EVALUATION OF THE MANY-BODY CONTRIBUTIONS TO THE INTERIONIC INTERACTIONS IN MGO

The predictions of an ''extended'' ionic interaction model for the pho nons and other properties of crystalline MgO are considered. In the ex tended model, the polarization interactions resulting from the dipoles and quadrupoles induced on the oxide ions by both Coulombic and short -range interactions with other ions are explicitly represented. Furthe rmore, the shapes of the oxide ions, as perceived through the short-ra nge repulsive interactions with their immediate neighbors, are allowed to respond to their instantaneous coordination environment. Shape def ormations of spherical (breathing), dipolar and quadrupolar symmetry a re allowed.;Each of these many-body effects is independently represent ed in the simulation model, so that the effect of each term on observe d: properties can be separately evaluated. It is shown-how the polariz ation and breathing effects may be directly parametrized from ab initi o electronic structure calculations, By consideration of the, symmetry of the individual phonons at high symmetry points, it can be shown th at the remaining effects may contribute only to particular phonons: Co nsequently they may be assigned values in an unambiguous manner. The i mportance of each many-body effect for the different phonon branches i s clearly delineated. (C) 1998 American Institute of Physics.