Table of contents

An attempt is made to explain the first-order phase transition (180.4 degrees C) and the presence of an intermediate phase (180.4 degrees C-300 degrees C) between the para- and ferro-phases in lead phosphate, Pb₃(PO₄)₂, by invoking the basic ideas of the three-dimensional renormalised Potts model. This involves the introduction of three order parameters with two different critical temperatures. A relaxational type flip mode is proposed as the driving soft mode. The abnormal behaviour of the spontaneous strain between 160 degrees C and 180 degrees C, as well as the anomalous behaviour of the elastic stiffness constants C₁₁ and C₂₂ during the phase transition, are explained.

A Green function diagrammatic method is developed for a system described by two hybridised electronic with Coulomb correlation energy at each site. It turns out to be a generalisation of a one-band Hubbard I approximation for this two-band system.

The effect of the matrix elements on the imaginary part of the Ehrenreich-Cohen permittivity component epsilon _{2 perpendicular to} (0, omega ) of ZrS₂ is calculated with the self-consistent Hartree-Fock-Slater method. It is found that it is necessary to calculate the matrix elements rigorously in the bulk of the first Brillouin zone to obtain a close agreement between the calculated and experimental optical spectra. From the comparison between these spectra it is found that the corrections needed to epsilon _{2 perpendicular to} of ZrS₂ should be opposite to the calculated local field and continuum excitonic corrections obtained recently for Si and TlCl. It is also found that no function of the form of omega ^-n epsilon _{2 perpendicular to} gives a satisfactory fit to the joint density of states in the whole energy range of 0-20 eV where the transition from the valence states are important.

The self-consistent theory of localisation of Kumar et al. (1975) is applied to the case of binary alloys. Two approximations are presented in the limit of small concentration (c=0.1). The second one appears in good agreement with the results predicted by other theories.

The temperature dependence of the resistivity of the intercalation complex VSe₂(N₂H₄)_0.6 has been measured between 4.2K and 450K. The resistive anomaly associated with charge density wave formation observed at 110K in pure VSe₂ is found at a higher temperature, of 165K, in the intercalated system. The authors discuss this behaviour in terms of the increased vanadium d-band occupation which results from charge transfer from the hydrazine.

A Hubbard model with randomised site energies is used to investigate electron localisation effect associated with both disorder and electron-electron interactions. The ground-state properties of this system are illustrated in a phase diagram. It is shown that electron localisation dominated by interactions is physically distinct from localisation dominated by disorder. Electrons localised through interactions have antiferromagnetic order (at T=0) and electrons localised primarily by disorder have a spin glass magnetic disorder. In this model, true non-magnetic Anderson localisation is a limiting case which applies only for non-interacting electrons.

Using the effective Hamiltonian method the authors investigate a model in which two kinds of magnetic atoms, Ising- and Heisenberg-like, interact with ferromagnetic forces. They determine and discuss the dependence of the critical temperature as a function of concentration of one kind of atoms, the ratio of the three coupling constants and the parameter characterising the interaction between Ising-like and Heisenberg-like spins.

The diffusional dynamics of particular points of a polymer chain is calculated in the Rouse approximation. The points considered correspond, in the case of incoherent neutron scattering, to the proton-labelled free ends of chains, cross-links between chains in rubbers or gels, or the central monomers in branched or star polymers. Results derived are also relevant to NMR, ESR and computer simulation experiments.

High-resolution neutron scattering experiments on deuterium-labelled model trifunctional networks show that the junction points move more slowly than the free chain centres by a factor of about two.

The effect of mass disorder on acoustical phonons temperature dependence has been investigated in anharmonic ZnS_1-zSe_z crystals.

At room temperature, dicalcium strontium propionate (DSP) is tetragonal with space group P4₁2₁2(D₄⁴) or P4₃2₁2(D₄⁸), with a=12.484 AA, c=17.231 AA, Z=4. Two of the propionate groups lie along a two-fold axis with their alpha and beta carbons statistically disordered in two sites on either side. The third propionate, which is in a general position, has 2-site disordered beta carbons, whereas the fourth propionate appear to be ordered. No sign of complete rotation of any groups is observed. The structure is compared with that of the Ba compound and the relationship between the two chiral forms is discussed. It is shown that the most important interactions are those of the Sr-O-Ca framework. DSP crystals with space group P4₃2₁2 are found to be dextrorotatory. A structural explanation is given for the observed ferroelastic and ferrogyrotropic orientation states.

The order-disorder transition in lithium ferrospinel (LFS) is studied in terms of the Landau theory of phase transitions. By analysing the ordering and the structural changes, two order parameters and corresponding Landau free energy are constructed. It is shown that: (i) the ordering or the Li ions and the structural changes occur simultaneously; (ii) the structural change is associated with softening of a phonon mode with k=(1,1,0) 2 pi /a; (iii) the transition is of the first order due to the occurrence of the cubic terms in the Landau potential. Assuming a weak coupling between the two order parameters the transition temperature, the hysteresis range and the values of the order parameters are estimated.

Calculates the energy of formation of alkali metal ion interstitials in zinc selenide by extending the use of defect modelling programs based on shell models. The results support suggestions that Li⁺ and Na⁺ are important defects in ZnSe and that it is the presence of these defects that cause the problems encountered in trying to make p-type zinc selenide. The calculations also allow an estimate of the defect migration energies, which suggest that these interstitials should be fairly mobile.

The approximate pseudopotential given by Bartram, Stoneham and Gah (1968) has been used in calculating the Z₁ centre absorption energy and absorption energy splitting. The results show that the ten-parameter trial wavefunctions give absorption energies that are in good agreement with experimental data. The theoretical results also enable the authors to explain the small absorption energy splitting.

Presents the low-temperature deep-level photoconductivity spectra of a wide range of n-type and semi-insulating Fe-doped InP crystals. The results differ in important respects from some recently reported photoconductivity work and conclusively demonstrate that Fe²⁺ rather than Fe⁺ is the stable charge state in n-type material. The electron ionisation threshold energy from the Fe²⁺ state to the lowest conduction band at 0.65 eV is found in all the samples studied. Only the semi-insulating show a strong photoconductivity threshold at 1.15 eV. This feature is absent in n-type InP(Fe). The authors tentatively assign this structure to an Fe-related level (Fe³⁺) 1.15 eV below the conduction band-edge.

Photocapacitance, DLTS, optical absorption and photoluminescence data are presented for iron- and iron-germanium-doped Czochralski single-crystal InP. The results demonstrate that the Fe²⁺ to Fe⁺ transition is not present within the InP band gap. A feature observed in photocapacitance and optical DLTS is attributed to the presence of the Fe⁴⁺ charge state, and features associated with the well known Fe³⁺ and Fe²⁺ states also occur. Photoluminescence measurements show an iron-related band near 0.5 eV, not previously reported in the literature.

Time-resolved and laser selective excitation experiments have been performed on CaF₂:Pr³⁺ crystals at seven concentrations ranging between 0.05 and 2 mole Pr³⁺%. The time dependence of the energy transfer between two Pr³⁺ ions sitting at different sites is independent of concentration. Using polarised laser selective excitation at various temperatures between 1.4 and 4.2K, it is shown that these ions exist as dimers in the crystals even at very low concentration. The existence of higher-order clusters of Pr³⁺ ions is demonstrated through high-resolution analysis of the absorption spectra and correlated to the variations of the emission lifetimes of the various sites as a function of concentration. The interactions of the single ions and dimers with the clusters are discussed. These experiments show that the classical hypothesis of a random distribution of the dopant in ionic crystals must be handled very carefully.

A first-principles self-consistent calculation of the energy band structure of (SN)_x has been made by using the numerical-basis-set LCAO method. The resulting Fermi surface consists of two electron and two hole pockets which have the shapes of flattened tubes or flattened ellipsoids. The calculated density of states agrees well with both the X-ray photoemission spectroscopy (XPS) data and the Fermi level density of states obtained from the specific heat measurement. On the basis of the band-structure results, the calculation of the imaginary part of the dielectric function of (SN)_x is carried out. The characteristic features of the observed optical absorption spectra of (SN)_x are satisfactorily explained by the theoretical results. Moreover, the Fermi surface of (SNBr_y)_x is calculated in terms of a rigid band model based on the present self-consistent band structure of (SN)_x. The resulting Fermi surface of (SNBr_y)_x, together with that of (SN)_x, clarifies the origin of the observed characteristic change of the physical properties of (SN)_x upon bromination.

Electrical resistivity of an interacting two-carrier system has been investigated by using the coupled Boltzmann equation. An exact formula for the resistivity due to carrier-carrier scattering is derived under a plausible angular dependence of the distribution function. This formula indicates that the interband carrier-carrier normal scattering acts so as to attenuate the relative motion of the two types of carriers, while the intraband carrier-carrier normal scattering acts only so as to renormalise the masses of two types of carriers. It is also shown that the centre-of-mass motion of the two types of carriers cannot be attenuated in the absence of carrier-carrier Umklapp scattering and other scattering mechanisms such as carrier-phonon scattering. Moreover, in order to consider both carrier-carrier and carrier-phonon scattering mechanisms simultaneously, an optimum formula for the resistivity is also given and applied to the case of (SN)_x and (SNBr_y)_x whose Fermi surface consist of highly anisotropic electron and hole pockets. The observed behaviour in the magnitude and the temperature dependence of the resistivity along the chain axis of these materials is satisfactorily explained by the calculated results.

A new theory of the Hall mobility in the hopping regime is presented. The theory is based on a microscopic description of diffusive transport. A formula is derived for the frequency-dependent Hall mobility when purely spatial disorder is present. The theory is valid for all densities and frequencies. The results of other authors who used percolation theory are easily recovered and interpreted. In the percolation regime the DC Hall mobility decreases exponentially with increasing critical hopping distance. The comparison of the theory with available computer simulation points is very encouraging.

The scattering of electrons in high-mobility liquids is discussed. The drift velocities and characteristic energies of hot electrons in liquid argon, krypton and xenon were calculated. The experimental drift velocity data available are shown to be describable in the framework of the Cohen-Lekner theory. In particular, the saturation of the drift velocity turns out to be due to reduction of the influence of the structure of the liquid on the scattering of few eV electrons. The possibility of getting information on energy and momentum transfer cross sections from the drift experiment data is discussed.

Energy-loss mechanisms of hot electrons are studied in n-InSb at low temperatures (T_L<50K) by magnetophonon and resistivity ( rho ) measurements. Two kinds of magnetophonon measurements of the '2TA' and 'impurity' series, at T_L=10K for various electric fields (E) and at a low E for T_L=10-20K, provide the 'electron temperature' T_e as a function of E. The results are compared with those determined from Miyazawa's method (1969) utilising rho (E) and rho (T_L) data, and a noticeable difference is found at higher E. Resonant cooling of hot electrons due to the pair emission of the TA phonon is found. The energy-transfer rate deduced from rho (E,T_L) data shows a significant deviation from the theoretical calculations assuming acoustic phonon scattering via piezoelectric and deformation potential couplings, and polar LO phonon scattering.

Polarised neutron diffraction data have been collected from single crystal samples of NiF₂ at 4.2K and in an external field of 1.4T. Subsidiary unpolarised neutron experiments carried out at a number of wavelengths have determined the extinction parameter and these have been used to correct the observed, polarised-beam flipping ratios. The geometry of the polarised neutron experiment was such that one group of reflections gives information solely on the distribution of the weak ferromagnetism, which corresponds to some 0.03 mu B/Ni²⁺ ion. The other reflections contain magnetic scattering from the antiferromagnetic component of the moment. Measurement of the flipping ratio of the (111) reflection has enabled the authors to verify the absolute configuration of the canted magnetic moments with respect to their octahedron of fluorine neighbors.

Spin diffusion and relaxation is considered in the framework of the Master equation. The formalism is applied to the nuclear spin-lattice relaxation in a-Si:H. In the framework of the model proposed by Carlos and Taylor (1980), the experimental data require that at least 10% of the hydrogen participates in local mode relaxation centres.

Ferroelectric domain walls have recently been considered as candidates for the occurrence of non-linear (soliton) excitations and roughening transitions. By means of X-ray or neutron scattering experiments, the domain walls may be observed directly and their width may be measured. The occurrence of a roughening transition would be observable by these techniques, and the roughening mechanism is suggested as an explanation of existing experimental results for Pb₅Ge₃O₁₁. A neutron scattering study of the domain walls in deuterated KH₂PO₄ is described. The domain walls in this system are shown to have a width of around two unit cells. This result implies that the continuum approximation used to describe nonlinear excitations in systems with domain walls is not applicable to KH₂PO₄, and quite probably to many other systems.

The A_g internal mode of the sulphate ion in potash alum is studied by Raman scattering as a function of temperature. This mode shows a splitting on account of the fact that a small fraction of S-O bonds point towards K⁺ while a larger fraction point towards Al³⁺. The area under each peak is a measure of the number of sulphate units having the corresponding orientation. Above 200K, the observed area ratio follows the expected exponential dependence on the temperature, and may be understood on the basis of phonon-assisted reorientations of the sulphate ion between the two configurations. At lower temperatures, there is a marked deviation from the exponential form. This is explained by invoking a temperature-independent contribution to the orientation rate. A detailed lineshape analysis is made and compared with experiment. The possible origin of the temperature-independent relaxation mechanism is also discussed.