Processus de clustérization au sein des solutions solides Ca 1−xLn x F 2+x (Ln=La, Nd, Gd) comportant un substituant trivalent lanthanidique de grande taille

Abstract

The model of the processes of clusterization proposed for the anion-excess CaF 2-type solid solutions M 1−x 2+M′ x 2+αF 2+α x (α=1, 2, 3) has been applied to the solid solutions Ca 1− x Ln x F 2+ x (Ln=La, Nd, Gd) involving rare earth substitutional cations of a large size. The formation of 1:0:3 clusters when x increases in the composition domain (0.02⩽ x⩽0.42) has been confirmed. Two sublattices of vacancies located inside the clusters ( y 1) and in the immediate neighbourhood ( y 2) are related respectively to the F″ and F‴ interstitial fluoride ions. The Ca 1− x Gd x F 2+ x phase characterized by the existence of the 1:0:3 clusters and the presence of the smallest substitute, has, for a given value of x, the highest percentage of vacancies y 2 and the weakest space constraints. It results in the best electrical performance for that solid solution.