Space charges and dipoles in rare-earth-doped SrF2.

Abstract

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 ${R}^{3+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ 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 ${R}^{3+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ 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.