Structural study of magnesium polyphosphate glasses

Magnesium polyphosphate glasses with molar ratios, y=n(MgO)/n(P2O5), ranging from 1.0 to 1.9 have been examined by X-ray and neutron scattering and 31P magic angle spinning nuclear magnetic resonance spectroscopy to extract information on their short-range, intermediate-range and submicroscopic structure. The depolymerization of the PO4 chains with rising MgO content is quantitatively described by the increasing concentration of Q1 and Q0 groups determined by NMR. In the pyrophosphate region the Q1 sites disproportionate to Q0 and Q2 groups, whereas there is no disproportionation of the Q2 sites at metaphosphate composition. The shortening of the real-space distances, rm, indicates that the structure of the glasses becomes more compact with progressive depolymerization which is due to the increasing connection of the MgOn polyhedra by sharing the non-bridging oxygen atoms. The Mg–O co-ordination sphere was found to change not significantly in dependence on composition. Heterogeneities about 1 nm in diameter exist in the glasses with MgO content exceeding 46.6 mol% indicated by a weak small angle X-ray scattering.