RELAXATION AND DYNAMICS OF THE (111) SURFACES OF THE FLUORIDES CAF2 AND SRF2

We have treated the relaxation and the dynamics of the (111) surfaces of the fluorides CaF2 and SrF2 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/Sr-F 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 CaF2 the calculations are compared with the HREELs data of Longueville et al.