HIGH TEMPERATURE THERMAL-EXPANSION OF ALKALI-HALIDES

Thermal expansion data for 11 sodium chloride structure-type solids have been evaluated with an empirical modified quasi-harmonic model for thermal expansion. Earlier it was shown for Group IV elements and 10 Grimm-Sommerfeld compounds that an empirical quasi-harmonic model fits experimental data from nearly absolute zero to intermediate temperatures in the vicinity of T-L, the approximate temperature, where all lattice modes are fully excited. At higher temperatures, experimentally measured thermal expansion is slightly higher than an extrapolation of this empirical relationship. The difference can be approximated with a second-order term that gives excellent quantitative agreement with experiment for the available data to approximately 90% of the melting temperature.