Tb3+ f-d absorption as indicator of the effect of covalency on the Judd-Ofelt Ω2 parameter in glasses

The two effects, covalency and asymmetry, on the Judd-Ofelt Omega(2) parameter have been separated by use of Tb3+ 4f-5d absorption peak wavelength as a measure of covalency. According to the Judd-Ofelt theory, 92 increases with increasing reciprocal energy difference between the 4f(N) and 4f(N-1)5d(1) configurations. In fact, a linear correlation between Omega(2) of EU3+ and the peak wavelength of the Tb3+ 4f(8) --> 4f(7)5d(1) absorption band has been found for the fluoride, fluoride phosphate and phosphate glasses investigated. The differences in the peak wavelengths are caused by differences in the polarizability of the ligands, leading to differences in the covalency of the rare earth sites. This behavior is responsible for the sensitivity of Omega(2) to the covalency of the bond between rare earth ions and their ligands. The deviation of the ultraphosphate glass investigated from the correlation between Omega(2) and the Tb3+ 4f-5d peak wavelength is attributed to a higher degree of asymmetry of the local crystal fields around the rare earth ions. (C) 1999 Elsevier Science B.V. All rights reserved.