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

The existence of a separate cubic phase ${\mathrm{RF}}_3\text{:3(}{\mathrm{MeF}}_2)$ , 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 ${\mathrm{SrF}}_2:{\mathrm{Ge}}^{\text{3+}}\hspace{0.17em}\mathrm{or}\hspace{0.17em}{\mathrm{SrF}}_2\mathrm{:\; (}{\mathrm{Gd}}^{\text{3+}},{\mathrm{Ce}}^{\text{3+}})$ a calculation estimates that such a phase could form at 0.36% total rare earth impurity concentration.