Space charges and dipoles in rare-earth-dopedSrF2

The high-temperature peak, observed with ionic thermocurrent (ITC) experiments performed on R${\mathrm{F}}_3$-doped ${\mathrm{SrF}}_2$, ${\mathrm{CaF}}_2$, and ${\mathrm{BaF}}_2$, is proved to be related to space charges. Measurements of $R^{3+}$-${\mathrm{F}}^{\mathrm{-}}$ dipole concentrations as a function of R${\mathrm{F}}_3$ doping concentration in ${\mathrm{SrF}}_2$ are presented (R=La, Ce, Nd, Eu, Sm, Gd, Dy, Er, Yb, and Lu). The results as a function of R${\mathrm{F}}_3$ doping concentration can be explained by assuming that only isolated $R^{3+}$-${\mathrm{F}}^{\mathrm{-}}$ dipoles are observed with ITC. The results as a function of $R^{3+}$ ionic radius can be explained by assuming an increasing clustering probability of the $R^{3+}$ ions as the radius of the rare-earth ions decreases. It is concluded that the large rare-earth ions are distributed randomly over the ${\mathrm{SrF}}_2$ lattice, while the small $R^{3+}$ ions tend to cluster during crystal growth.