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

Abstract A series of (La 2 O 3 ) x (P 2 O 5 ) 1− x glasses with 0.05⩽ x ⩽0.28 was characterized by X-ray diffraction. A change of the LaO co-ordination number, N LaO , 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 N LaO is a consequence of the reduction in the number of terminal oxygen (O T ) atoms available for co-ordination to the La 3+ ions. With increasing x , isolated LaO n polyhedra are replaced by LaO n groups that share O T s. 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 La 2 O 3 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 N LaO 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 O T s of a common PO 4 neighboring unit.