Alkali and alkali–lead oxynitride phosphate glasses: a comparative structural study by NMR and XPS

Abstract

Nitrided phosphate glasses are characterized by tetrahedral units P(O,N) 4 in which nitrogen atoms have substituted for both bridging and non-bridging double bonded oxygen atoms. 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) shows that PO 4, PO 3N and PO 2N 2 tetrahedra may coexist within the glass network. The relative proportion of these structural units as a function of the N/P ratio depends on the composition of the oxide base glass, as illustrated in sodium, lithium–sodium and lithium–sodium–lead phosphate glasses. Furthermore, 31P double quantum (DQ) MAS NMR shows that the nitrogen/oxygen substitution is not a random process. The modifier cations influence the connections between tetrahedra throughout the overall nitrided glass network, and, therefore, the final structure. N 1s X-ray photoelectron spectroscopy (XPS) shows that nitrogen atoms may exist in the P(O,N) 4 tetrahedra as doubly coordinated (–N= ) and triply coordinated (–N<) species, bonded to two and three phosphorus atoms, respectively. The relation between both kinds of nitrogen as a function of the N/P ratio depends also on the oxide-base glass composition. In this work, the thermal nitridation in flowing ammonia of alkali and alkali–lead metaphosphate glasses is studied. The results deduced from the NMR and XPS experiments make it possible, in addition to a comparison between the nitridation kinetics, to follow and to compare the structural evolution of oxynitride glasses resulting from a progressive nitrogen incorporation. In particular, the important role of PbO in the nitridation mechanism is revealed, demonstrating in this case that the nitridation is not random, its beginning included.