Oxygen in oxide form is a common contaminant of BaFBr and BaFCl crystals. Upon x-irradiation of oxide-containing crystals of both materials, ${\mathrm{O}}_{\mathrm{F}}^{\mathrm{-}}$ centers are formed, as has been shown previously by electron paramagnetic resonance (EPR) methods. It is shown here by EPR that upon x-irradiation ${\mathrm{O}}^{\mathrm{-}}$ centers are also formed on the ${\mathrm{Br}}^{\mathrm{-}}$ and ${\mathrm{Cl}}^{\mathrm{-}}$ sites, respectively. With electron nuclear double resonance spectroscopy, hyperfine interactions with the nearest ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ neighbors have been determined for ${\mathrm{O}}_{\mathrm{Br}}^{\mathrm{-}}$ in BaFBr. Their interpretation leads to the conclusion that the ${\mathrm{O}}^{\mathrm{-}}$ ion is displaced along the c axis from the ${\mathrm{Br}}^{\mathrm{-}}$ site by about 1 Å, in qualitative agreement with previous theoretical calculations. The EPR hyperfine data for ${\mathrm{O}}_{\mathrm{Cl}}^{\mathrm{-}}$ centers in BaFCl indicate that a similar displacement occurs. Also by EPR, (${\mathrm{O}}_2$${)}_{\mathrm{Cl}}^{\mathrm{-}}$ centers could be identified in x-irradiated BaFCl crystals containing ${\mathrm{O}}^{2\mathrm{-}}$ enriched with the magnetic ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ isotope.

• Received 23 January 1995

DOI:https://doi.org/10.1103/PhysRevB.52.3941