Effect of non-stoichiometry on the luminescence properties of lanthanum oxyfluorides

Abstract

The UV and argon-ion laser-excited high-resolution luminescence spectra of the Eu3+ ion were used as structural probes to investigate the effect of non-stoichiometry on the structure of the lanthanum oxyfluoride, LaOF, system. The spectra of the two modifications of LaOF:Eu3+ are different. For the stoichiometric, rhombohedral LaOF the spectrum is in agreement with the crystallographic C3ν point symmetry of the La3+ site. The crystal field analysis, based on the 7F0-2 levels, revealed only slight distortions from the ordered cubic fluorite phase. In the tetragonal LaO1-xJCF1+2x phase a symmetry slightly lower than C4ν was found for the La3+ site, in disagreement with structural data. The Eu3+ level scheme in this phase corresponds approximately to those in the tetragonal rare-earth (RE) oxysalts with the (REO)nn+ complex cation. The X-ray powder diffraction patterns of the rhombohedral and the tetragonal phases are similar, resembling the ordered cubic fluorite pattern. In addition to the weak splitting of the major peaks, the position and intensity of secondary reflections differed only slightly.