Effects of the multipole polarization on the elastic constants and ionic conductivity in cubic structure crystals

The contributions from the multipole polarization of the ions to the elastic constants are calculated for the cesium–chloride structure crystals. The multipole polarizabilities of the ions are calculated by the self-consistent field treatment of the local density approximation and the spherical solid model. The calculated contributions CM,11, CM,12 and CM,44 from the multipole polarization to the elastic constant are about 7, 12 and less than 2% of the experimental values CE,11, CE,12 and CE,44 in the cesium–chloride structure crystals, respectively. The contributions from the multipole polarization to the elastic constants are significant in the silver halide crystals and the fluorite structure crystals which exhibit high ionic conductivity, while they are less significant in the cesium–chloride structure crystals which exhibit low ionic conductivity. It is found that the ionic conductivity correlates with the deviation from the Cauchy relation ΔCE = CE,12 − CE,44, and becomes higher with increasing ΔCE in the silver halide and alkali halide crystals with positive ΔCE. The calculated values ΔCM = CM,12 − CM,44 of the deviation from the Cauchy relation due to the multipole polarization are in reasonable agreement with the experimental values ΔCE in these crystals. These facts indicate that strong non-central forces due to the induced polarization play an important role in the ionic conductivity.