Relaxation and dynamics of the (111) surfaces of the fluorides CaF₂and SrF₂

The authors have treated the relaxation and the dynamics of the (111) surfaces of the fluorides CaF 2 and SrF 2 in the framework of shell models, in which the short-range interactions are represented by Born-Mayer potentials. The relaxation in the outer layer of the crystal, consisting of three closely spaced F-Ca/SrF layers, is significant. The calculated normal vibrations of a 19-layer slab are presented in 3-dimensional diagrams of the normalized surface squared amplitude, which provide a very direct and pictorial display of the surface localization of vibrational states. For CaF 2 the calculations are compared with the HREELS data of Longueville et at. (1990).