Table of contents

The electron-density distribution of A15-type Cr was obtained by a combination of powder-X-ray-diffraction data and the maximum-entropy method (MEM). Fine powder, in which about 13 vol% BCC Cr was contained, prepared by evaporation of Cr in a low-pressure atmosphere of Ar, was used. Though the powder is composed of the two phases, the final R and R_W factors were 0.21% and 0.29%, respectively, and therefore excellent MEM maps were obtained. The MEM maps indicate that the main interaction is a strong covalent bond between adjacent 6c-site Cr atoms within the infinite linear chains. The MEM maps are compared with the valence-electron-density maps of superconducting V₃Si and non-superconducting Cr₃Si having an A15-type structure. The structure factors for some forbidden reflections, which originate from asymmetric bonding-charge distribution, were also calculated from the MEM densities.

The electronic structure of cadmium chalcogenide clusters is calculated within the molecular orbital framework using the extended Huckel technique with charge self-consistency. The size of clusters calculated corresponds to one to three elementary cells (the largest cluster is Cd₅₄S₆₃). The results of the calculation show the tendency towards semiconductor-like band-structure formation. Explicit bands in the energy spectrum appear for clusters containing more than 100 atoms. There is good correspondence to experimental features of clusters in the optical and X-ray photoelectron spectra. The relative stability of clusters with different geometries and sizes is evidence of possible structure transformations in the case of small clusters. Most stable structures of small clusters are distinct from the bulk.

We report directional Compton profiles of KHCO₃ measured along the pi pi direction (parallel to the HCO₃ layers and parallel to the H bonds), the pi sigma direction (parallel to the layers and orthogonal to the H bonds), and the sigma sigma direction (orthogonal to the layers and orthogonal to the H bonds) using a high-resolution Compton spectrometer at the LURE-DCI synchrotron facility in France operating at 19.9 keV. These measurements confirm the significant anisotropy of the electron distribution in KHCO₃, which we associate with the H bonds. This assumption is supported by the fact that the free-atom model (anions HCO₃^- without H bonds) is not able to describe our experimental results. A molecular orbital-LCAO-self-consistent model has been used to calculate the anisotropies with different size clusters, and taking into account or not the environment of the dimeric anion (HCO₃)₂^2$containing two H bonds. The calculated anisotropies based on free K₂(HCO₃)₂ yield the main features of the experimental anisotropies with respect to the pi pi direction. More sophisticated calculations involving K₄(HCO₃)₄ and K₆(HCO₃)₆ with the dimeric anion (HCO₃)₂^2$surrounded by point charges have not improved the agreement with the experiment. Moreover, none of these calculations is able to describe the small measured anisotropy between pi sigma and sigma sigma directions.

A microscopic approach to considering the influence of atomic local static displacement (ASD) on binary alloy thermodynamic properties has been developed. An equation that permits the calculation of ASD amplitudes from first principles has been derived. The value of the ASD amplitude is shown to depend on the difference in the effective interatomic potentials of the alloy components. The expression for the alloy free energy, which takes the ASD into account, is obtained by the collective variables method. The theoretical results are illustrated by numerical calculations, performed for alloys of the K-Rb system. The dependence of the ASD squared, averaged over configurations, on temperature and alloy concentration has been investigated, in particular.

The PrGe_1.6 compound shows two coexistent crystallographic structures, which are ordered and disordered variants of the ThSi₂ structure, the ordered phase resulting from the ordering of Ge vacancies. Comparison is made with CeGe_1.6 and Ce and Pr silicides, which exhibit the same type of phase separation.

For 30 diamond- and zincblende-structure semiconductors, the bond length d, bond polarity alpha _p, bulk modulus B, elastic shear constants (c₁₁-c₁₂)/2 and c₄₄, bond-stretching force constant alpha , bond-bending force constant beta , internal displacement parameter zeta , effective atomic charge Z*, transfer parameter beta *, transverse charge e_T*, and piezoelectric charge e_p* are calculated from bond orbital calculations based on the tight-binding method. The results are compared with previous theoretical calculations and experiments.

The internal-friction spectrum of high-purity polycrystalline and single-crystal iron is studied at 5, 10 and 30 MHz, over the temperature range 50-250 K. The results lead to the following conclusions: the temperature of the peak is essentially independent of the orientation; the temperature of the peak decreases with deformation and then starts to increase slightly with further deformation; the peak height increases with increasing deformation. Measurements on polycrystalline iron subjected to successive doses of neutron and gamma irradiation showed a reduction in the relaxation strength by increasing the irradiation doses.

We present results of elastic band structure for two-dimensional composite materials, composed of periodical square arrays of parallel cylinders in a background. We reveal, for the first time, the existence of several very large complete band gaps in the band structure of a material of practical interest such as a C fibre reinforced epoxy composite. Within these gaps the propagation of acoustic waves is forbidden. The influence of the geometry of the cylinders and the effect of the composition of the composite material on the band structure are studied. We also compare these results with those obtained for metallic composites such as W (Al) cylinders in an Al (W) matrix. The complete band gaps are observed in the cases of C cylinders in epoxy or W cylinders in Al, but not in the opposite situations. We discuss the existence of these gaps in relation to the physical parameters of the materials involved.

We report on experimental valence and conduction electronic distributions of quasicrystalline Al-Mn-Pd alloys probed using the soft x-ray emission and absorption spectroscopy techniques. The various partial distributions are adjusted in the binding energy scale in order to investigate the electronic interactions characteristic in the material. In the valence band, interaction exists between Al states and Mn 3d states near the Fermi level and with Pd 4d states in the middle of the band. At the Fermi level the intensity of the Al 3p states is very low and a rather wide pseudo-gap is observed. In the conduction band, interaction exists also between Al p-Mn d states in the energy range of the absorption edge. Pd d-s states are found about 2 eV beyond the Fermi level. It is suggested that unlike the case of Mn, a small charge transfer may exist from Al to Pd states. Adjustment of Al 3p and Al p distributions at the same intensity at the Fermi level shows that the density of available Al p conduction states is dramatically decreased in the quasicrystal in comparison to pure Al. We propose that this could be connected to the existence of numerous narrow gaps and spikes in the densities of states as predicted theoretically elsewhere for crystal approximant phases with very large unit cells.

In this work, the electronic, ground-state and vibrational properties of both alpha -GaN (i.e. wurtzite structure) and the recently fabricated beta -GaN (i.e. zincblende structure) have been studied using the ab initio pseudopotential method within the local density approximation and a simple quasi-particle scheme. The calculated equilibrium lattice constants, bulk moduli, the pressure derivatives of the bulk moduli, and the Al TO( Gamma ) phonon frequency are in good agreement with available experimental and other recent ab initio theoretical results. The self-energy band gap corrections are found to be highly k-dependent. The calculated fundamental band gap is direct in both cases and for the experimental lattice constant is calculated to be 3.36 eV in beta -GaN and 3.48 eV in alpha -GaN, in excellent agreement with experiment.

The dielectric screening function epsilon (k, omega ) of the fluid of charged bosom at zero temperature is evaluated in a range of low-to-intermediate coupling strength, in view of recent data on static screening and fluid structure from quantum Monte Carlo methods. Correlations beyond the random phase approximation are included within a class of approximations which are well known for the electron fluid, i.e. by introducing a frequency-independent local field factor to be determined through self-consistency requirements connecting various aspects of the physics contained in the dielectric function. The static dielectric function epsilon (k,0) is negative over a range of wavenumbers at all values of the density, leading to oscillatory screening of a foreign charge and to an effective long-range attraction between equi-charged impurities. Quantitative agreement with the Monte Carlo data on static screening is achieved by imposing self-consistency on the compressibility of the fluid in addition to self-consistency on the pair distribution function. Good agreement is also obtained on the pair distribution function and the correlation energy. Within the present class of approximations, the dispersion relation of longitudinal excitations takes the Feynman form, starting at the plasma frequency with a negative dispersion coefficient and going through a minimum before ending at the single-particle recoil frequency.

Positron states and annihilation rates are calculated for nine different II-VI compound semiconductors. Positron annihilation from the delocalized states in the perfect lattice as well as from the localized states at vacancies and divacancies is considered. The calculations are based on the local-density approximation (LDA) for the electron-positron correlation effects. The calculations are performed using non-self-consistent electron densities and electrostatic potentials obtained by atomic superposition and solving for the three-dimensional positron wavefunctions by a relaxation method. For the perfect lattices, also self-consistent electron densities and positron states (linear muffin-tin orbital method within atomic-spheres approximation, LMTO-ASA) are calculated. The results show that positron annihilation with the outer d electrons of the group II metal atoms plays an important role. The positron lifetimes calculated for perfect lattices are a few per cent shorter than the experimental ones, indicating an LDA overestimation of the d-electron enhancement around the positron. These bulk lifetimes can be corrected efficiently by a semiempirical model introduced in this work. In the case of positrons trapped by defects, the present theoretical description is less satisfactory, because atomic relaxations due neither to rearrangements in the electronic structure nor to the localized positron are taken into account. However, the calculated annihilation probabilities with core and valence electrons will serve as an important database for methods in which defects can be identified using the positron angular correlation or Doppler broadening measurements.

Density, DC conductivity and ¹¹B wide-line NMR investigations have been carried out for xFe₂O₃-(50-x)PbO-50B₂O₃ glasses (0<or=x<or=20 mol%). The NMR spectra indicated the formation of non-bridging oxygen ions in the borate network upon introducing Fe₂O₃. The DC conductivity and the activation energy for conduction change markedly with Fe₂O₃ content. The high-temperature conductivities can be described by Mott's model for the phonon-assisted hopping of small polarons. The polaron hopping is shown to be non-adiabatic. The low-temperature conductivities are characterized by the variable-range hopping mechanism. For Fe₂O₃ contents greater than 15 mol%, glass-crystal composites are formed, changing greatly the electrical properties, and other conduction mechanism is to be assumed.

The spectrum of a two-dimensional electron system in a square lattice potential subject to a perpendicular magnetic field is considered. Well known symmetry considerations show that if the magnetic field flux per unit cell (measured in units of the flux quantum hc/ mod e mod ) is rational and non-integer, the electron energy can be expressed as a function of a generalized momentum k and the spectrum consists of translationally degenerate bands in k-space. In this work it is shown that if the chemical potential is located at a logarithmic Van Hove singularity of the density of states then it is possible that a periodic lattice distortion is energetically favourable. This instability persists when the flux is changed within a small interval. The width of this interval as well as the critical temperature marking the onset of the instability can be estimated with logarithmic accuracy. The effects of electron-electron interactions are considered in the Hartree approximation.

Effects of a single scatterer on the transport properties of a leaky electron waveguide are investigated theoretically. The delta -function is used to simulate the scattering potential. The results show that the presence of even a single scatterer located in the waveguide will lead to obvious distortion of the shape of conductance steps, and will greatly influence the oscillations of the tunnelling current observed in clean waveguides. However, the effects of the scatterer being located outside the tunnelling barrier on either the conductance steps or the oscillations of the tunnelling current are negligible.

The superconducting oxide spinel system Li_1+xTi_2-xO₄ (0<or=x<or=1/3) has been studied in the temperature range 160-340 K by nuclear magnetic resonance (NMR) of the ⁷Li nucleus. Elucidation of the two different Li sites (tetrahedral and octahedral) for x>0 has been achieved via dilution of the ⁷Li nuclei with ⁶Li. NMR linewidths show a narrowing with increasing temperature for low x, indicating that the Li ions are becoming mobile around room temperature. Observed chemical shifts show an increase at the metal-insulator transition, demonstrating that the Li nuclei act as weakly coupled probes of the bulk paramagnetic susceptibility. Analysis of these shifts, together with nuclear spin-lattice relaxation times, has allowed the Knight shift due to the Li(2s) contact to be calculated. This reveals that there is approximately 7% Li 2s character in the unpaired-electron wave function at the Fermi surface in LiTi₂O₄.

C₆₀ samples as well as C₆₀ containing O₂, C₇₀ or both were studied. The dielectric loss and constant of the pressed pellets have been measured at 1 and 10 kHz as the temperature was cycled between 85 and 273 K. The low-temperature parts of the dielectric-loss spectra of doped C₆₀ contain Debye relaxational features that have been analysed by other authors. Our data suggest that the electric dipoles responsible relate not only to intrinsic orientational defects but also to dissolved O₂. At the higher temperature of 256 K, sharp peaks appeared in the spectra of both dielectric loss and constant, but only during the heating cycle and only if the sample had previously stayed at 85 K for 2 h. Our explanation for their total absence from the cooling curves invokes the masking effect of the random dipolar fields arising from orientational disorder, suppressed by annealing at below 90 K.

Using polarized neutrons we have studied the magnetization density arising from unpaired electrons in the compound UCu₂P₂. This phosphide crystallizes in the hexagonal CaAl₂Si₂-type structure (P3m1) and is a ferromagnet with the highest Curie temperature (T_c=216 K) among all known 5f-electron compounds. Analysis of the experimentally derived form-factor function f(sin theta / lambda ) indicates an admixture of the 6d states with the 5f states due to the overlapping and covalency effects. In addition, we have found that this form factor is affected by the crystal-field effect occurring either for the U⁴⁺ or U³⁺ ion. In agreement with the molecular approach developed by Watson and Freeman to correlate covalency with magnetization density, we found a real occupation of the 6d states with the spatial symmetry of the U site, which explains an important anisotropy of the magnetic form factor at low values of sin theta / lambda .

We report spin-wave results for the sublattice magnetization (to order O(1/S²)) and two spin stiffnesses and susceptibilities (to order O(1/S)) for the two-dimensional triangular Heisenberg antiferromagnet. These stiffnesses and susceptibilities are used as input parameters in scaling functions for various observables. The scaling results for uniform susceptibility are compared with recent numerical data.

The fully self-consistent discrete variational method within the local-spin-density framework has been employed to obtain the electronic structure and magnetic moments of the Mn-containing cubic Laves-phase pseudobinary compound of YFe₂. The embedded-cluster model was used to simulate the local environments around Fe in this compound. The calculated results show that the substitution of Mn atoms on Fe sites weakens the ferromagnetism of this compound. With the change of the local environments around Fe. We find that when Fe has one or two Mn atoms in its nearest-neighbour sites, its magnetic moment decreases rapidly, while the Mn moment is 071 mu _B, parallel to the Fe moment; when Fe has four or more than four Mn atoms in its nearest-neighbour sites, the moments of these Mn atoms are reversed and antiparallel to the Fe moments and meanwhile the Fe moments increase. In addition, calculations for the cluster with an Mn atom occupying a Y site indicates that such an Mn atom is in a high-spin state. Based upon our results, the various experimental data have been satisfactorily explained.

We report the results of our dielectric measurements carried out systematically on polyemeraldine base (PEB) in an Al-PEB-Al configuration, as functions of both frequency (5 Hz-13 MHz) and temperature (77-380 K). An attempt has also been made to understand the transport of charge in PEB in terms of the polaron-hopping model applicable to amorphous semiconductors. Analysis of the AC conductivity data indicates the operation of two relaxation mechanisms in this conducting polymer.

The z(yz)y Raman spectrum of a single-domain BaTiO₃ crystal in the tetragonal phase (z is the ferroelectric axis) is carefully measured and analysed to determine the quasimodes. In this scattering geometry, the phonon wave vector q is at 45 degrees to the ferroelectric axis, and only mixed modes of A₁+E character are observed. We show that a modification of the Merten equation is necessary to take account of the large damping of the lowest E(TO₁) soft mode. We describe an attractive application of these calculations to the accurate frequency determination of the heavily damped soft mode from knowledge of the well defined frequency of the other modes. This careful analysis lets us make unambiguous the presence of a central component in the Raman spectrum of BaTiO₃.

The spectroscopic properties of Ag⁺-doped strontium fluoride crystals were investigated at various temperatures, using absorption and fluorescence spectroscopies. The system exhibits a strong ultraviolet emission upon excitation into the two principal absorption bands. The azimuthal dependence of the degree of polarization of this luminescence is analysed, as well as its dynamics. The monovalent silver ions are shown to substitute for a host cation, with cubic symmetry. This is the first reported example of a cubic coordination for the Ag⁺ ions in an insulator. This cubic field, together with the strong ionic character of the framework, confers rather original spectroscopic properties to this system. The luminescence mechanisms are interpreted on the basis of the measured decay times and with the aid of energy diagram calculations. Two closed thermalized spin-orbit levels, with symmetry A_2g and T_2g respectively, are involved in the luminescence processes. The pure spin triplet A_2g only emits at low temperatures (T<15 K), whereas the T_2g level ( approximately 2% spin singlet character) emits in turn upon warming the crystal. One-dimensional configuration coordinate diagrams are proposed to interpret the peculiar temperature dependence of the emission band maximum.

The emission from KCl:Bi³⁺ excited in the X absorption band (i.e. between the A and B absorption bands) was measured as a function of temperature. The polarized emission spectrum and the angular dependence of polarization ratio were also investigated. The X-band excitation produces two emission bands peaking at 345 and 395 nm, the centres of which are not the same. The loosely bonded Cl^- ion arising from the substitution of Bi³⁺ ions into the KCl lattice is assumed to be the colour centre for the X-band emission. The definitive assignment of the two emission bands is presented in terms of the relaxed excited states of the perturbed Cl^- ion, in which the Jahn-Teller interaction coupling to the B₁ and B₂ vibronic modes is taken into account.

A novel method of determination of the longitudinal strains at a very local scale in the case of materials that develop a plastic neck under tensile drawing is presented. It relies on the analysis of the distortions of a linear target drawn on a thick thermoplastic polymer film by means of computer-assisted video-controlled extensometry. An equation is given for the target slope as a function of the local principal strains. True-stress and true-strain determinations can be performed along the neck as it forms and propagates. In situ measurements of the volume variations are afforded as a function of strain during the necking process.