Three-body interaction effects on cohesive properties of alkali halides

Abstract

I t is now well known that three-body interactions (1.3) arise due to the overlap of adjacent ions and play a significantly impor tant role in the description of lattice dynamical, dielectric and elastic properties (4.~) of ionic crystals. However, the problem of investigation of their role to predict the cohesive properties has received very scant at tent ion. I t happened only recently when SIIA~.~A et al. (v) have studied the effect of three-body interactions (TBI) on cohesive energy of alkali halides via Singh and Vcrma model (4). The results obtained by them have led to the conclusion that the effect of TBI incorporated in the manner of SINGH and V~RMA (4) introduce large deviations while their inclusion through Basu and Sengupta model (s) is free from such limitations. With such remark in mind we have recalculated the cohesive energy of rocksalt and cesium-chloride type alkali halides on the basis of Singh and Verma model with Coulomb, Born-Mayer type overlap repulsive and three-body interactions. The results obtained by us show excellent agreement with measured cohesive energies for all the alkali halides except for LiF and RbI. Itowever, the deviation in these extreme solids also is not more than 14%. The aims of this paper are twofold. The aim of this work is to introduce TBI in the manner indicated by Singh, because the numerical calculations performed by Sharma have been found in error. We will also show that the inclusion of Van tier Waals interactions and zero-point energy improves the agreement remarkably. The agreements thus obtained are almost exact and comparable to those already obtained by Sarkar and Sengupta (8) using three-body potential of Basu and Sengupta (s) together with vdW dipole-dipole and dipole-quadrupole interactions and second-neighbour short-range interactions.