bands are assigned to the defects and their charge states. It is based on simultaneous measurements of the light-induced changes of the optical absorption and of the corresponding EPR signals. Moreover, indirectly optical bands of EPR-silent defects can also be labelled in this way, strongly widening the scope of EPR based defect studies. We apply this method to the infrared-sensitive photorefractive system BaTiO₃:Rh, where illumination leads to recharging among the valence states Rh⁵⁺, Rh⁴⁺ and Rh³⁺. The values of all parameters governing the charge transfers responsible are inferred from the magnitude of the absorption bands, the absolute determination of their absorption cross-sections and the kinetics of the absorption changes under illumination. In contrast to previous investigations, these parameters are deduced independently of photorefractive measurements.

We report the results of an extended x-ray absorption fine structure (EXAFS) study of a sample of ZrO₂ prepared by high-energy ball milling. X-ray diffraction showed that the sample contained nanocrystals that were predominantly monoclinic with a particle size of 15 nm. The EXAFS for the sample was strongly attenuated in comparison to that for bulk monoclinic ZrO₂. This has been interpreted as the ball-milled sample containing a large level of disorder whose possible origins are discussed. In contrast, our previous EXAFS studies of nanocrystalline oxides prepared by sol–gel methods have shown that these samples contain well-ordered crystallites with grain boundaries similar to those in bulk materials. It is concluded that ball-milled samples are very different from oxide nanocrystals produced by other techniques.

We study the transfer of vibronic excitation energy in helical forms of proteins. The steric structure of the helix protein is modelled by a three-dimensional network of oscillators representing peptide groups. The covalent and hydrogen bonds between the peptide groups are described by pair interaction potentials. Each peptide group possesses one internal vibrational (excitonic) degree of freedom embodying the amide-I mode. The transfer dynamics of an amide-I exciton along the helix is expressed in terms of a tight-binding system. In the first part of this paper we study a reduced system arising when the vibrations of the covalent bonds are neglected. For the resulting system consisting of the exciton coupled to the hydrogen bond vibrations oriented along the helix axis we construct polaron solutions. Subsequently we investigate the mobility of the polarons within the complete protein matrix including deformations of the covalent bonds too. In particular we show that, during a phase of adaptation going along with internal energy exchange between the exciton and the bond vibrations, a relaxation into a new steady regime takes place. The newly reached equilibrium state is characterized by a localized exciton breather and is attributed local deformations of the steric peptide cage in the form of phonobreathers. Finally, coherent motion of an exciton breather is initiated through suitable injection of kinetic energy. In this way the long-range transfer of vibronic amide-I energy in the steric protein cage is provided. Interestingly, the α-helix possesses better facilities in supporting mobile localized excitons compared to the 3–10-helix form of proteins.

We studied the charge ordering transition of LiMn₂O₄ by means of anelastic spectroscopy, measuring the Young's modulus, E^', and the elastic energy loss function, Q⁻¹. Our measurements confirm the occurrence of a phase transition at 296 K on cooling, showing a large hysteresis on heating. The anelastic spectroscopy results also indicate that the transformation is characterized by the coexistence of two phases. The shape of the real part of the Young's modulus seems to indicate that the elastic constant which is more strongly affected by the transition is c₄₄, like in the case of Fe₃O₄. The time evolution of E^' in the coexistence region was measured. The characteristic time, τ, of this evolution rapidly decreases from 296 to 290 K and then increases on further cooling. This behaviour of τ can possibly be related to the temperature dependence of E^'. The absolute value of the Young's modulus of LiMn₂O₄ obtained considering the effect of the porosity of the sample was estimated to be of the order of 10 GPa. Finally a new thermally activated peak, centred around 100 K at a measurement frequency of ∼1 kHz, has been observed, which cannot be described by the Debye or Fuoss–Kirkwood model. This relaxation may be ascribed to the stress-induced jumps of charges from Mn³⁺ to Mn⁴⁺ sites, similarly to an analogous process found in Fe₃O₄, or to possible small movements of the octahedra and tetrahedra composing the crystal structure.

Thermal properties of ordered and disordered Pb(In_1/2Nb_1/2)O₃ (PIN) single crystals were investigated. For the ordered PIN, typical thermal anomalies were observed at the antiferroelectric transition temperature. These anomalies were degraded and disorder appeared during heating processes. The defect-induced phase separation provided a good account of the instability of the relaxor and antiferroelectric states of the PIN crystals.

The dependence of the lattice parameter of rare-gas (Lennard-Jones) solids on isotopic mass has been studied by the path-integral Monte Carlo method. Simulations were carried out in the isothermal–isobaric ensemble, which allows us to study this isotopic effect as a function of temperature and pressure. In the limit T → 0 and at ambient pressure, the difference Δa between lattice parameters of isotopically pure crystals with lightest and heaviest isotopic mass is found to range from 8.7 × 10⁻³ Å for Ne to 1.2 × 10⁻³ Å for Xe. This isotopic effect decreases appreciably upon increasing temperature. At 80 K, Δa for Ar, Kr and Xe is found to be less than one-third of the corresponding low-temperature value. An applied hydrostatic pressure also causes an important decrease in Δa. For Ne and Xe, a pressure of 30 kbar reduces this difference by a factor of 6.5 and 3.1, respectively. These differences in lattice parameter are larger than the sensitivity limit presently achieved in experimental studies, even for solid xenon at its Debye temperature.

The shifts of the fundamental and optical bandgap energies as functions of dopant concentration in heavily n-type and p-type doped Si_1−xGe_x (x ≤ 0.3) have been investigated theoretically. The band structure of the intrinsic crystal was described by the k · p-perturbation method, where the Kohn–Luttinger parameters were determined from a first-principles and full-potential band-structure calculation. The doping-induced effects on the bandgap were thereafter calculated using a zero-temperature Green function formalism within the random phase approximation and with a local field correction of Hubbard. We found only small effects on the bandgap energies due to variation of composition x. The calculated bandgap narrowing of Si and of Si_0.82Ge_0.18 were found to be in good agreement with photoluminescence measurements.

By means of electron-correlation-corrected density functional calculations for GdBaCoO_5.5, it has been found that the pdσ hole delocalization in the 'almost high-spin' state is responsible for the antiferromagnetic–ferromagnetic-like transition and insulator–metal transition in isostructural LBaCo₂O_5.5 (L = Sm, Eu, Gd, Tb, Dy, Y), and that the simultaneous octahedral Co1 3z² − r² /x² − y² (pyramidal Co2 x² − y²) → Co1 (Co2) yz electron transfer gives rise to the lattice anomaly at T_IM. The present study, supporting the author's prior work (Wu H 2001 Phys. Rev. B 64 092413), provides a unified explanation for a number of experimental findings for LBaCo₂O_5.5.

The properties of Ce³⁺ in CaBPO₅, LiCaPO₄ and Li₂CaSiO₄ have been studied in the 100–400 nm wavelength region with time resolved spectroscopy at temperatures between 10 and 300 K. Emphasis is on the relationship between the energies of the five 5d levels of Ce³⁺ and the crystalline environment. Good agreement between predicted and observed average energy of the 5d configuration is demonstrated. In addition values for the bandgap of the host materials, Stokes shifts and luminescence decay time data are presented.

Measurements have been made to test the proposal that the INVAR effect is associated with a non-collinear ferromagnetic state. Neutron scattering experiments with polarization analysis of the incident and scattered beams have been made to obtain the absolute spin-dependent scattering cross-sections from the archetypal INVAR alloy Fe₆₅Ni₃₅. The measured average spin-flip cross-section for this alloy has been found to be close to zero, independent of the scattering vector Q and effectively constant as the sample temperature is reduced from 300 to 4.2 K and the magnetic field increased from 1.4 to 2.0 T. All this suggests that the INVAR sample is a collinear ferromagnet. In addition, the measured spin-flip cross-section is also in poor agreement with calculated curves based on models of the proposed non-collinear state. The magnitudes of the magnetic moments on the iron and nickel atoms in the collinear state have been obtained from the intensities of the Bragg peaks in the non-spin-flip cross-sections. The diffuse scattering between the Bragg peaks has also been analysed to determine the magnetic disorder present. These data have been discussed in the context of different models of the magnetic structure and the results also support the conclusion that Fe₆₅Ni₃₅ is a collinear ferromagnet.

The results of specific heat, magnetization and magnetic susceptibility measurements for the antiferromagnetic (AF) compound Gd₃Rh are presented. Below its Néel temperature T_N = 112 K, this compound exhibits a field-induced phase transition from the AF to the ferromagnetic state in a relatively small magnetic field (∼0.4 T). Specific heat measurements revealed a giant enhancement of the coefficient of the T-linear specific heat in Gd₃Rh (γ = 118 mJ mol⁻¹ K⁻²) in comparison with that observed for the isostructural nonmagnetic partner Y₃Rh (γ = 11 mJ mol⁻¹ K⁻²). An anomalous behaviour of the heat capacity as well as a set of peculiarities of magnetic properties in Gd₃Rh are associated with the existence of spin fluctuations induced by f–d exchange interaction in the d-electron subsystem that arises from features of the crystal and electronic structure of the R₃M-type compounds.

We report on the electron paramagnetic resonance (EPR) and optical absorption and fluorescence spectroscopy of YAl₃(BO₃)₄ single crystals doped with 0.2 mol% of trivalent chromium. From EPR we determine that the Cr³⁺ ions reside in sites of essentially octahedral symmetry with an orthorhombic distortion. The ground state ⁴A ₂ splitting is determined to be 2√D² + 3E² ≈ 1.05 ± 0.04 cm⁻¹, where D and E are fine-structure parameters, and we can attribute this splitting to the combined effect of a low-symmetry distortion and spin–orbit coupling. The g-values and fine-structure parameters D and E of the ground state ⁴A₂ are measured to be g_x ≈ g_y ≈ g_z = 1.978 ± 0.005, |D| = 0.52 ± 0.02 cm⁻¹ and |E| = 0.010 ± 0.005 cm⁻¹ respectively. From 10 K optical absorption we have measured the position and crystal-field splittings of the ²E, ²T ₁, ⁴T ₂, ²T ₂ and ⁴T ₁ states with the ⁴T ₂ and ⁴T ₁ levels appearing as vibronically broadened bands.

A theoretical model has been developed to calculate the superparamagnetic relaxation rate for small magnetically ordered particles of uniaxial symmetry having size 50–120 Å. On a microscopic level the relaxation process involves an exchange of energy between magnon and phonon sub-systems via magnon–phonon scattering induced by dynamic exchange. The model has been applied to ultra-fine iron compounds and the calculated superparamagnetic relaxation rates at low temperatures are usually consistent with those which are frequently observed by Mossbauer spectroscopy. One finds that the relaxation rate increases with temperature in a complicated manner besides its dependence on the number of magnetic ions in the particle.

The role of domain-orientation effects in forming the piezoelectric coefficient d₃₃ of polydomain KNbO₃ single crystals in the mm2 phase is studied. Non-180^° domain wall displacements in the electric field E ∥ [001] are analysed, and their contributions to d₃₃ are evaluated. A link between these contributions and crystallographic characteristics of domain structures is revealed for the first time for different 60^°, 90^° and 120^° domain structures.

Y₂O₃:Eu³⁺ and Li-doped Y₂O₃:Eu³⁺ luminescent thin films have been grown on sapphire substrates using a pulsed laser deposition technique. The thin-film phosphors were deposited at a substrate temperature of 600^°C under the oxygen pressures of 100, 200, and 300 mTorr. The films grown under different deposition conditions have been characterized using microstructural and luminescence measurements. The crystallinity and photoluminescence (PL) of the films are highly dependent on the oxygen pressure. The PL brightness data obtained from Y₂O₃:Eu³⁺ films grown under optimized conditions have indicated that sapphire is a promising substrate for the growth of high-quality Y₂O₃:Eu³⁺ thin-film red phosphor. In particular, the incorporation of Li⁺ ions into Y₂O₃ lattice could induce a remarkable increase of PL. The highest emission intensity was observed with LiF-doped Y_1.84Li_0.08Eu_0.08O₃ (Y₂O₃LiEu), whose brightness was increased by a factor of 2.7 in comparison with that of Y₂O₃:Eu³⁺ films. This phosphor may hold promise for application in flat-panel displays.

The systematic in the 4f ⁿ → 4f ⁿ⁻¹ 5d transition energy of divalent lanthanides in inorganic compounds has been studied. Energies were derived from excitation and optical absorption spectra gathered from the literature. Going through the lanthanide series from La²⁺ to Yb²⁺, a characteristic variation is observed similar to that known for the trivalent lanthanides and the free atoms and free ions. Once the transition energy is known for one lanthanide in a compound, that of all others in that same compound can be predicted. The value for the Stokes shift between fd excitation and df emission appears independent of the lanthanide ion. For Gd²⁺ to Yb²⁺ both the spin-forbidden and the spin-allowed fd-transition energies are discussed.

Temperature-dependent high-resolution ultra-violet photoemission spectroscopy (PES) measurements combined with tight-binding linear muffin-tin orbital (LMTO) atomic-sphere approximation (TB-LMTO-ASA) band-structure calculations are carried out to study the polar ordering in the low-dimensional ferroelectric CuInP₂Se₆. In the paraelectric phase, the energy-dependent PES spectra are in good agreement with our LMTO calculations. Below T_c ≈ 230 K, evidence for a strong redistribution of the experimental density of occupied states (DOS) is given. The LMTO calculations, accounting for the off-centre position of Cu atoms in the low-T phase, allow us to interpret this behaviour in terms of a redistribution of the partial Cu 3d DOS. This strong interplay between electronic and crystallographic instabilities is analysed in the light of a second-order Jahn–Teller effect acting as the driving force of the ferroelectric transition.

The longitudinal and shear velocities, and their pressure dependences, for various bulk amorphous materials, including bulk metallic glasses and non-metallic glasses, have been measured by the ultrasonic pulse echo overlap method under hydrostatic pressure (maximum: 2 GPa) or uniaxial compression in ambient conditions. The second- or third-order elastic constants and the long-wavelength acoustic mode Grüneisen parameter are calculated. The results reveal that all metallic glasses have positive pressure dependences of both the longitudinal and shear velocities, while for most non-metallic glasses both the longitudinal and shear velocities decrease with increasing pressure. Thus, the Grüneisen parameters evaluated for the mean mode and shear mode for metallic glasses and non-metallic glasses have opposite signs. This indicates that the influences of the short-range-order structure and bonding, correlating closely with the atomic configurations in the various amorphous materials, play an important role in the properties determined by vibrational anharmonicity. The negative and positive Grüneisen parameters exhibit mode softening and stiffness under high pressure, respectively. The results also clearly demonstrate that the Grüneisen parameter is affected by the pressure derivative of the shear modulus, especially for non-metallic glasses.