EVOLUTION OF THE CRYSTAL-FIELD EFFECTS IN (LA, GD)OBR - EU-3+ SERIES

A complete series of gadolinium-substituted lanthanum oxybromides doped with Eu3+ ion, (La1-xGdx)OBr:Eu3+ (0≤x≤1), were prepared and characterized by X-ray powder diffraction (XRD) and photoluminescence spectroscopy. The XRD analyses revealed the complete solid solubility between La3+ and Gd3+ host cations. The luminescence spectra of (La1-xGdx) OBR:Eu3+ powder samples were recorded at 21 K between 475 and 770 nm (13,000 and 21,000 cm-1). The 5D0-1→7F0-4 transitions were analyzed and the 7F0-4 energy level schemes were assigned according to the selection rules between the crystal field (c.f.) sublevels of the 5D0-1 and 7F0-4 levels for the C4v point symmetry of the RE3+ site in the RE oxybromides. The spectral analysis confirmed the formation of complete La-Gd solid solutions except for the splitting of a few E levels. These splittings were concluded to result from a decrease in the site symmetry from C4v to C2v owing to the ordered substitution of the La3+ ions with Gd3+. The c.f. analysis yielded sets of five nonzero Bq k parameters which reproduced the experimental 7F0-4 energy level schemes within rms deviations between 5 and 8 cm-1. The parameters vary smoothly (except the B0 6 parameter) as a function of the Gd content. The c.f. parameter sets calculated by using the electrostatic point charge model showed no correlation to the phenomenologically determined sets. The electrostatic model must thus be considered inadequate to account for the interactions in RE oxybromides. © 1992 Academic Press, Inc.