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. He re an analytical expression for the thermal defect contribution and it s dependence on pressure is derived. We show that such contributions, which are significant at high temperature and atmospheric pressure, be come negligible at pressures above approximately 0.25 to 0.35 of the b ulk modulus at standard conditions. At very high pressure, based on Bi rch's (1968) relationship between high and low pressure thermal expans ion, and assuming a K-T is independent of pressure, NaCl thermal expan sion can be calculated within the constraints of a semi-empirical quas i-harmonic perfect crystal model. The calculations are compared with a vailable theoretical and experimental values over an extended temperat ure/pressure regime. The method should have broad applicability for ot her ionic crystals.