High-energy X-ray diffraction study of La co-ordination in lanthanum phosphate glasses

A series of (La2O3)x(P2O5)1−x glasses with 0.05⩽x⩽0.28 was characterized by X-ray diffraction. A change of the LaO co-ordination number, NLaO, from 8.2±0.4 (x≅0.13) to 6.2±0.3 (x≅0.25) was extracted by Gaussian fitting of the first-neighbor distances. The decrease of NLaO is a consequence of the reduction in the number of terminal oxygen (OT) atoms available for co-ordination to the La3+ ions. With increasing x, isolated LaOn polyhedra are replaced by LaOn groups that share OTs. These changes in the La environments explain the compositional dependence of glass properties, including mass density. The positions of the first maxima in the X-ray scattering intensities also depend on composition. The position of the main peak shifts from 13.5 to 15.5 nm−1 for La2O3 additions up to x∼0.11, is constant for 0.11⩽x⩽0.25, and increases again when x>0.25. The constant peak position correlates with the compositional range where NLaO decreases and a plateau in the mass density occurs. For glasses with x>0.11 a prepeak occurs at ∼10.5 nm−1. This prepeak is related to distances between two La ions which are co-ordinated to different OTs of a common PO4 neighboring unit.