MODEL CALCULATION OF THE PRESSURE DERIVATIVE OF THE HIGH-FREQUENCY DIELECTRIC-CONSTANT OF ALKALI-HALIDE CRYSTALS

The mechanism of electronic polarization in insulators is still an unsettled issue. Quite contradictory mechanisms (either strictly intraionic or fully interionic excitations giving rise to polarization) envisaged in the different models appear to determine equally satisfactorily the high-frequency dielectric constants. In the present work we propose to calculate the pressure dependence of the electronic dielectric constant within all the existing models. It is interesting to note that though all of these describe the dielectric constant in a somewhat satisfactory way, their predictions for the pressure dependence of the same are quite different from one to the other and from observation. Next we suggest a modification of the shell-model dielectric theory by including the idea of an effective excited state of the anion in a crystal. The calculations based on a simple version of the new model which appears to favor partly intraionic and partly interionic excitations responsible for the electronic polarization show distinct improvement in agreement with experiment over all the existing models. © 1979 The American Physical Society.