Neutron diffraction studies of the actinide oxides II. Thermal motions of the atoms in uranium dioxide and thorium dioxide between room temperature and 1100 °C

Abstract
The mean-square thermal displacements of the atoms in U 0 2 and T h 0 2 have been determined as a function of temperature from the analysis of two-dimensional diffraction data. In the range investigated, 20 to 1100°C, the oxygen atoms vibrate more strongly than the metal atoms and at each temperature the displacements of the metal and oxygen atoms are slightly less in T h 0 2 than in U 0 2. The interpretation of these results gives a Debye characteristic temperature which is independent of tem temperature above 400 °C and equal to 377 °K for U 0 2 and 393 °K for T h 0 2. As the temperature rises, the oxygen atoms tend to be displaced from the fluorite-type sites at 1/4 1/4 1/4. . . towards the large interstitial holes at 1/2 1/2 1/2.... At 1000°C the mean atomic co-ordinates of the oxygen atoms are 1/4+ 8 1/4+8 1/4 ...where, 8 — 0*016 for UO 2 and 8 = 0.014 for ThO 2 . This relaxation effect indicates either that the oxygens are disordered or that they vibrate anharmonically across the 1/4 1/4 1/4. . . positions.