³¹P, ¹¹B, ²⁹Si and ²³Na solid state NMR studies of phospho-borosilicate glasses: towards the understanding of crystal formation

The aim of this work is to understand the main driving force of the crystallisation behaviour that occurs with the addition of P₂O₅ to a Na₂O–B₂O₃–SiO₂ glass system with fixed component ratio. We apply ³¹P, ¹¹B, ²⁹Si and ²³Na solid state NMR in combination with previously published x-ray diffraction (XRD) patterns and Rietveld analysis on the same samples to characterise the distribution of bridging and nonbridging oxygens in the glass matrices and the crystal compositions. We use the quantitative fractions of bridging and nonbridging oxygens per nucleus as obtained from the NMR experiments to characterise the oxygen partitioning between the nuclei. The strength of this analysis is that no information about the nature of the next nearest neighbours is necessary. Given the constant^[3]B:^[4]B ratio (within signal-to-noise) our analysis shows that the oxygen requirements of P₂O₅, which depolymerises compared to its starting component, is met by the silica network, which in turn polymerises. Our estimates indicate that the depletion of nonbridging oxygens from the silica network initiates the crystal formation in the form of, first, Na₃PO₄ domains, and then Na₄P₂O₇ crystals, consistent with phosphorus’s highest cation field strengths between components.