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 Li 0.5 Na 0.5 PO 3−3x/2 N x (0<x⩽0.55) , was investigated using X-ray photoelectron spectroscopy (XPS), ³¹ P magic angle spinning (MAS) and double quantum (DQ) nuclear magnetic resonance (NMR) spectroscopies. XPS results show that nitrogen N ³⁻ exists as two-coordinated –N atoms and three-coordinated –N atoms, respectively bonded to two and three phosphorus atoms, thus forming mixed PO ₃N and PO ₂N ₂ tetrahedra. Nitrogen atoms substitute for bridging –O– as well as non-bridging O oxygen atoms and a correlation was observed between the O _1s and N _1s spectra. ³¹P MAS NMR results show that PO ₄, PO ₃N and PO ₂N ₂ 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 PO ₄ and a PO ₃ N tetrahedron. Consequently, oxynitride micro-domains grow at the expense of oxygenated areas in such a way that connectivity between PO ₄ tetrahedra is retained, even for the most nitrogen-rich glass compositions. Our data demonstrate that the nitrogen substitution for oxygen is not random.