Corrélations entre propriétés électriques et ordre à courte distance au sein des solutions solides Ca 1− xY xF 2+ x et Ca 1− xLu xF 2+ x

Abstract

A model has been proposed to correlate the composition dependence of electrical properties and the progressive cluster growth with increasing x in a fluoride anion excess CaF 2-type solid-solution of M 2+ 1− x M′ 2+ α x F 2+ αx ( α = 1,2,3). According to the model, the density of interstitial fluorine ions and the density of vacancies at normal sites are respectively represented by the functions y int. and y □ which depend on three parameters called λ, m, and x e: Y int. = (λ−α)mx 3+λ(m−α)x 2 e (λ−α)x 2+(m−α)x 2 e Y □ = (λ−α)(m−α)(x 3+x 2 e x (λ−α)x 2+(m−α)x 2 e The λ and m parameters respectively define the clustering conditions for the lowest and highest values of x. The x e parameter is the degree of substitution for which a maximum of conductivity in the composition dependence of the electrical properties is encountered. This model, applied to the solid-solutions Ca 1− x Y x F 2+ x (0 ⩽ x ⩽ 0.38) and Ca 1− x Lu x F 2+ x (0 ⩽ x ⩽ 0.40), shows the progressive transformation of 4:4:3 monomer clusters into cubooctahedral 8:12:1 clusters when x increases. The percentage of cubooctahedral clusters in these solid-solutions quenched from 950°C has been determined as a function of composition.