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

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 ₀^thermal=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.