Pressure-induced structural transformations in phosphorus oxynitride glasses

• Sodium metaphosphate oxynitride glasses with varying N/P ratio are subjected to hot compression at 1 GPa.
• Raman, 31P, 23Na NMR spectroscopy, density, and indentation measurements are performed.
• N/P ratio influences the type and extent of pressure-induced changes in structure and properties.
• Mechanism for the pressure-induced structural transformations is proposed.

The structure-property relations of permanently densified sodium metaphosphate oxynitride glasses (NaPO3 − 3x/2Nx) with varying N/P ratio are investigated. Densification of the bulk samples is achieved through 1 GPa isostatic compression at the glass transition temperature. The pressure-induced structural transformations are characterized by Raman spectroscopy and solid state 31P and 23Na NMR spectroscopy, whereas the glass hardness and brittleness are quantified through Vickers indentation. Nitridation of the ambient pressure sodium metaphosphate glass (NaPO3) results in a step-wise conversion of PO4 units into PO3N and PO2N2 oxynitride species. Upon hot compression, the glasses become permanently denser and harder, but also more brittle. This pressure-induced densification is not accompanied by changes in the types or fractions of the structural units for the NaPO3 glass. In contrast, densification of the oxynitride glasses (N/P > 0) is accompanied by pressure-induced structural transformations between the oxynitride species. Based on the 31P NMR results, a structural mechanism for these transformations is proposed.