The effects of three-body interactions on crystal properties and defect energies in silver halides

Abstract

The currently available two-body interionic potentials for AgCl and AgBr may be used to calculate the temperature dependence of the Frenkel defect formation energy in good accordance with the experimentally measured transport properties. These potentials have, however, shortcomings, especially with respect to the Cauchy violation, the phonon dispersion and the Schottky energy. The reasons for these failures are ascribed to the lack of many-body terms in the potentials and to the assumption of spherically-symmetric cations. The first of these factors is examined here in terms of two models: one based on angle-bending forces and the other based on the triple-dipole interaction. Both new potentials lead to Schottky energies which are in agreement with anion transport data and they both give significantly better predictions of the pronounced Cauchy violation in silver halides. However, the triple-dipole model is to be preferred on the grounds of its greater stability in defect calculations. Both models lead to some slight improvement in the calculated phonon dispersion, particularly in the acoustic modes, but a better prediction of the optic modes must await the implementation of an ion-deformation model in perfect crystal and defect computer codes.