Neutron scattering studies of fluorite oxides at high temperatures

The high temperature behaviour of the fluorite oxides U02, Th02 and Y203-stabilised Zr02 have been investigated using a variety of neutron scattering techniques. Bragg diffraction measurements have shown that a growing fraction of anions vacate their regular sites above 2100K in U02 and above 2300K in Th02, attaining vacancy concentrations of approx. 20% at 2900K in both materials. The elastic constants of U02 which have been determined up to 2930K from measurements of the long wavelength acoustic phonons, show an increased rate of softening above 2400K. The zone-centre optic phonons broaden rapidly above 2000K in U02. The results provide the first direct, unambiguous evidence of thermally-induced Frenkel disorder in U02, which is analogous to the disorder observed in the fast-ion phase of the fluorite halides, such as CaF2. Quasielastic scattering techniques have also been used to study the defect structure of Y203-stabilised Zr02 between 293K and 2000K. (author)