High-temperature disorder and the elastic constants in fluorite crystals

The effect of nearest-neighbour central force constant changes near vacancies and interstitials in crystals with fluorite structure is investigated by ATA. It is found that they cannot account for the experimentally observed difference in the temperature dependence of the elastic constants c 11 (large decrease near the transition temperature to the state with good ionic conductivity) and c 12 and c 44 (no measurable decrease). Including Coulomb force constant changes to lowest order in perturbation theory, suggests that they may be responsible for the different behaviour.