Strain dependence of static and high frequency dielectric constants of some alkali halides

Abstract

The strain derivatives of the static and high frequency dielectric constants of some alkali halides with the NaCl structure have been computed on negative ion polarisable shell models. The shells are assumed to be undeformed during straining. Three models are considered Model I assumes a central overlap interaction of Born-Mayer form to extend to the nearest neighbours. In Model II, besides the above interaction between nearest neighbours, a central overlap interaction of Born-Mayer form is taken to act between second neighbour anions. Model III differs from Model II only in taking the first neighbour interactions to be non-central. The strain derivative W⁽⁰⁾_11,11 of the static dielectric constant is an order of magnitude larger than W⁽⁰⁾_11,22 and W⁽⁰⁾_12,12 in these alkali halides. Some general trends in the behaviour of the strain derivatives are brought out. In the calculation of the strain optical constants, the present models enable one to include the strain polarisability effect due to the third order parameter in the nearest neighbour interaction. It is shown that the inclusion of this effect brings the value of p_11,11 closer to the experimental values. It is also concluded that a deformation of the ionic shell on straining the crystal will play a dominant role in the strain polarisability contributions to p_11,22.