Importance of defect clustering in the analysis of BaF2:Er3+optical spectra

Abstract

A description of the site distribution in rare-earth-doped Ba${\mathrm{F}}_2$ crystals cannot be complete without the inclusion of cluster sites made up of two or more interacting dopant ions. Villermain-Lecolier et al. observed two transitions in the far-infrared spectrum of Ba${\mathrm{F}}_2$: ${\mathrm{Er}}^{3+}$ which they assigned to two different sites, possibly the trigonal and tetragonal sites. They described the absorption band at 131 ${\mathrm{cm}}^{-1\mathrm{}}$ considering contributions from only single ${\mathrm{Er}}^{3+}$-${\mathrm{F}}_I^{-}$ sites. An alternative explanation for their observations is presented considering the existence of cluster sites in the crystal and the validity of a description including only single pair sites is questioned.