Anharmonicity in the alkaline earth fluorides

The temperature dependence of the q approximately=0 infrared active phonons in CaF 2 , SrF 2 and BaF 2 is reported in the range 5-500 K, and the pressure dependence of the long wave transverse optic phonons at 290 K, in the range 0-12 kbar. These measurements have been used to determine the variation of q approximately=0 transverse optic phonon frequencies at constant volume from 5 to 500 K and also the dependence on volume at 290 K. The results are discussed in terms of the complex selfenergies of this phonon and in terms of the volume dependence of the effective charges for these solids.