ANOMALOUS RARE-EARTH EMISSION IN MAGNESIUM FLUOROBORATE

The luminescence of some rare-earth ions in magnesium fluoroborate, Mg3F3BO3, is reported and discussed. The luminescence spectra of Pr3+, Eu3+, Gd3+ and Dy3+ reveal emissions which, especially in the case of Eu3+ and Dy3+, are different from what is observed usually. The luminescent centre appears to be a rare-earth ion which is associated with an O2- ion replacing a F- ion in the lattice. The covalent character of the luminescent centre and a strong (linear) crystal-field explain part of the anomalous emission lines in the spectra. The covalency accounts also for the low-energy position of the Eu3+-O2-charge-transfer transition and for the low efficiency of the Eu3+ luminescence under charge-transfer excitation (20%).