The local field distribution of Gd3+ in transition metal fluoride glasses investigated by electron paramagnetic resonance

The EPR spectra of Gd3+ in two transition metal fluoride glasses (PZG and PBI) have been extensively studied as functions of temperature, Gd3+ concentration and microwave frequency (S, X, K and Q bands). The marked changes observed in the EPR spectra with temperature and concentration imply the existence of pairs of Gd3+ ions. The spectra of isolated Gd3+ ions were obtained at low temperature for samples with low Gd3+ doping concentration. Isolated Gd3+ ions are characterized by a fine-structure b(2)(0)-parameter distribution within the range 0.02-0.15 cm(-1). The simulations of these spectra are computed with a Czjzek finestructure parameter distribution (this solution prohibits the existence of axial symmetry sites). It indicates a decrease of the number of geometrical constraints compared to those for Cr3+ and Fe3+ (octahedral coordination in a fluoride medium), in agreement with the higher Gd3+ coordination numbers. Gd3+-ion sites are found to be more distorted in PZG than in PBI, suggesting that the glass network has an influence on the Gd3+ polyhedron distortion amplitude.