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

• A change from GeO4 to GeO6 units is detected with increasing P2O5 content.
• The Na+ ions find full charge compensation from side of the non-bridging oxygens.
• A general equation describes the evolution of the Ge-O coordination numbers.
• The model explains the pre-peaks at the unusually small Q = ~ 8 nm− 1.

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 (24 mol%). 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 37 mol% GeO2. The peak shape of the P–O distances changes strongly between NaPO3 and the NGP glass of 24 mol% 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 (7.5 nm− 1). Here, similar pre-peaks are observed (at 8 nm− 1) which are interpreted with an analogous model. Na-rich and Ge-rich phosphate nano-domains with distances of ~ 1.0 nm for repetition of like domains are assumed to explain the pre-peaks.