Brillouin scattering and theoretical studies of high-temperature disorder in fluorite crystals

Crystals with the fluorite structure show a specific heat anomaly at a temperature Tc well below the melting temperature. This premelting phenomenon is due to generation of extensive disorder in the anion sublattice. The present investigation shows, using Brillouin scattering methods, that the disorder at Tc results in a large decrease in the elastic constant C11 and a much smaller change in C44 and C12. Comparison of both lattice and lattice-defect energy calculations with experimental data suggests a molar defect concentration of interstitials of order 10% just above Tc. The calculations also provide a qualitative explanation of the limitation of the disorder giving rise to the specific heat anomaly.