Surface lattice dynamics of potassium halides

The Green function method for the surface dynamics of ionic crystals, developed in a previous paper, is applied to the (001) surface of potassium halides. An interesting feature of these crystals is the existence of a surface sagittal optical mode, originated by the bulk LO band, which is is always below the Lucas modes. It is also found that the Rayleigh wave frequency at the M? point depends strongly on the ionic polarizabilities. The condition for the occurrence of surface instability induced by a change of ionic polarizabilities is briefly discussed.