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.

Mixed Network Former Effects in Oxide Glasses: Spectroscopic Studies in the System (M2O)1/3[(Ge2O4)x(P2O5)1–x]2/3

“…Ge (5) and Ge (6) units (with bond valences of 0.8 and 0.67) may be stabilized by interaction with P (3) units (bond valence 1. 25), and this interaction may contribute to the preference for heteroatomic P−O−Ge linkages in this glass system, as it is found experimentally. On the other hand, we cannot rule out the formation of ring structures based on Ge− O−Ge linkages being partly responsible for the T g maximum.…”

“…Overall the general conclusions are consistent with those drawn from our previous study of the NaPO 3 −GeO 2 system. 25 For the present system, however, the NMR data also provide new indirect conclusions regarding the germanate anomaly. On the one hand the 31 P NMR data clearly indicate that the germanate network is partially modified for x ≥ 0.4, on the other hand, neither Raman spectroscopy nor XPS provide any clear evidence for the formation of nonbridging oxygen atoms associated with depolymerized Ge (3) or Ge (2) units, except for the x ≥ 0.8 samples.…”

“…These fits The resonances of the various P (n) mGe units are systematically shifted to higher frequencies as n is decreased and m is increased, in agreement with previous observations in sodium germanophosphate glasses of different compositions. 25 Furthermore, the signals in the NaGeP glasses are found at slightly higher frequencies and have a slightly larger width (full width at half-maximum, fwhm) than those of the KGeP glasses. The fwhm of individual signal components tend to be larger in those glasses in which a large number of different P (n) mGe units is found than in those glasses in which one or two units dominate the structure.…”

“…Figure 3 shows the O-1s X-ray photoelectron spectra. In accordance with a previously developed analysis procedure 25 they can be fitted using five Gaussian lines represening the P−O−P, P−O−Ge, Ge−O−Ge, NBO(P), and NBO(Ge) species. 25,28,29 The respective binding energies of all but the P−O−Ge oxygen species are obtained from the spectra of the binary glasses.…”

“…25 Figure 5 shows such double quantum filtered spectra, based on J-couplings detected via 31 P-1D refocused INADEQUATE spectroscopy. 31 The spectrum proves the absence of any P− O−P linkages in the glass with x = 0.8, where the phosphate units occur exclusively as P (2) 2Ge and P…”