Clusters of group-III ions in activated fluorite-type crystals

The data on optically detected EPR in absorption bands of alkaline-earth fluoride crystals doped with rare-earth (Er, Tm, Yb, Lu) or yttrium activators indicate that these crystals contain clusters similar to Y6F37, which is the structural unit of the naturally occurring mineral tveitite and of synthetic yttrofluorite [(CaF2)(1 - y)(YF3)(y)] crystals, whose lattices exhibit a superstructure at certain values of y. Starting from a rare-earth ion concentration of the order of 0.1%, the greater part of the rare-earth impurity is concentrated in these clusters, having a tendency to aggregate into larger clusters. Clusters are also of considerable importance in semiconducting fluorite-structure crystals of CdF2 doped with Ga, In, or Y ions, as indicated by microwave and IR radiation absorption in these crystals. (C) 2002 MAIK "Nauka / Interperiodica".