K. Clausen

Investigation of thermally induced anion disorder in fluorites 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.

Quasielastic diffuse neutron scattering from yttria-stabilized zirconia at elevated temperatures

Coherent diffuse scattering from single crystals of six concentrations of Y2O3 in ZrO2 has been studied at temperatures between 293 K and 2780 K. The results are interpreted in terms of the scattering calculated from a model of the crystal comprising a vacancy-free tetragonal region, and a region in which there are vacancies and aggregates of vacancy pairs with a range of sizes. The vacancies and possibily the smaller aggregates appear to be most mobile as the temperature increases.

Inelastic neutron scattering investigation of the lattice dynamics of ThO2 and CeO2

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.

A Study of the Disorder in Heavily Doped Ba1-Xlaxf2+X by Neutron-Scattering, Ionic-Conductivity and Specific-Heat Measurements

The ionic disorder in single crystals of the fluorite-type solid solutions Ba1-xLaxF2+x (with x=0.209 and x=0.492) has been studied in the temperature range from room temperature to 800 degrees C by diffuse neutron scattering, ionic conductivity, and specific heat measurements. From the diffuse neutron scattering it was found that the disorder was dominated by 222 clusters, which at low temperatures (T >10-10s), in agreement with NMB results which suggest a jump frequency below 75 MHz. The temperatures at which the steepest slopes are found in the loss of correlations and in the conductivity coincide at approximately 650 degrees C. At this temperature no clear anomaly is observed in the specific heat. Based on these findings the authors propose a conduction mechanisms where F- ions are moving through the lattice by means of rearrangements of the 222 clusters.

Quasielastic diffuse neutron scattering from fluorites in the fast ion phase

Abstract Quasielastic diffuse coherent neutron scattering arising from the dynamically disordered anions in CaF 2 and PbF 2 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.

Coherent diffuse neutron scattering from UO2 and ThO2 at temperatures above 2000 K

The structure of the View the MathML source reconstruction induced by adsorption of View the MathML source of a monolayer of Sn on the Si(111) surface has been determined using surface X-ray diffraction. The in-plane projection of the surface structure, obtained from structure factors near zero perpendicular momentum transfer, indicates substantial lateral displacements of the Si atoms. Intensity profiles of the fractional order rods give information concerning displacements normal to the surface. The adatoms are shown to occupy sites above second layer Si atoms. Incorporation of elastic strain minimisation into the structure factor analysis enables the identification of relaxations extending six layers into the bulk.

Heavily doped M1−xUxF2+2x fluorites studied by quasielastic neutron scattering(M=Ba) and specific heat measurements (M=Pb).

Abstract The thermal generation of defects in pure and doped Pb 1−x U x F 2+2x (x=0, 0.05and0.10) has been studied by specific heat measurements between 475 k and 875 k. A simple phenomenological mean field thermodynamic model has been developed and used to interpret the data. At low temperatures the result of doping is an increase of the thermally generated defect concentration; at high temperatures the effect is reversed. The microscopic defect structure of Ba 0.9 U 0.1 F 2.2 has been studied by diffuse quasielastic neutron scattering experiments at room temperature. The experimental observations are in good agreement with a model calculation based on a defect structure of 212-clusters.

The defect structure of yttria-stabilized zirconia, studied by quasielastic diffuse neutron scattering

The static defect structure of the oxygen ion conductor Y203 stabilized zirconia has been studied at room temperature by coherent diffuse neutron scattering from single crystal samples containing nominally 9.4, 12, 15 and 18 mol% Y203. There are two principal contributions to the observed diffuse intensity. The first arises from tetrahedral distortions in small vacancy free regions of the crystal which decrease in volume as the dopant level increases. The second arises from correlated vacancies and their associated relaxed ions in the remainder of the crystal. The 9.4 mol% sample has been studied at elevated temperatures. The scattering becomes partly quasielastic, but the correlations persist to the highest temperatures studied (1900°C). The temperature and Q-dependence of the energy width has been studied at selected positions in reciprocal space.

Structure and dynamics of disordered solids: A neutron scattering study of Ba1−xLaxF2+x

Abstract Single crystals of the fluorite-type solid solutions Ba1−xLaxF2+x, x = 0.209 and x = 0.492, show strong anisotropic diffuse neutron scattering which is quantitatively explained in terms of simple models involving defect clusters of the 222 type. The concentrated sample shows additional evidence for short correlations between such clusters which tend to form larger, roughly linear aggregates. The aggregated dissociate at temperatures above 500°C. The implications for the conduction mechanism are discussed.

The transition from zero- to first-sound phonon propagation in lead at high temperatures

Wavevector dependences of the transverse phonon energy widths, determined by neutron scattering, in monocrystalline lead provide evidence for a transition from a zero-sound, collision-free mode of high-frequency phonon propagation to a first-sound, collision-dominated regime at about 500 to 550 K. Hence the propagation of phonons having wavelengths on the scale of the interatomic spacing is heavily damped, similarly to that in the liquid state, prior to melting.