Simple models for super-ionic disorder were analyzed with regard to the gradual transition of AB2 fluorite or anti-fluorite structures to the highly conductive state. The approach was based upon the use of excess enthalpy, entropy and free energy. It was established that the change in defect concentration, as a function of temperature, in the super-ionic region was characterized by deviations from an Arrhenius-type dependence and was consistent with the analogous behavior of ionic conduction. At the melting point, the degree of disorder was still only 10 to 20% in the case of the chloride and 20 to 40% in the case of the sulfide. On the other hand, the models did not explain all of the peculiarities of the super-ionic transition in fluorites.

B.M.Voronin: Journal of the Physics and Chemistry of Solids, 1995, 56[6], 839-47