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

Abstract

Phosphates, as aluminosilicates, are a major family of glasses characterized by XO 4 tetrahedral units, in which nitrogen can substitute for oxygen, thus forming oxynitride glasses. As shown by X-ray photoelectron spectroscopy (XPS), nitrogen N 3− 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 O 1s and N 1s spectra. 31P magic angle spinning nuclear magnetic resonance (MAS NMR) results show that three kinds of tetrahedra, PO 4, PO 3N and PO 2N 2, 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 `NaPON' and `LiNaPON' phosphorus oxynitride glass series of increasing nitrogen content is presented.