Multiphonon Absorption in the Alkaline Earth Fluorides

The temperature dependence of the intrinsic multiphonon absorption in CaF_2, SrF_2 and BaF_2 was obtained at 10.6 um from 300°K to 800°K. These results were compared with two different theoretical approaches, both of which have given reasonable agreement for the alkali halides. Best agreement with experiment was obtained with the approach involving a quantum-mechanical perturbative treatment of the potential whose parameters are determined from compressibility and lattice frequency at each temperature