The structure of CaO–Na2O–MgO–P2O5 invert glass

The short and intermediate range order of invert glasses of the system CaO–Na2O–MgO–P2O5 have been examined by using X-ray and neutron diffraction and 31P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The depolymerization of the phosphate chains with decreasing P2O5 mol% content from 39.4 to 28.7 and simultaneous addition of MgO from 0 to 22.8 mol% are quantitatively described by the increase in the concentration of Q1 and Q0 groups. With progressive depolymerization the structure of the glasses becomes more compact indicated by the shortening of the distances between the MeOn (Me stands for the metal ion) polyhedra and between the decreasingly long PO4 chains, which correlates with the increase in packing density, w. These changes have as a consequence that the glass transition temperature, Tg, and the chemical durability continuously increase. Two effects could be a factor contributing to the glass-forming tendency of the high network modifier (>60 mol%) compositions of the invert glasses: (i) The Mg–O co-ordination number, which does not significantly change in dependence on composition, was found to be smaller in the glasses (NMgO∼5) than in the related crystal structures (NMgO=6). (ii) In the pyrophosphate region, a disproportionation of the Q1 species in the glass melt to Q0 and Q2 is indicated by the NMR results. In addition, the concentration of the Q0 and Q1 sites towards orthophosphate composition deviates from the chemical ordering model which can be explained by the occurrence of unreacted MgO. A small angle scattering effect indicating the presence of heterogeneities of electron density between 0.8 and 1.0 nm in diameter is observed for the samples, with the exception of the glass near the pyrophosphate stoichiometry. Additional information on their nature is gained from examinations of heat treated samples.