Table of contents

Due to boundary conditions imposed by the finite thickness of crystals used in electron diffraction experiments the change in wavevector of the fast electron need not precisely equal the wavevector of the crystal excitation involved in the scattering process. It is shown that this does not, however, lead to resonance effects. The degree of partial coherence between different processes is calculated for a perfect crystal.

A comparison is made of the radial distribution functions for NaI calculated by molecular dynamics simulation from consistent rigid ion and shell model interionic potentials. Substantial differences are observed.

Optical rotatory power, birefringence and thermal expansion were measured near the structural phase transition point of CsCuCl₃. All these quantities show discontinuous changes at the transition point. Enantiomorphic domains were observed in the low-temperature phase and irreversible temperature dependence of the optical rotation angle is ascribed to this domain formation. Helical atomic displacements are explained in terms of the condensation of doubly degenerate E₁ phonon modes at +or-q0=(0 0+or-2 pi /3c) which is induced by the cooperative Jahn-Teller effect.

Room-temperature electrical resistivity and Hall effect measurements as a function of pressure are reported on p-type MoS₂ and on n-type MoS₂, MoSe₂ and MoTe₂. In each case, the resistivity decreases under pressure, due to an increase in the carrier concentration. The Hall mobility is relatively pressure-independent. The data are consistent with the predominance of extrinsic conduction in these semiconductors until well above room temperature. The impurity activation energy and its pressure dependence are given, together with estimates of the intrinsic indirect bandgap obtained from high temperature conductivity measurements, photoemission studies and band structure calculations.

Correlations near the critical point of a face-centred cubic lattice are studied by series expansion methods. The amplitude of the second moment correlation length xi ₁ is calculated for several sets of critical exponents.

The author investigates how the Landau theory may break down at a first-order transition (FOT). This may happen because the transition temperature T_c is located near two critical points: the metastable critical point (MCP), which is the stability limit of the ordered phase, and the fluid-like critical point (FLCP), at which the conjugate H of the order parameter is not null. Using the Ginzburg criterion, it is found that one inequality expresses the condition that T_c be outside the two critical regions. The results are applied to two types of FOT: one located near a tricritical point and the other located near an isolated critical point. When T_c is inside a critical region, it is found that the critical indices of the MCP are beta ₁= beta (critical index of the FLCP) and alpha ₁= gamma ₁=1- beta . The critical region (0.2 degrees C) of BaTiO₃ near the FLCP is determined and the experimental results of SbSI and KMnF are discussed.

For pt.I see abstr. A60719 of 1974. Following the generalization of the CPA introduced by Bishop and Mookerjee, the CCPA averaged density of states for a 50-50 disordered binary alloy on a three-dimensional diamond lattice is presented. The cluster considered consists of a central vertex with its four nearest neighbours. This is embedded in a CP medium.

A new parametrization is proposed which enables the intensities of transitions between crystal field split levels to be related to parameters representing the components of a vector field. Unlike the conventional parameters which are related to the odd parity components of the crystal field coupling the 4f to the nd and ng excited states, the new parameters are defined in a way which is explicitly independent of the physical processes involved. Use of these parameters enables the authors to explain a paradox in the interpretation of the odd parity crystal field.

The interaction of itinerant conduction band electrons with the localized spins of an antiferromagnetic lattice is investigated in the spin-wave approximation. The interaction results in: (a) removal of the spin and site degeneracy of the atomic levels; and (b) the appearance of an effective resonance energy of two neighbouring sites. The thermal average of the latter suggests a non-exponential dependence on the temperature and coupling strength, in contrast to the earlier studies of Wolfram and Callaway (1962).

The dynamics of critical phenomena in the molecular field approximation are discussed and a calculation of the nuclear spin-lattice relaxation time (T₁) for ³⁵Cl nuclei in NH₄Cl at temperatures close to the lambda transition point is described. The calculation is based on a linearized Ising model treatment and the approach should be fairly widely applicable. The T₁ expression may be expected to fail very close to the critical point. Measurements have been made of the ³⁵ClT₁ in NH₄Cl and ND₄Cl at temperatures both below and above the lambda point. The results for NH₄Cl are compared with those obtained by previous workers. The data are analysed using the theoretical expression mentioned above and it is shown that the temperature variation of the order parameter extracted from the results leads to an anomalously low exponent value beta =0.15.

This paper reports the accurate measurement of dielectric constant, dielectric loss and DC conductivity on two crystal samples, cut from the same boule, over a wide range of temperatures (from liquid-nitrogen temperature to +400 degrees C). Dielectric anomalies were observed at three transitions. The application of Devonshire's phenomenological theory of ferroelectrics reveals that the ferroelectric-paraelectric transitions, observed at -1 degrees C and -4 degrees C for the two crystal samples, are very close to second order in character. The peak in conductivity corresponds to a peak in the dielectric constant in the region of the Curie temperature. The spontaneous polarization was obtained from the measured values of pyroelectric current below +30 degrees C. Variation of residual polarization with applied field was studied at a fixed temperature from hysteresis loop data. Results for the frequency dependence of dielectric constant and loss, and for the field dependence of conductivity, are also presented. An attempt has been made to explain the observed physical properties on the basis of donor concentrations found to be present, though in small quantities, in the candidate samples.

The authors present the results obtained by wavelength and thermally modulating the reflectivity of the layer compound GaTe up to about 5 eV. The E₁ and E₃ structures are explained in terms of a two-dimensional M₀+M₁ metamorphism and an M₂ two-dimensional critical point respectively. Furthermore a certain amount of information on the excitonic nature of some observed structure is presented and a clear relationship is found with the optical spectra of GaS and GaSe.