Structure of Na2O–GeO2–P2O5 glasses by X-ray and neutron diffraction

Abstract

Large fractions of six-coordinated Si are detected in Na2O–SiO2–P2O5 glasses with large P2O5 contents, and so six-coordinated Ge is expected in the structures of Na2O–GeO2–P2O5 (NGP) glasses with similar P2O5 contents. This behavior is investigated by X-ray and neutron diffraction. The Ge–O coordination numbers vary in the range 4.45≤NGeO≤5.6. Most GeO6 octahedra exist in the glass with the smallest GeO2 fraction (24mol%). The Na+ ions are associated only with non-bridging oxygens (NBO) on phosphate tetrahedra, even though the ratio n(NBO)/n(Na) decreases from two for NaPO3 glass to ~1.3 for a glass with 37mol% GeO2. The peak shape of the P–O distances changes strongly between NaPO3 and the NGP glass of 24mol% but it is unchanged for the other NGP glasses. Atomistic models of the glasses with the maximum possible NGeO, with Na+ ions that find full charge compensation on NBO neighbors successfully reproduce the experimental NGeOs. The corresponding behavior is described by a general equation which is based on the mole fractions of Na2O, P2O5, and GeO2 and the ratio n(NBO)/n(Na). An earlier study of similar K2O–GeO2–P2O5 glasses with greater GeO2 content revealed the existence of strong pre-peaks in the scattering intensities at very small Q-values (7.5nm−1). Here, similar pre-peaks are observed (at 8nm−1) which are interpreted with an analogous model. Na-rich and Ge-rich phosphate nano-domains with distances of ~1.0nm for repetition of like domains are assumed to explain the pre-peaks.