Influence of surface relaxation on the surface vibrational modes of some ionic crystals

In this paper we evaluate the effects of surface relaxation on the surface vibrational modes of the (001) surfaces of NaCl, NaI and MgO, using bulk shell models which have been modified to allow for distance dependence of the short-range interactions. It is found that these dynamical shell models do not lead to convergent results for the static surface relaxation. In spite of this inconsistency, it is found that surface relaxation gives rise to a new structure of relaxationinduced surface modes, which is common to the three crystals and independent of the details of the relaxation. By using assumed (convergent) relaxation configurations for the MgO (001) surface it is shown that relaxation can account for the high-frequency peak in the surface-excess density of vibrational states of MgO, as found in inelastic neutron scattering experiments on MgO microcrystals. In general, surface relaxation has a profound effect on surface vibrational phenomena, and better shell models will have to be developed to deal with the static and dynamical surface phenomena in a consistent fashion.