Model for Locally Compensated Cubic Sites of Tripositive Rare Earth Ions in Fluorite Crystals

The existence of a separate cubic phase RF3:3(MeF2), which provides a cubic symmetry site with local charge compensation for a tripositive rare earth ion in a fluorite lattice, is examined in order to explain the increase in cubic sites with increasing rare earth impurity concentration. An expression for the minimum concentration at which such a phase would be stable is presented. For the case of SrF2:Ge3+ or SrF2: (Gd3+, Ce3+) a calculation estimates that such a phase could form at 0.36% total rare earth impurity concentration.