Fluoride ion disorder and clustering in superionic PbF₂

Computer simulations of the β-phase of PbF 2 using a polarizable ion interaction potential are described. Studies of KF- and YF 3 -doped PbF 2 , as well as the pure material are included. The simulations reproduce the macroscopic observables associated with the transition to superionic behaviour well, including the heat capacity, lattice constant and conductivity. An explanation is provided of the familiar observation of the similarity of the conductivities of the superionic solid just below and of the melt just above the melting transition. Comparisons are made with diffraction and diffuse scattering studies, which confirm that the nature of the fluoride ion disorder above the transition temperature in the simulations is very similar to that deduced from experiments. This involves a cooperative excitation of the fluoride sublattice, which results in the creation of a large number of vacancies and interstitials. Detailed studies of the positional correlations between these defects reveal a high degree of order associated with specific clustering effects. These positional correlations appear to be stronger than would be anticipated from recent mean-field descriptions of the interactions between charged defects in superionic materials. The nature of the correlations is compared with that associated with the interstitial clusters found in moderately YF 3 -doped PbF 2 at low temperatures.