Quantification of Short and Medium Range Order in Mixed Network Former Glasses of the System GeO2–NaPO3: A Combined NMR and X-ray Photoelectron Spectroscopy Study

Abstract

Glasses in the system xGeO2–(1–x)NaPO3 (0 ≤ x ≤ 0.50) were prepared by conventional melting–quenching and characterized by thermal analysis, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and 31P nuclear magnetic resonance (MAS NMR) techniques. The deconvolution of the latter spectra was aided by homonuclear J-resolved and refocused INADEQUATE techniques. The combined analyses of 31P MAS NMR and O-1s XPS lineshapes, taking charge and mass balance considerations into account, yield the detailed quantitative speciations of the phosphorus, germanium, and oxygen atoms and their respective connectivities. An internally consistent description is possible without invoking the formation of higher-coordinated germanium species in these glasses, in agreement with experimental evidence in the literature. The structure can be regarded, to a first approximation, as a network consisting of P(2) and P(3) tetrahedra linked via four-coordinate germanium. As implied by the appearance of P(3) units, there is a moderate extent of network modifier sharing between phosphate and germanate network formers, as expressed by the formal melt reaction P(2) + Ge(4) → P(3) + Ge(3). The equilibrium constant of this reaction is estimated as K = 0.52 ± 0.11, indicating a preferential attraction of network modifier by the phosphorus component. These conclusions are qualitatively supported by Raman spectroscopy as well as 31P{23Na} and 31P{23Na} rotational echo double resonance (REDOR) NMR results. The combined interpretation of O-1s XPS and 31P MAS NMR spectra shows further that there are clear deviations from a random connectivity scenario: heteroatomic P–O–Ge linkages are favored over homoatomic P–O–P and Ge–O–Ge linkages.