Neutron scattering studies of fluorite compounds

The nature and mobility of defects in ionic materials with the fluorite structure have been studied using neutron scattering techniques. These systems model the behavior of the fission fuel UO2 at elevated temperature. A powder sample of β-PbF2 has been investigated using neutron diffraction, which gives the time-averaged occupation of sites in the unit cell. The temperature dependence of the lattice parameter, the concentration of Frenkel defects, and the thermal parameters of both fluorine and lead ions, have been determined at temperatures from ambient to well above the transition to the fast-ion phase. The defect structure of the anion-excess fluorite (Sr,Y)Cl2.03 has been studied using the coherent diffuse scattering from single-crystal samples. Consistent and complementary information on the diffusion of chlorine ions in (Sr,Y)Cl2.03 has been obtained from the quasielastic energy broadening of the single-crystal incoherent scattering measured at elevated temperature. Comparison with previous results from SrCl2 shows that chlorine diffusion is faster in (Sr,Y)Cl2.03, and that the diffusional process in the anion-excess fluorite resembles that found when the level of thermally generated disorder is high in the pure compound. The coherent scatterer UO2+δ (δ = 0.13, 0.14) transforms from a mixture of oxides at ambient temperature to a single, fluorite-structured phase at roughly 850K. Considerable hysteresis is associated with this transition, in which regular arrays of cuboctahedral units break into isolated clusters of the type first postulated by Willis in 1963. At 1573K the distribution of quasielastic diffuse scattering is reminiscent of that observed for (Sr,Y)Cl2.03 in the fast-ion phase. The oxygen self diffusion coefficient in UO2+δ is comparable to that found in pure UO2 some 1000K higher