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 Y 6 F 37 , which is the structural unit of the naturally occurring mineral tveitite and of synthetic yttrofluorite [(CaF 2 ) 1− y (YF 3 ) 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 CdF 2 doped with Ga, In, or Y ions, as indicated by microwave and IR radiation absorption in these crystals.