X-ray photoelectron spectroscopy and nuclear magnetic resonance structural study of phosphorus oxynitride glasses, ‘LiNaPON’

In this study, the structure of mixed alkali nitrided phosphate glasses ‘LiNaPON’, of composition Li0.5Na0.5PO3−3x/2Nx (0<x⩽0.55), was investigated using X-ray photoelectron spectroscopy (XPS), 31P magic angle spinning (MAS) and double quantum (DQ) nuclear magnetic resonance (NMR) spectroscopies. XPS results show that nitrogen N3− exists as two-coordinated –N atoms and three-coordinated –N atoms, respectively bonded to two and three phosphorus atoms, thus forming mixed PO3N and PO2N2 tetrahedra. Nitrogen atoms substitute for bridging –O– as well as non-bridging O oxygen atoms and a correlation was observed between the O1s and N1s spectra. 31P MAS NMR results show that PO4, PO3N and PO2N2 tetrahedra coexist within the glass network. From our 31P MAS and double quantum NMR results, we propose a model of nitrogen/oxygen substitution in which nitrogen atoms preferentially substitute for oxygen atoms shared by a PO4 and a PO3N tetrahedron. Consequently, oxynitride micro-domains grow at the expense of oxygenated areas in such a way that connectivity between PO4 tetrahedra is retained, even for the most nitrogen-rich glass compositions. Our data demonstrate that the nitrogen substitution for oxygen is not random.