Table of contents

Abnormally large variations in the values of epsilon _a, epsilon _b, and epsilon _c are observed in (NH₄)₂SO₄ at 151.5 degrees C with a thermal hysteresis of 9 degrees C, confirming the phase transition suggested earlier by the authors from conductivity and TSC studies. The transition temperature is 162 degrees C for (ND₄)₂SO₄, implying that ammonium ions are involved in the process.

Recently published experimental measurements of Sn and Te self-diffusion in SnTe are discussed in terms of the defect chemistry of SnTe. It is concluded that Te self-diffusion cannot be attributed to a single point defect mechanism and that Sn self-diffusion involves at least two separate charge states of the Sn vacancy.

Using existing data, the authors emphasise that in the range T_v>T> approximately T_v/2 the electrical conductivity sigma of Fe₃O₄, and some other mixed-valence oxides, is described better by sigma varies as exp(T) than by an Arrhenius law. A possible explanation of this dependence is offered, based on incoherent tunnelling of electrons between neighbouring sites.

The authors present a new field theoretic formulation of the quantum mechanics of disordered systems. The problem is converted to an explicit field theory by changing variables in the functional integral over all random potentials to an integral over all possible wave-functions. Unlike previous formulations this field theory has the novel feature of having the 'right' sign for the coefficient of the phi ⁴ term. Thus the important excitations are kinks rather than instantons. In addition, it exhibits singular solutions with finite action. The method is illustrated with an exact calculation of the asymptotic density of states in a one-dimensional gaussian white noise potential.

An effective medium theory is used to calculate the conductivity activation energy and the Seebeck coefficient for nearest-neighbour hopping in a band of Anderson localised states at the Fermi level. The results of these calculations are compared with experimental observations on cerium sulphide, fluorine-substituted magnetite and 'pure' magnetite Fe₃O₄.

A semiclassical theory of non-radiative transitions is proposed here which is an extension of the Landau-Zener theory to low and intermediate parameters. It is shown, by considering the vibrational overlap factor, that in this regime the transition occurs by tunnelling through a potential barrier as suggested by Holstein (1978) for the small polaron motion. The formulae which are derived justify the method suggested earlier for the determination of potential curves from the non-radiative transition rate. It is shown that, to leading order in p, the present method gives correct results for the Huang-Rhys model (1950) of potential curves.

All subgroups of the magnetic axial point groups are derived, and the invariant ones are indicated. These subgroups can be used in the determination of the symmetry of the low-temperature phase in phase transitions of crystals that are highly anisotropic in one direction (the ferroelectric transition of the chain-type crystal SbSI at 295K is considered as an example) and of quasi one-dimensional systems (where the order of the principal rotational axis may be any integer).

The deformation dipole model, the shell model, and also extended versions of these models have been investigated for the lattice dynamics of LiH and LiD. A deformation dipole model with 13 adjustable parameters gave the best fit to the phonon dispersion of LiD known from neutron and Raman experiments. The model has been used to compute elastic and dielectric constants, Szigeti effective charges, phonon densities of states, Debye temperatures and second-order Raman spectra of LiD and LiH. Good agreement with the experimental data was obtained. The contributions of short-range three- and four-body forces to the model force constants are discussed. First calculations of the phonon dispersion curves of the hydrides and deuterides of Na, K, Rb and Cs, which are based on shell models, are presented.

When a condensate within a host lattice is coupled to lattice strain, the phase-solid phase change is Delta ( phi _c- phi _l)=2 pi /p, where phi _c is the phase of the condensate, phi ₁ is the phase of the host lattice relative to the unstrained lattice, and p is the order of the commensurability. The results differ from those of Bak and Timonen (1978) because the commensurability energy is of the form cos p( phi _c- phi ₁), explicitly allowing for strain-modified positions of the lattice; the coupled Euler-Lagrange equations for phi _c and phi _l are solved exactly for the sinusoidal potential; and the exactly determined soliton phase change Delta phi _c is not equal to 2 pi /p. The stability of the constant-wavevector incommensurate state is examined. In an approximate solution that becomes exact when the lattice rigidity becomes negligible there occurs a soliton distortion, primarily of the lattice, separating regions of constant-wavevector incommensurability rather than commensurability.

The thermal expansion of VSe₂ from 40-1175K has been determined by X-ray diffraction. The lattice expansion is linear with temperature from 40 to about 300K with alpha _a=18.6*10^-6K^-1 and alpha _c=9.8*10^-6K^-1. The behaviour alpha _a> alpha _c is anomalous for a layer compound. From the relation between the thermal expansion and the elastic constants it follows that either the elastic constants of VSe₂ are anomalous for a layer compound or the elastic constants are normal but there is a large anisotropy in the Gruneisen functions gamma _a and gamma _c ( gamma _a> gamma _c), together with a relatively large value of the cross compliance constants s₁₃. The latter possibility is probably true. The physical origin lies in the electronic structure; the exceptionally high ratio c/a (1.823 at 40K, 1.818 at 300K) due to a band Jahn-Teller type distortion, decreases at increasing temperature. The expansion above about 300K is no longer linear; with increasing temperature the expansion of the a axis increases while that of the c axis becomes almost zero. The non-linear behaviour is ascribed to the formation of Frenkel defects; intrasandwich vanadium is transferred to the interstitial sites in the Van der Waals gap.

Thermostimulated luminescence and conductivity of X-ray-irradiated doped CsI crystals are measured from 11K up to room temperature. It is shown that, at the first peak of thermoluminescence around 60K, no peak of thermostimulated conductivity corresponds. It is concluded that the V_k-electron recombination corresponding to this first peak of thermoluminescence is not due to a linear V_k migration. This conclusion is confirmed by other experimental results such as the isothermal decay of thermostimulated luminescence. An interpretation for this recombination is given, which is based on different lattice relaxations for the different states of the electron-hole pair.

An efficient method for obtaining the off-diagonal Green function matrix elements for a band secular matrix is presented. This enables one to study localisation phenomena in systems like polymers or random mass chains. Calculations are presented for some model systems and the results are related to some eigenstates obtained by inverse interaction.

The authors present detailed calculations of the bandstructure, density of states and total bandstructure energy of a 2H layer compound in the 3 × 3 charge density wave (CDW) state. The calculations are performed for a variety of amplitudes and phases revealing the size and form of the energy lowering in the CDW and also the effect of the phase. It is found that a phase of -¹/₄ pi +²/₃n pi gives the largest gain and that cluster formation is not energetically favoured. The energy gain can be fitted to a BCS form of Ax²lnn(x/c) but the parameters required are unrealistic, indicating that the energy gain is not of a simple BCS form.

The authors present a somewhat unorthodox approach to the periodic Anderson model in which both the on-site correlation and the single-particle hybridisation terms are included in the perturbation. They show that it is possible to utilise the known exact solutions for the cases where either the on-site correlation or hybridisation is zero and make use of the definition of irreducibility for mixed species graphs, to show how a sum over all graphs can be attempted for both localised and conduction electron propagators. The results are internally consistent and in particular they find that the energy of the system can be determined either by thermal averaging the Hamiltonian or from a density-of-states picture. For a particular choice of parameters it is statistically favourable to make a comparison with the exact results for a three-site, six-electron model and for this system the number of localised holes and the specific heat have been calculated numerically.

The optical polaron bounded in a Coulomb potential is investigated within the modified variational scheme of the Lee-Low-Pines theory (1953) which is valid for the whole range of the electron-phonon coupling. The ground state energy, the mean number of phonons in the cloud around the electron and the spatial localisation of the electron for the optical polaron change abruptly at certain values of the electron-phonon coupling constant alpha and of the Coulomb potential beta . The polaron changes its state according to the values of the parameters alpha and beta . Such features are summarised in the phase diagram.

Describes a relativistic generalisation of the spin density functional formalism. The full Feynman propagator is presented and discussed. Effective single-particle Dirac equations for the polarised gas are given. Effective one-particle potentials for the magnetised gas are calculated in the local density scheme. These results are displayed graphically and their significance discussed.

The diffusion equations of electric carriers and heat are first examined with the help of thermodynamics of irreversible processes. Analytical formulae are deduced and give electric carriers and heat fluxes as a function of the lattice temperature, T_L, and the excited carrier concentration, n, in the case of Si. At constant T_L, it is shown that heat flux occurs, due to the strong electron-phonon coupling. It arises also that the thermal conductivity, at constant n, varies like (1- beta n)², where beta is constant. It results from this work that thermal energy confinement occurs simultaneously with the electron-hole plasma confinement.

The authors consider the stark effect in a crystal field for crystals with rare-earth ions R of the type La_1-xR_xZ, Lu_1-xR_xZ or RZ. They compute the shift in the crystal-field levels with temperature due to phonon interactions. Their computations differ from earlier ones since they consequently use the crystal symmetry and express their results in terms of observable thermodynamic quantities for crystals of the LaZ, LuZ type.

For pt.I see ibid., vol.14, p.4311 (1981). The authors compute the corrections to the thermal properties of crystals which contain rare-earth ions. These corrections arise fom magnetoelastic interaction. It is shown that different thermal properties contain the same free parameters and hence fitting must be done simultaneously.

Infrared absorption spectra of various samples of hydrogen beta alumina are discussed with respect to the alternative concerning the existence of hydroxyl (OH^-) and oxonium (H₃O⁺) ions in this compound. Some results of inelastic and quasi-elastic neutron scattering are also given. It can be shown that the presence of different entities depends on the method of preparation. In the case of silver beta alumina more or less reduced by hydrogen Al-O(5)-H⁺, Al-O(i)-H⁺, H₃O⁺ and H₂O⁺, H₂O species have been identified while hydrogen beta alumina prepared by treatment of the sodium derivative with sulphuric acid or by conversion of ammonium beta alumina does not contain any Al-O-H⁺ groups attached to aluminium but contains oxonium ions or higher species. The existence of OH^- ions in hydrogen beta alumina is very unlikely.

Photoluminescence spectra of undoped and Cu-doped crystals of the layered semiconductor GaSe have been measured in the temperature range from 80K to room temperature. The main modification of the spectra in doped crystals with respect to those of undoped samples is the appearance of two new bands and the enhancement of light emission. A detailed analysis of spectra, by varying excitation intensity and temperature, enabled the authors to analyse the defect levels involved in the transitions. A simple kinetic model for indirect free and bound excitonic transitions in doped samples is proposed.

The authors present a study of the influence of correlations on the two-particle spectra of transition metals using a generalization of Cini's method (1977). These two-particle spectra are shown to be strongly sensitive to the presence of empty electron states in the d band. Indeed, as soon as the d hole concentration differs from zero, the XVV Auger spectra are much broader than the quasi-atomic shape observed on Cu. The U values which account for this effect are found to be in agreement with those deduced from experimental photoemission data.

Based on the current space-charge theory for ionic crystals and the existence in real experiments of an inevitable gap between crystal and electrode surfaces a 'surface polarisation' model is developed. An analysis of thermally-stimulated current and dielectric relaxation data of CdF₂ by 'fitting' shows a good agreement between experiment and theory and results of DC conductivity measurements.