Analysis of the elastic properties of alkali halides

A quantitative explanation for the breakdown of the Cauchy relation for elastic constants of alkali halides has been presented in terms of overlap integrals for neighbor ions using the Lowdin-Lundqvist formulation of the many-body potential. Values of the Cauchy deviation ($c_{12}-c_{44}$) are found to present close agreement with experimental data. The analysis has been extended to evaluate the third-order elastic constants and the pressure derivatives of the second-order elastic constants of NaCl-structure alkali halides. An adequate model for the interionic potentials has been developed by considering the short-range repulsive interactions between first and second nearest neighbors and van der Waals dipole-dipole interactions. The results obtained in the present paper compare well with recent experimental data.