EPR of Gd³⁺ and Eu²⁺ in fluorozirconate glasses

EPR spectra at 9.1 and 30 GHz are reported for the iso-electronic ions Gd³⁺ and Eu²⁺ in glasses of composition 0.50 ZrF₄, 0.20 BaF₂, 0.055 AlF₃, 0.045 LaF₃ and 0.10 NaF or LiF. The spectra are similar to the 'U' spectra familiar in many oxide glasses, indicating very low and disordered site symmetries with a broad distribution of crystal fields. For europium the mean crystal-field parameter, b₂⁰ approximately=0.075 cm^-1, is 30% larger than that for gadolinium, b₂⁰ approximately=0.058 cm^-1, suggesting that the two ions occupy significantly different sites. Whereas the trivalent gadolinium will substitute readily into La³⁺ sites, it is likely that divalent europium ions replace Ba²⁺ in sites where they are more weakly bound and have a higher fluorine coordination.