Thermal expansion of alkali halides at high pressure: NaCl as an example

Thermal defects in ionic materials can have important effects on their thermal expansion at high temperatures. Earlier treatments of thermal expansion generally have neglected or not considered such effects. Here an analytical expression for the thermal defect contribution and its dependence on pressure is derived. We show that such contributions, which are significant at high temperature and atmospheric pressure, become negligible at pressures above approximately 0.25 to 0.35 of the bulk modulus at standard conditions. At very high pressure, based on Birch's (1968) relationship between high and low pressure thermal expansion, and assuming αKTis independent of pressure, NaCl thermal expansion can be calculated within the constraints of a semi-empirical quasi-harmonic perfect crystal model. The calculations are compared with available theoretical and experimental values over an extended temperature/pressure regime. The method should have broad applicability for other ionic crystals.