Etude des proprietes de conduction ionique des solutions solides Ca 1− xTh xF 2+2 x et Ca 1− xU xF 2+2 x

Abstract

A comparative study of the ionic conductivity properties of the solid solutions Ca 1− x Th x F 2+2 x (0 ≤ x ≤ 0.18) and Ca 1− x U x F 2+2 x (0 ≤ x ≤ 0.19) has been undertaken and correlations between the electrical properties and the short-range order within these phases have been established. The model of the processes of clustering proposed for the anion excess CaF 2-type solid solutions M 2+ 1− x M′ 2+α x F 2+α x (α = 1, 2, 3) has been applied to the solid solutions Ca 1− x M″ x F 2+2 x ( M″ =Th, U). The formation of the 1:0:3 clusters is confirmed when x increases in the composition domain (0.01 ≲ x ≤ x L) ( x L: upper limit of substitution rate). Two sublattices of vacancies are located inside the 1:0:3 clusters (y 1) and in the immediate neighborhood (y 2); these are related respectively to the F″ and F‴ interstitial fluoride anions. The Ca 1− x U x F 2+2 x phase, involving the smallest tetravalent cation, has a larger number of vacancies y 2 and weaker space constraints for the same substitution rate. It results in better electrical properties for that solid solution than for Ca 1− x Th x F 2+2 x . The solid solutions Ca 1− x M″ x F 2+2 x ( M″ =Th, U) are characterized by the same process of clustering as the solid solutions Ca 1− x Ln x F 2+ x ( Ln =La, Nd, Gd) involving rare earth substitutional cations of large size.