The nature of disorder in the superionic state of fluorites

In fluorites, the principal intrinsic lattice defect at ordinary temperatures is the anion Frenkel pair, with the interstitial at a cube-centre site. However, quasielastic neutron scattering and molecular dynamics studies show that this defect structure is not important in the heavily disordered superionic phase of fluorites. Here the authors discuss the structure and stability of cluster of anion vacancies and interstitials (2:2:2 type cluster) and show that it is sufficiently stable to exist in a transient fashion at high temperatures and to account for some of the effective reduction in anion Frenkel energy necessary to generate the disorder in the superionic phase. It also accounts to some extent for the q dependence of quasielastic scattering intensity in the superionic phase (Clausen et al. 1981).