The phonon dispersion relations of ThO2 and CeO2 have been measured in the three principal crystallographic directions at 293 K using inelastic neutron scattering. After correction for instrumental resolution effects, the data were fitted by rigid ion and shell models of the interionic forces, and the corresponding parameters determined. The results indicate a general similarity between the lattice dynamics of the two compounds.

Frenkel defect formation in the oxygen sublattice and the excitation of electronic defects in the form of small polarons are considered. A brief summary is given of the results of a recent investigation into possible oxygen lattice disorder in UO//2 and ThO//2 at temperatures up to 2930 K, using neutron scattering techniques.

Some materials with the fluorite structures show a pronounced specific heat anomaly well below their melting temperature. This anomaly is a consequence of lattice disorder and is associated with the onset of fast-ion conduction. This paper presents the results of a series of experiments in which the coherent diffuse quasielastic neutron scattering from single crystals of three such fluorite compounds PbF2, SrCl2 and CaF2, was investigated. The diffuse scattering intensity, and its energy width, increases with temperature into the fast-ion phase, and when integrated over energy transfer the intensity exhibits a characteristic variation with scattering vector, falling on an anisotropic shell in reciprocal space and peaking in certain directions. The diffuse intensity indicates that dynamic correlations exist between the defective anions in the fast-ion-phase. A model of short-lived clusters comprising anion Frenkel interstitials, anion vacancies and relaxed anions has been developed which satisfactorily accounts for the distribution of intensity.

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High resolution triple-axis neutron spectroscopy has been used to make a detailed investigation of the incoherent scattering from mobile anions in the fast-ion phase of strontium chloride. The quasielastic energy broadening has been measured as a function of the scattering vector at temperatures of 1001, 1053 and 1087 K. The data have been analysed in terms of the jump diffusion model of Chudley and Elliott, and the encounter model of Wolf. A vacancy diffusion mechanism is found to predominate in the fast-ion phase, with anion hops mainly to nearest neighbour and next-nearest neighbour regular anion sites.

Energy broadening of the incoherent quasielastic neutron scattering from a single crystal of the fast-ion conductor strontium chloride at 1053K has been measured for scattering vectors along the three principal crystal symmetry directions. The data are well accounted for by either the random-jump diffusion model of Chudley and Elliott (1961) or by Wolf's encounter model. A vacancy diffusion mechanism is found to predominate in the fast-ion phase, with anion hops mainly between nearest and next-nearest neighbour regular anion sites and a mean residence time of 25-29*10-12s.

Preliminary diffraction measurement, diffraction and scattering in the nature of phonon modes up to 2673 K. The results do not indicate the formation of Frenkel defects before 2400 K, but the formation of defects above this temperature n is not excluded

Quasielastic diffuse coherent neutron scattering arising from the dynamically disordered anions in CaF2 and PbF2 has been investigated as the temperature is increased into the fast ion phase. The characteristic variation with scattering vector Q of the integrated intensity, S(Q), can be accounted for by a model in which the most probable instantaneous configuration of the defective anions is a cluster centered at the mid-point of nearest-neighbour regular anion sites. The observed S(Q,ω) is Lorentzian in ω and the width has a marked increase with temperature.