Thermal expansion at low temperatures of the alkaline earth fluorides and PbF2

Abstract

Thermal expansions have been measured from 2 to 90K on four compounds of fluorite structure. Data at the lowest temperatures lead to the following limiting values of the Gruneisen parameter: gamma 0thermal=1.0 (CaF2), 0.64 (SrF2), 0.23 (BaF2) and 0.5-0.8 (PbF2). These agree within limits of error with those calculated from the pressure dependence of elastic moduli. For the three alkaline earth fluorides gamma (T) increases monotonically with temperature from gamma 0 to gamma 293 approximately 1.5-1.8, but for PbF2 there is a marked maximum near 25K where gamma approximately=3.5, consistent with the effect of an optic mode of characteristic temperature theta E approximately=68K.