Re-analysis of single-crystal neutron-diffraction data on UO2 using third cumulants

The single-crystal neutron diffraction data on UO2, recorded by Willis [Proc. R. Soc. London Ser. A, (1963), 274, 134-144] over the temperature range 293 to 1373 K have been re-analysed. The analysis was carried out with a structure-factor equation which included the third-cumulant coefficient, c123, of the O atom. (The symmetry of the fluorite structure reduces the remaining third-cumulant coefficients of O and all the third cumulants of U to zero.) Apart from the overall scale factor, there were three parameters to be refined at each temperature: the isotropic B factors of U and O and c123 of O. The temperature variation of the B factors is in reasonably close agreement with that predicted from lattice dynamics [Dolling, Cowley & Woods (1965), Can. J. Phys. 43, 1397-1413]. If c123 is to be ascribed to the influence of anharmonic forces on the motion of the atoms, an Einstein model of the interatomic potential predicts that c123 should be approximately proportional to T2, where T is the absolute temperature. The experimental data are consistent with this prediction, but data of higher accuracy are required for a rigorous test.