Theory of surface rumpling in rock-salt structured ionic crystals

Abstract

Surface lattice relaxations at finite temperatures are studied on the basis of Verwey's model for rock-salt structured ionic crystals with an (001) surface plane, with particular reference to sodium halide crystals. The temperature dependence of the ionic displacements, mean-square displacements (MSD) and induced moments of the surface ions are obtained in the classical limit with the framework of the extended self-consistent Einstein model. The MSD of cations perpendicular to the surface is found to hardly exceed that of the bulk ions. Unless the degree of rumpling is small at T=0K, the rumpling becomes enhanced and the induced moments of the anions grow at finite temperatures. Brief discussions on the mechanisms of the rumpling are also given.