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 = ∼0.25 and with R = Nd, Sm, Gd for x = ∼0.15. 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, N_PO, 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, N_RO, decrease from ∼8 to ∼6.5 when x is increased from ∼0.15 to ∼0.25. N_OO and N_PP decrease with increasing R₂O₃ content indicating the network disintegration. The numbers N_RO of the metaphosphate glasses (x = ∼0.25) decreases from ∼7 to ∼6 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 N_RO with increasing R₂O₃ content is due to the reduction in the number of terminal O (O_T) available for coordination of the R³⁺ ions (six at metaphosphate composition). Especially for smaller R³⁺ ions sharing O_T between two R sites is not favored. The decrease by ∼0.04 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.