A model for the onset of fast-ion conduction in fluorites

The quasielastic neutron scattering from fast-ion conductors with the fluorite structure has been interpreted alternatively as being due to either clusters of defects or to the motion of defects and their associated strain field. In this paper, we examine the hypothesis that the sharp rise in ionic conductivity that occurs in fluorites near the transition temperature (Tc) from normal to superionic conductivity, is associated with defect clustering. By using lattice simulation techniques it is shown that a wide variety of clusters are stable in SrCl2, CaF2, SrF2, BaF2 and β-PbF2. A cluster consisting of two vacancies, two relaxed lattice ions and three interstitials (which we shall designate 2| 2| 3) is particularly stable. It is shown that the formation of 2| 2| 3 clusters results in an increase in the concentration of vacancies and that this increase can account quantitatively for the rise in conductivity near Tc. Our concern is thus with the possible connection of the onset of superionic conduction with defect clustering and we do not attempt, in this paper, any interpretation of the quasielastic neutron scattering.

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