Transition from a phosphate to niobate network structure in vitreous Nb2O5-NaPO3

The structure of (Nb2O5)x(NaPO3)1-x glasses was re-visited by combining neutron and high energy x-ray diffraction with Raman scattering over a wide composition range. The results were interpreted by reference to the phosphorus atom speciation found from a novel analysis of 31P magic angle spinning nuclear magnetic resonance spectra [Ensuncho et al., J. Am. Chem. Soc. 147, 31147 (2025)]. The results indicate a distorted octahedral coordination environment for the Nb atoms across the composition range. The measured x-dependence of (i) the mean numbers of non-bridging oxygen (NBO) atoms and P-O-P and P-O-Nb connections per phosphate group, and (ii) the fraction of oxygen atoms in Nb-O-Nb connections, are described by a self-consistent analytical model in which there is a preferential formation of heteronuclear P-O-Nb bonds within a network structure formed by 4- and 6-coordinated P and Nb atoms, respectively, such that P-O-P connections are absent when the niobia content exceeds x ∼ 0.22. At smaller x, the non-bridging oxygen atoms are distributed among the P- and Nb-centered polyhedral units. The model provides a connectivity density that accounts for the rapid increase in the glass transition temperature with increasing Nb2O5 content and shows that the enhancement to the non-linear optical properties for x > 0.2 is related to a more rapid increase with x in the fraction of oxygen atoms involved in polarizable Nb-O-Nb connections. The methodology also suggests that the dissolution rate measured for the (Nb2O5)x(Na2O)0.4(P2O5)0.6-x glass series is dependent on the proportion of P-O-P linkages.