Structure of alkali magnesium, zinc, and calcium metasilicate glasses

The structure of the metasilicate composition glasses (A2O)x(XO)0.50-x(SiO2)0.50, with A = Na or K, X = Mg, Zn, or Ca, and x = 0.25 or 0.33, was investigated by combining neutron and high-energy x-ray diffraction with Raman scattering and 29Si and 25Mg magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The latter employed the rotor-assisted population transfer approach for signal enhancement. The diffraction results show a substantial population of four-coordinated X2+ cations in the majority of the magnesium- and zinc-bearing glasses. Based on the average degree of polymerization of the silicate networks obtained from the solid-state 29Si NMR results, and supported by the findings from Raman spectroscopy, no compelling evidence could be found for a network-forming role for the four-coordinated Mg2+ and Zn2+ species. The relationship between the Mg-O coordination numbers measured by diffraction and the mean isotropic chemical shifts found from 25Mg MAS NMR spectroscopy is considered for a variety of silicate glasses. A clear correlation between these parameters could not be found.