Temperature Dependence of the Surface Energy of the Low-index Planes of CaF2, BaF2 and SrF2

The temperature dependence of the surface energy γ of the low-index (111), (110) and (100) planes of CaF2, BaF2 and SrF2 was evaluated for the first time, using a simple method which is based on physical and thermodynamic quantities and considerations. The extrapolated to 0 K surface energy values agree well with the available theoretical data reported in the literature for the (111) plane and are close to those for the (110) plane, whereas they are significantly lower than those for the (100) plane. The calculated γ111 values for 298 K agree well with the corresponding experimentally determined literature values. The calculated γ100/γ111 ratio revealed formation of crystals with truncated octahedron structure in thermodynamic equilibrium. This structure remains practically constant with temperature. The surface energy values of fluoride melts formed by melting of octahedron structures, calculated via extrapolation of the surface energy of the solids to the melting points, were compared with experimentally determined literature values for melts resulting from single or polycrystalline materials.