Oxygen defects in BaFBr and BaFCl

Abstract

Oxygen in oxide form is a common contaminant of BaFBr and BaFCl crystals. Upon x-irradiation of oxide-containing crystals of both materials, ${\mathrm{O}}_{\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ centers are formed, as has been shown previously by electron paramagnetic resonance (EPR) methods. It is shown here by EPR that upon x-irradiation ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ centers are also formed on the ${\mathrm{Br}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ sites, respectively. With electron nuclear double resonance spectroscopy, hyperfine interactions with the nearest $^{19}\mathrm{F}$ neighbors have been determined for ${\mathrm{O}}_{\mathrm{Br}}^{\mathrm{\ensuremath{-}}}$ in BaFBr. Their interpretation leads to the conclusion that the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ion is displaced along the c axis from the ${\mathrm{Br}}^{\mathrm{\ensuremath{-}}}$ site by about 1 \AA{}, in qualitative agreement with previous theoretical calculations. The EPR hyperfine data for ${\mathrm{O}}_{\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ centers in BaFCl indicate that a similar displacement occurs. Also by EPR, (${\mathrm{O}}_{2}$${)}_{\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ centers could be identified in x-irradiated BaFCl crystals containing ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ enriched with the magnetic $^{17}\mathrm{O}$ isotope.