Fluoride ion disorder and clustering in superionic PbF2

Computer simulations of the beta -phase of PbF2 using a polarizable ion int eraction potential are described. Studies of KF- and YF3-doped PbF2, as wel l as the pure material are included. The simulations reproduce the macrosco pic observables associated with the transition to superionic behaviour well , including the heat capacity, lattice constant and conductivity. An explan ation is provided of the familiar observation of the similarity of the cond uctivities of the superionic solid just below and of the melt just above th e melting transition. Comparisons are made with diffraction and diffuse sca ttering studies, which confirm that the nature of the fluoride ion disorder above the transition temperature in the simulations is very similar to tha t deduced from experiments. This involves a cooperative excitation of the f luoride sublattice, which results in the creation of a large number of vaca ncies and interstitials. Detailed studies of the positional correlations be tween these defects reveal a high degree of order associated with specific clustering effects. These positional correlations appear to be stronger tha n would be anticipated from recent mean-field descriptions of the interacti ons between charged defects in superionic materials. The nature of the corr elations is compared with that associated with the interstitial clusters fo und in moderately YF3-doped PbF2 at low temperatures.