Elastic constants of some NaCl type alkali halides

Abstract

Abstract Second and third order elastic constants of NaCl, NaBr, KF, KCl, KBr, and RbBr have been calculated using the Born model of ionic solids. Short range repulsive interactions have been included up to second nearest neighbors using an expression of the form A exp (‐ r/b ) where b is the repulsive parameter. A distinction is made between the like and unlike ion interactions. Van der Waals terms are also included. When the room temperature lattice parameter is used, the elastic constants satisfy the Cauchy relations. Experimental data show that many of the alkali halides fail to satisfy the Cauchy relations. This aspect is examined by using the quasi‐harmonic approximation as developed by Leibfried and his associates. Linear temperature coefficients of the elastic constants are also calculated in the high temperature limit. In order to assess the contributions of the van der Waals terms, elastic constants are also calculated by dropping the van der Waals terms in the expressions for the elastic constants. Calculated values of the elastic constants are compared with the available experimental data.