Structural roles of calcium in alkaline and alkaline-earth aluminosilicate glasses by solid-state 43Ca, 17O and 27Al NMR.

Structural roles of Ca in aluminosilicate glasses have been investigated by solid-state 43Ca, 17O, and 27Al NMR spectroscopy. In 15Al2O3-55SiO2-15CaO-15RO (R=Mg, Ca, Sr, and Na2) (mol%) glass systems wherein half of the alkaline plus alkaline-earth cations charge-compensate (AlO4)‒ tetrahedra and the other half modify the glass network, with decreasing cation field strength (CFS) in the order of Mg2+>Ca2+>Sr2+>Na+, the isotropic chemical shift (δiso) of 43Ca moves to a higher frequency and the quadrupolar coupling constant (PQ) of 43Ca decreases. The change in the δiso of 43Ca is more sensitive to the role of Ca than that in the PQ of 43Ca. The two possible roles (network modifier and charge compensator) of Ca in the glass with R=Ca are not distinguished in the 43Ca 3QMAS and 5QMAS spectra. The 17O 3QMAS results demonstrate that the cation with higher CFS (e.g., Mg2+ in the R=Mg glass and Ca2+ in the R=Na2 glass) dominantly creates non-bridging oxygen, even though there is slight cation mixing. With increasing CFS in the glass, the PQ of both 43Ca and 27Al also increases, indicating that the cation with higher CFS tends to degrade the structural symmetry.