Table of contents

High-pressure phases of C₆₀ fullerene similar to those reported by Iwasa et al. were obtained at a non-hydrostatic pressure of 3 GPa and different temperatures in the 100-500 degrees C range. The substances were investigated by means of infrared, optical and micro-Raman spectroscopies. Compared with pristine C₆₀, the spectra of the transformed material reveal multiple changes which depend systematically on treatment conditions. They evidence a substantial modification of selection rules for allowed optical transitions apparently due to formation of C₆₀ molecules with deformed and/or partially destroyed cages.

A theoretical approach to homogeneity characterization by means of composition variance analysis is presented. A multiscale sampling image analysis is proposed to determine, using the model, both the intensity and the scale of segregation in a binary particle mixture. The method is applied to characterize the homogeneity evolution of a WO₃-NiO grain mixture with mixing time. The results show that the homogenization is controlled by a kind of diffusion process.

Ion transport mechanisms in Mg²⁺-stabilized Na⁺- beta ''-alumina were investigated by molecular dynamics simulations. Two compositions, different Mg²⁺ distributions, and different system sizes were used in the temperature range from 200 K to 1700 K. The trajectory plots of Na⁺ ions and the Na⁺-Na⁺ radial distribution function demonstrated the formation of a vacancy superlattice at low temperature. The stability of the superlattice was found to depend crucially on the Mg²⁺ distribution. The ionic motions in the Na⁺- beta ''-alumina were found to be highly correlated. The sodium tracer self-diffusion coefficient and zero-frequency ionic conductivity were found to depend strongly on the Mg²⁺ distribution. This explains why the conductivity depends on the thermal history of Na⁺- beta ''-alumina samples. The simulations show that the electrical conductivity can be increased by one order of magnitude at room temperature if the Mg²⁺ ions occupy optimal sites. The activation energy for Na⁺ transport was shown not to be a static single-ion energy barrier, but it is numerically close to the pseudo-potential energy difference along conduction paths at high temperature. Correlated ionic motion was suggested to be the main reason for the very low activation energy in Na⁺- beta ''-alumina.

The effect of Mg impurities on the FCC Al density of states in phonons is treated directly by introducing Mg in supercells of increasing size. A simple substitutional impurity is treated first, and the method is compared to the more usual Green function approach. Small clusters are then studied, and the additivity of pair effects on the lattice specific heat of the alloy is considered.

We report on the thermopower of (Ce_xPr_1-x)Cu₆ for 0<or=x<or=1 from 2 K up to room temperature. Over the studied temperature range, it is found that the thermopower is positive and show a maximum except for pure PrCu₆. Besides the maximum, we notice a shoulder at low temperature. With increasing Pr concentrations, the shoulder becomes more apparent and almost comparable in size to the main maximum. Using the Nordheim-Gorter rule, we have been able to estimate the magnetic contribution of Ce to the thermopower, which is found to be 50 mu V K^-1 at the maximum. We also report the thermopower of PrCu₆ for the first time. It is positive over the studied temperature range. PrCu₆ also shows a modest maximum at higher temperature. This maximum, however, is much broader and smaller than that for the Ce-rich side of CeCu₆.

In the present paper we report on the results of the Monte Carlo simulation of the time of flight (TOF) experiment for r and r- epsilon hopping transport in highly defected very thin crystalline layers. The defects are considered as localized states, between which a hopping motion of the injected carriers is possible. The total (integrated over energy) density of defects is assumed to be spatially non-uniform on the macroscopic scale, i.e. the scale comparable with the layer thickness. In particular, we consider an exponential dependence of the total density of hopping centres on the distance from the layer contacts. The results of simulations performed for various defect concentrations, various defect distributions in energy, and various degrees of the layer spatial non-uniformity are discussed. It is shown that both r and r- epsilon hopping transient currents measured in the classical TOF experiment are highly sensitive to the spatial macroscopic scale variations of the total centre concentration. The detailed shape of the transients depends in a complicated way on the system dilution, the energetic centre distribution, and the character of the spatial variations of the total centre density. The existence of the spatial non-uniformity of the layer could be recognized experimentally by observation of the qualitative changes of the current shape with increasing temperature, which leads to lower dispersion, and thus more pronounced characteristic features of the x-dependent total centre density, such as a higher polarity dependence, or the appearance of the current maxima or plateaux just before the effective TOF.

We use strong-coupling perturbation theory and the extrapolation of small-cluster exact-diagonalization calculations to describe the ground-state properties and possible intermediate-valence transitions of the spinless Falicov-Kimball model with a generalized type of hopping. It is shown that, even for relatively small values of the interaction strength, the Falicov-Kimball model undergoes only a few (discrete) intermediate-valence transitions, and this result is demonstrated to be independent of finite-size effects.

An ESR study of vanadium-doped paratellurite indicates that vanadium enters the lattice as V⁴⁺, equally populating the four inequivalent cation sites. The 3d¹ electron exhibits hyperfine interaction with the I=7/2 ⁵¹V nucleus and superhyperfine interaction with two neighbouring I= 1/2 nuclei identified as protons by ENDOR experiments. Spin Hamiltonian parameters computed from ESR measurements show that each V⁴⁺ ion has a single C₂ symmetry axis along one of the (110) directions. The neighbouring hydrogens appear to enhance the symmetry and stability of the defect structure. Group theoretical arguments show that there are two possible linear combinations of d wavefunctions that could represent the ground state of V⁴⁺ in alpha -TeO₂. However, a determination of the lower-lying state was not feasible using only the ESR data.

The results of an electron paramagnetic resonance study of a new product, namely ethylenediammonium copper(II) diphosphate monohydrate of chemical formula Cu(NH₃(CH₂)₂NH₃)P₂O₇.H₂O, are presented. The X-ray diffraction has shown the existence of two types of copper(II) coordination polyhedron for each elementary mesh in this compound. Each polyhedron consists of a copper ion surrounded by five oxygenated ligands stoning from two P₂O₇ groups and a water molecule. Fair agreement between these results and those obtained by X-ray diffraction is observed. In particular the organization of oxygen atoms as pyramids with a square plane around the copper(II) ions is confirmed. The existence of two magnetically nonequivalent sites of copper per mesh is evidenced.

The influence of the size of the cation impurity (M) on the dynamics of the ZnCl₄ anion and on the structural stability of the (Rb_1-xM_x)₂ZnCl₄ system (M=Na, K, NH₄ and Cs) is investigated through ³⁵Cl NQR frequency and relaxation studies as a function of temperature. The mean square amplitude of torsional motions of ZnCl₄ tetrahedra about an axis parallel to the a-axis of the crystal structure and about axes perpendicular to this direction, as well as the corresponding activation energies, are estimated from the experimental data. The results show that, while parallel motions are insensitive to the presence of impurity, the perpendicular motions develop an anharmonic potential when the impurity cation is larger than the host cation, which also suppresses the structural phase transition temperature. Smaller impurity cations, on the other hand, have minimal effect on the anion dynamics and on the phase transition temperature.

A crystallographic analysis of the ferroelectric/ferroelastic domains and phase boundaries in thin platelets of Pb(Mg_1/3Nb_2/3)O₃, associated with the electric-field-induced phase transition from the disordered cubic phase to the induced 3m phase, is reported. Calculations have been performed by taking into account the internal stresses and their relation to the unit cell parameters. The results have permitted us to interpret the experimentally observed polar macrodomain structure, phase boundary and switching of the induced polarization. A diagram connecting the possible domain types and the shapes of the m3m from or to 3m phase boundaries has been proposed. The role of the several domain types and features of their elastic matching have also been analysed.

The presence of hydrogen (deuterium) defects in oxide ABO₃ crystals is revealed by a characteristic OH (OD) stretching vibration near 3500 cm^-1 (2600 cm^-1). Low-frequency OH (OD) librational modes have not been reported so far for these crystals. However, we have found in a number of ABO₃ crystals (LiNbO₃, LiTaO₃, SrTiO₃ and KTaO₃) weak high-energy sidebands to the stretching vibration with positions shifted by about 950 cm^-1 for OH and about 700 cm^-1 for OD from the vibrational band position. The bandshape of these new OH/OD absorption bands is similar to the bandshape of the OH/OD absorption bands of the stretching vibration in the corresponding ABO₃ crystals. The polarization of these sidebands in general is different from the stretching vibration and reflects more the properties of a three-dimensional oscillator. While the OH/OD stretching vibrations in LiNbO₃ and LiTaO₃ are completely polarized perpendicular to the ferroelectric z-axis, the libration+vibration combination bands exhibit in both crystal systems an intensity ratio I/sub ////I_{perpendicular to} approximately 0.55 for OH and an intensity ratio I/sub ////I_{perpendicular to} approximately 0.75 for OD with respect to the ferroelectric z-axis. These different intensity ratios for OH and OD support the identification of the new absorption bands as libration+vibration combination bands of the OH (OD) oscillator. The OH/OD libration+vibration combination bands have also been detected as sidebands to higher vibrational transitions in proton- (deuteron-) exchanged LiNbO₃. In a first approximation the energetic positions of the observed OH and OD combination bands can be well described with a simple oscillator model for the libration. The intensity of these sidebands is directly proportional to the intensity of the stretching vibration (or the concentration) of the H (or D) centres.

The scintillation properties of Ce³⁺ doped CsGd₂F₇ and CsY₂F₇ crystals have been studied at various temperatures. Upon X-ray excitation CsY₂F₇:5 mol% Ce³⁺ shows fast ( tau approximately=32 ns) Ce³⁺ 5d to 4f luminescence with a yield of 1400 photons MeV^-1 at room temperature. The most intense Ce³⁺ luminescence of about 7300 photons MeV^-1 is found in CsGd₂F₇:20 mol% Ce³⁺. The scintillation decay curve of CsGd₂F₇:Ce³⁺ consists of at least two non-exponential components, one of the order of tens of nanoseconds and one in the microsecond region. Both CsY₂F₇ and CsGd₂F₇ show a broad STE emission band at around 445 nm, especially at low temperatures. VUV photon excitation measurements at 300 K and low temperatures revealed the presence of two inequivalent types of Ce³⁺ centres in both CsY₂F₇ and CsGd₂F₇. The presence of Gd³⁺ appears to have a dominating influence on the luminescence characteristics of CsGd₂F₇:Ce³⁺, due to the occurrence of both Ce³⁺ to Gd³⁺ and Gd³⁺ to Ce³⁺ energy transfer as well as energy migration over the Gd³⁺ sublattice. On the basis of the optical studies, models of the scintillation mechanisms of CsY₂F₇ and CsGd₂F₇ are developed which account for the scintillation properties of these crystals.