PROCESS OF CLUSTERIZATION IN SOLID-SOLUTIONS CA(1+X)LN(X)F(2+X) WITH A LARGE LANTHANIDE TRIVALENT SUBSTITUENT

The model of the processes of clusterization proposed for the anion-excess CaF2-type solid solutions M1-x2+M'x2+alpha-F2+alpha-x (alpha = 1, 2, 3) has been applied to the solid solutions Ca1-xLnxF2+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 less-than-or-equal-to x less-than-or-equal-to 0.42) has been confirmed. Two sublattices of vacancies located inside the clusters (y1) and in the immediate neighbourhood (y2) are related respectively to the F" and F"' interstitial fluoride ions. The Ca1-xGdxF2+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 y2 and the weakest space constraints. It results in the best electrical performance for that solid solution.