The dynamic behaviour of ions in fluorite and tysonite-type solid solutions

Abstract

Metal flourides with the fluorite (CaF 2) structure are known to accept large amounts of LaF 3, rare earth (III) fluorides, ThF 4, or UF 4. On the other hand, metal fluorides with the tysonite (LaF 3) structure are known to dissolve large amounts of the alkaline earth fluorides. The electrical properties of these anion-excess fluorite-type, and anion-deficient tysonite-type solid solutions have attracted considerable attention in recent years. Nominally pure alkaline earth fluorides are rather poor anionic conductors, in contrast to the nominally pure tysonites. With regard to the composition dependence of the isothermal ionic conductivity, significant differences exist between both types of solid solution. The ionic conductivity of concentrated fluorite-type solid solutions increases exponentially with increasing solute content. In several of these systems a maximum is observed in the conductivity isotherm, while in all extensive defect clustering is assumed to occur. Up to about 5 mole% a linear increase occurs in the tysonite-type solid solutions. Like the anion-deficient oxyfluorites, the conductivity isotherms reveal a maximum. In fluorites both fluoride ion vacancies, and fluoride interstitials carry the current. This has some unexpected consequences for the composition dependence of the ionic conductivity. In the tysonite-type solid solutions only fluoride ion vacancies are mobile. However, in this structure two types of anion lattice site need be taken into account. Recently, several conduction models have been proposed for both types of solid solution. These models will be discussed in relation with possible defect structures, which do not only affect the extrinsic conductivity, but in the case of the fluorite-type solid solutions also the high-temperature liquid-like conduction mode. The discussion will not be restricted to diffusive motion of the defects. In addition, it will be shown that the study of localized motion with thermal depolaarization techniques can contribute significantly to our understanding of the conduction mechanisms of both types of solid solution.