Thermal Expansion Coefficients of Alkali Halides and Alkaline-Earth Fluorides

The thermal expansion coefficients of alkali halides and alkaline-earth fluorides have been estimated and analyzed using Ruffa's method by reevaluating the parameter used. The Morse potential has been used to evaluate the thermal expansion coefficient assuming the Debye model for the frequency distribution of the Morse oscillator. The parameter of the Morse potential has been determined by fitting the thermal expansion coefficients of alkali halides at 293 K to the experimental values. Using the parameter the thermal expansion coefficients of alkali halides and alkaline-earth fluorides in a wide range of temperatures have been calculated. The calculated thermal expansion coefficients thus obtained are in excellent agreement with the experimental ones except for the excess contribution due to vacancy formation at high temperatures. The excess term has been analyzed and the enthalpies of vacancy formation due to Schottky defects have been obtained. The enthalpies of vacancy formation are shown to be linearly dependent on Debye temperature and cohesion energy for alkali halides and alkaline-earth fluorides separately.