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

摘要

Nitrided phospahte glasses are characterized by tetrahedral units P(O,N)_4 in which nitrogen atoms have substituted for both bridging and non-briding double bonded oxygen atoms. ~(31)P 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, ~(31)P double quantum (DQ) MAS NMR shows that the nitrogen/oxygen substitution is not a random process. The modifier cations influence the connections between tetrahedra through-out 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 coordianted (-N =) and triply coordianted (-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.