PRESSURE-DEPENDENCE OF THERMAL EXPANSIVITY FOR ALKALI-HALIDES

An isothermal equation of state is developed to study the compression of lithium and sodium halides at pressures ranging from zero to their structural transition pressures. The calculations are performed by developing an ionic model based on Harrison's quantum mechanical treatment of overlap repulsive potential which takes into account the interactions up to second neighbours. The van der Waals dipole-dipole and dipole-quadrupole interactions calculated by more accurate methods are also included in the model. The compression data thus obtained are used to investigate the pressure dependence of the volume thermal expansion coefficient by using the formula due to Anderson derived on the basis of thermodynamic analysis by making some crude approximation. The results are compared with the available experimental data. The crudeness of the approximations used in the present work is discussed in the light of recent work.