Nitridation of alkali metaphosphate glasses: a comparative structural analysis of the Na–P–O–N and Li–Na–P–O–N systems

Phosphates, as aluminosilicates, are a major family of glasses characterized by XO4 tetrahedral units, in which nitrogen can substitute for oxygen, thus forming oxynitride glasses. As shown by X-ray photoelectron spectroscopy (XPS), nitrogen N3− may exist in P(O,N)4 tetrahedra as doubly coordinated (N) and triply coordinated (N<) atoms, respectively, bonded to two and three phosphorus atoms. They replace non-bridging (OP) as well as bridging (P–O–P) oxygen atoms, thus inducing a more covalent and cross-linked structure, and a correlation is observed between the O1s and N1s spectra. 31P magic angle spinning nuclear magnetic resonance (MAS NMR) results show that three kinds of tetrahedra, PO4, PO3N and PO2N2, coexist within the glass network. The thermal nitridation in flowing ammonia of alkali metaphosphate glasses is particularly studied here and a comparative XPS and NMR structural analysis of `NaPONʹ and `LiNaPONʹ phosphorus oxynitride glass series of increasing nitrogen content is presented.