Experimental and theoretical studies of effects of anharmonicity and high-temperature disorder on Raman scattering in fluorite crystals

Abstract
Some crystals with the fluorite structure, e.g. CaF2, SrF2, BaF2, SrCl2 and PbF2, are known to exhibit a specific heat anomaly at temperatures Tc well below the melting point. It is generally assumed that this anomaly is associated with the development of extensive disorder in the anion sublattice. To study this disorder we have carried out polarized Raman scattering investigations of these crystals in the temperature range 4-1420 K. Fluorite crystals have one Raman-active phonon with T2g symmetry. We find th at the position and shape of this line below Tc can be explained in detail by using third- and fourth-order anharmonicity. At Tc and above additional scattering develops on the low-energy side of the T2g phonon and this is accounted for by a theory of defect-induced scattering which includes effects of both anion vacancies and interstitials. Both force-constant and polarizability changes are considered in a nearest-neighbour approximation.

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