HIGH TEMPERATURE THERMAL-EXPANSION OF ALKALI-HALIDES

Thermal expansion data for 11 sodium chloride structure-type solids ha ve been evaluated with an empirical modified quasi-harmonic model for thermal expansion. Earlier it was shown for Group IV elements and 10 G rimm-Sommerfeld compounds that an empirical quasi-harmonic model fits experimental data from nearly absolute zero to intermediate temperatur es in the vicinity of T-L, the approximate temperature, where all latt ice modes are fully excited. At higher temperatures, experimentally me asured thermal expansion is slightly higher than an extrapolation of t his empirical relationship. The difference can be approximated with a second-order term that gives excellent quantitative agreement with exp eriment for the available data to approximately 90% of the melting tem perature.