Temperature Dependence of the Surface Energy of the Low Index Planes of UO2 and ThO2

The temperature dependence of the surface energy γ of the low index (111), (110), and (100) planes of UO2 and ThO2 was calculated for the first time, using a simple method based on physical and thermodynamic quantities and considerations. The extrapolated to 0 K surface energy values agreed well with the available theoretical data reported in literature for the (111) and (110) surfaces, whereas they were significantly lower for the (100) surface. The γ 100/γ 111 ratio for UO2 and ThO2 revealed the formation of crystals with truncated octahedron structure in thermodynamic equilibrium. This structure shifts to octahedral with increasing temperature. Comparison with experimentally determined surface energies of the polycrystalline oxides showed that their values are in the range between γ 111 and γ 100.