Structural studies of NaPO3–WO3 glasses by solid state NMR and Raman spectroscopy

Abstract

Vitreous samples were prepared in the (100 − x) NaPO₃–x WO₃ (0 ≤ x ≤ 70) glass forming system using conventional melting–quenching methods. The structural evolution of the vitreous network was monitored as a function of composition by thermal analysis, Raman spectroscopy and high resolution one- and two-dimensional ³¹P solid state NMR. Addition of WO₃ to the NaPO₃ glass melt leads to a pronounced increase in the glass transition temperatures, suggesting a significant increase in network connectivity. At the same time Raman spectra indicate that up to about 30 mol% WO₃ the tungsten atoms are linked to some non-bridging oxygen atoms (W–O⁻ or WO bonded species), suggesting that the network modifier sodium oxide is shared to some extent between both network formers. W–O–W bond formation occurs only at WO₃ contents exceeding 30 mol%. ³¹P magic angle spinning (MAS)-NMR spectra, supported by two-dimensional J-resolved spectroscopy, allow a clear distinction between species having two, one, and zero P–O–P linkages. The possible formation of some anionic tungsten sites suggested from the Raman data implies an average increase in the degree of polymerization for the phosphorus species, which would result in diminished ³¹P/²³Na interactions. This prediction is indeed confirmed by ³¹P{²³Na} and ²³Na{³¹P} rotational echo double resonance (REDOR) NMR results, which indicate that successive addition of WO₃ to NaPO₃ glass significantly diminishes the strength of phosphorus–sodium dipole–dipole couplings.