Neutron scattering studies of fluorite oxides at high temperatures
The high temperature behaviour of the fluorite oxides UO2, THO2 and Y2O3-stabilised ZrO 2 have been investigated using a variety of neutron scattering techniques. Interest has centred on the cause of the anomalously large enthalpy of UO2 at temperatures above 15OOK, an understanding of which is important in view of its use as a fission reactor fuel. High temperature techniques have been developed which enable the performance of neutron scattering at temperatures up to 3OOOK. Bragg diffraction measurements have shown that a growing fraction of anions vacate their regular sites above 21OOK in UO2 and above 23OOK in ThO2 attaining vacancy concentrations of ~2O% at 29OOK in both materials. Quasielastic scattering investigations have confirmed the occurrence of anion Frenkel disorder in UO2 at high temperatures and have shown that the disorder is of a dynamic nature. Both sets of results may be interpreted in terms of fluctuating, dynamic clusters of vacancies and inter stitials, having lifetimes of a few phonon periods. The elastic constants of UO2, which have been determined up to 293OK from measurements of the long wavelength acoustic phonons, show an increased rate of softening above 24OOK. The zone-centre optic phonons broaden rapidly above 2OOOK in UO2- The full phonon dispersion relation of ThO2, measured at 293K, resembles closely that of UO2. The results provide the first direct, unambiguous evidence of thermally-induced Frenkel disorder in UO2, 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 Y 2O 3 -stabilised ZrO 2 between 293K and 2OOOK. Many general features of the scattering observed at room temperature may be explained in terms of nearest neighbour relaxations around oxygen vacancies. The diffuse scattering broadens in energy above 1OOOK, signifying that these clusters of relaxed ions have a finite lifetime of ~5ps at 15OOK. The results have been interpreted in terms of a vacancy hopping model which is consistent with the enhanced conductivity observed in this temperature range.