Table of contents

Many glasses and liquids exhibit a first sharp diffraction peak that is evidence for intermediate-range order caused by regularities in the packing of structural units. These tend to cluster around particular values of the wavevector when scaled by the nearest-neighbour distance or the mean inter-atomic spacing. The first sharp diffraction peak is distinguished from the pre-peak in binary metallic glasses, which occurs at higher wavevectors when scaled in this way.

The accurate calculation of defect entropies is necessary to obtain concentrations of defects and the rates of defect-controlled processes. The authors compare two methods of calculating such entropies-the large crystallite and the supercell methods-and show that they give the same answer for the trial cases considered. Some problems encountered when considering charged systems are discussed.

The velocity of sound has been measured as a function of composition and temperature in liquid In-Te alloys. The compressibility deduced from the velocity of sound for Te-rich alloys has a maximum when plotted against temperature. This is interpreted as being due to the existence of In₂Te₃ which persists in the liquid and yet which undergoes a type of structural transition or dissociation with temperature.

By means of accurate measurements of birefringence, phase diagrams in the (E,T) plane have been constructed around the lock-in type of transition between the incommensurate and the commensurate phase of K₂SeO₄ and Rb₂ZnCl₄. While the features characterising the first-order nature of the transition are insensitive to E up to 40 kV cm^-1, T_c follows the law T_c(E)=T_c(0)-A mod E mod ln(B mod E mod ). Under an applied field, the permittivity anomaly at T_c gradually disappears. This can be understood by assuming that the soliton lattice response becomes clamped. The behaviour of permittivity along complex trajectories in the (E,T) plane has been measured and is discussed on the basis of a generalised phase diagram.

Weak features between 16000 and 19000 cm^-1 in the absorption spectra of Cs₂ReCl₆ and K₂ReCl₆ are assigned as cooperative absorptions involving coupled excitations of the Gamma ₈(⁴A_2g) to Gamma ₆(²T_1g), Gamma ₈(²T_1g), Gamma ₈(²E_g) transitions on neighbouring complex ions. The intensity mechanism of these transitions is discussed. Below the Neel temperature of K₂ReCl₆, the single-ion transitions are accompanied by magnon sidebands with a magnon wavenumber of about 45 cm^-1.

In the presence of a DC bias the influence of electric field inhomogeneity in the space charge region of doped GaAs on electroreflectance spectra has been studied by model calculations which are compared with experimental results. Field inhomogeneity effects are shown to modify the spectra near the gap in a characteristic manner. The predictions of the theory concerning lineshape 'rotation' with a DC bias, estimation of the gap energy and doping dependence f the spectra are confirmed by measurements on a set of doped GaAs samples. It is shown that the field inhomogeneity and not the effect of excitons is mainly responsible for the observed lineshape 'rotation' at room temperature. However, owing to the same bias dependence of both mechanisms there is no simple way of distinguishing between them.

It is demonstrated that the usual perturbation approach is not applicable when considering systems with strong inter-electron correlations (with Hubbard sub-bands). In particular, the expansion in the electron occupation numbers in the low-density limit yields contributions to the ground-state energy which diverge at U to infinity , with U being the Hubbard intra-site repulsion. The dynamic magnetic susceptibility in the Hubbard model with U to infinity is calculated self-consistently using many-electron Hubbard operators. The saturation magnetisation, spin-wave spectrum, Curie temperature and the paramagnetic susceptibility containing the Curie-Weiss contribution are obtained. The results are used to interpret the properties of the system Fe_1-xCo_xS₂. The method developed is generalised in order to consider the narrow-band s-d exchange model.

Measurements of the static paramagnetic susceptibility chi _p and magnetisation M have been performed on p-type manganese-doped GaAs. Pronounced deviations from a pure Curie-type ( chi _p) or Brillouin-type (M) behaviour were observed. These can be assigned to a low-lying energy level at about 2.5 meV resulting from exchange coupling of the delocalised hole with the 3d⁵ manganese acceptor core.

The electron paramagnetic resonance (EPR) parameters of BaZnF₄:Mn have been studied theoretically. The substitution of Mn²⁺ ions for the Zn²⁺ ions has been determined by using a method different from that of other workers. The contribution of next-nearest neighbours to the EPR parameters has been discussed and the results obtained are different from those of previous authors. In addition, various mechanisms which contributed to EPR parameters have been considered for comparison.

A Monte Carlo computing procedure different from the standard least-squares fitting method is presented for evaluating the set of parameters which best fit the effective spin Hamiltonian. The parameter g and the electric-field parameters b_l^m are fitted using the experimental data obtained from the electron paramagnetic resonance (EPR) spectra. This method is very powerful as regards decision making so that the labelling of the quantum levels involved in a transition is optional in most cases. The different steps in the simulated 'annealing' program are described in detail and a sample calculation is given for Gd³⁺ in a CaF₂ matrix when it is located in tetragonal or cubic sites. This centre was chosen as the results obtained can be compared with previous determinations reported in the literature by using more conventional fitting procedures. This method is applicable even when the off-diagonal elements of the effective spin Hamiltonian matrix are large. The use of Monte Carlo optimisation methods, even if time consuming, is a modern approach that is easily available as it can be run on a microcomputer; this offers clear advantages for fitting the EPR data.

Potassium titanyl orthophosphate KTiOPO₄ has been studied by Raman and infrared reflectivity spectroscopy in the range from a low temperature to room temperature and for various scattering configurations. The absence of a structural phase transition below room temperature is confirmed. The phonon modes are classified within their symmetry species, and their parameters such as damping, oscillator strength and integrated intensity are obtained. On the basis of an internal and external mode analysis and by comparison with related compounds such as RbTiOPO₄ and KH₂PO₄, the origin of the various vibrational groups is discussed. The main part of the scattered intensity is shown to be due to vibrations of the Ti-O bonds.

The influence of surface ripplings and periodical fluctuations of the refractive index in the layer closest to the surface on the phenomenon of light diffraction by surface acoustic waves (SAW) is investigated. The analysis includes a study of perpendicular interactions in deuterated ammonium dihydrogen phosphate crystals. The cases of external reflection, internal reflection, and transmission of light from/by the surface on which the SAW is propagating are considered. Numerical calculations of the diffraction efficiency for the two basic states of light polarisation (parallel and perpendicular) make it possible to interpret the experimentally observed effects.