Structure of Rare-Earth Phosphate Glasses by X-ray and Neutron Diffraction

Previous diffraction studies of the structures of rare-earth phosphate glasses (R2O3)x(P2O5)1−x are extended to glasses with smaller R3+ ions with R = Sm, Gd, Dy, Er, Yb, Y for x = not, vert, similar0.25 and with R = Nd, Sm, Gd for x = not, vert, similar0.15. Parameters for the P-O, R-O and O-O first-neighbor peaks were obtained by Gaussian fitting. P-P and R-P distances were estimated from the positions of peak maxima. Effects of residual silica or alumina contents present as a result of glass processing were taken into account for selected samples. The P-O coordination number, NPO, and the P-O, O-O, P-P distances are consistent with the presence of phosphate tetrahedra and are insensitive to the R species and the R2O3 content. Rare-earth coordination numbers, NRO, decrease from not, vert, similar8 to not, vert, similar6.5 when x is increased from not, vert, similar0.15 to not, vert, similar0.25. NOO and NPP decrease with increasing R2O3 content indicating the network disintegration. The numbers NRO of the metaphosphate glasses (x = not, vert, similar0.25) decreases from not, vert, similar7 to not, vert, similar6 when R is changed from La to Yb. This change is also indicated by the behavior of the R-O distances and by constant number densities of atoms. The decrease in NRO with increasing R2O3 content is due to the reduction in the number of terminal O (OT) available for coordination of the R3+ ions (six at metaphosphate composition). Especially for smaller R3+ ions sharing OT between two R sites is not favored. The decrease by not, vert, similar0.04 nm of the prominent R-R first-neighbor distance with a change of R from La to Yb at the metaphosphate composition is indicated by a shift to higher magnitude of scattering vector of the shoulder occurring in front of the first main diffraction peak