Surface relaxation and surface dynamics of crystals of the rutile structure: MgF 2 (110)

Abstract

We extend to crystals of the rutile structure our previous studies of the surface dynamics of ionic crystals within the framework of the shell model. These crystals have many interesting features due to their relatively open structure. Among the crystals of this type, MgF 2 has a closed-shell electronic configuration; we therefore expect that the surface properties can be calculated using potentials which are obtained from bulk dispersion relations. We find that the bulk dynamics can be described very satisfactorily, if the static charge distribution of the shell of the fluorine ion is shifted with respect to the core. The value of this shift is determined by the equilibrium condition. Using this model we calculate the surface relaxation pattern and dynamics of the (110) surface. Due to the open nature of the structure, we find relaxations both perpendicular to the surface and parallel to the surface. The calculated surface dynamics reveals a variety of surface modes which will be discussed and compared with the results for other ionic crystals.