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.