Computer simulations were made of the β-phase by using a polarizable ion interaction potential. Studies of KF- and YF3-doped, as well as pure, material were included. The simulations also reproduced the macroscopic observables that were associated with the transition to superionic behavior. These included the heat capacity, lattice constant and conductivity. An explanation was suggested for the similarity of the conductivities of the superionic solid when just below the melting point, and of the melt when just above the melting point. A comparison with diffraction and diffuse scattering studies confirmed that the simulated nature of the fluoride-ion disorder above the transition temperature was very similar to that deduced from experiment. It involved a cooperative excitation of the fluoride sub-lattice, which resulted in the creation of large numbers of vacancies and interstitials. A study of the positional correlations between these defects revealed a high degree of order that was associated with specific clustering effects. It was suggested that these positional correlations were stronger than would have been expected on the basis of previous mean-field descriptions of the interactions between charged defects in superionic materials. The nature of the correlations was compared with that associated with the interstitial clusters found in moderately YF3-doped material at low temperatures.

Fluoride Ion Disorder and Clustering in Superionic PbF2. M.J.Castiglione, P.A.Madden: Journal of Physics - Condensed Matter, 2001, 13[44], 9963-83