title: Structural roles of calcium in alkaline and alkaline-earth aluminosilicate glasses by solid-state43Ca,17O and27Al NMR.
abstract: Structural roles of Ca in aluminosilicate glasses have been investigated by solid-state43Ca,17O, and27Al 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) of43Ca moves to a higher frequency and the quadrupolar coupling constant (PQ) of43Ca decreases. The change in the δisoof43Ca is more sensitive to the role of Ca than that in the PQof43Ca. The two possible roles (network modifier and charge compensator) of Ca in the glass with R=Ca are not distinguished in the43Ca 3QMAS and 5QMAS spectra. The17O 3QMAS results demonstrate that the cation with higher CFS (e.g., Mg2+in the R=Mg glass and Ca2+in the R=Na2glass) dominantly creates non-bridging oxygen, even though there is slight cation mixing. With increasing CFS in the glass, the PQof both43Ca and27Al also increases, indicating that the cation with higher CFS tends to degrade the structural symmetry.
authors.affiliation: Nippon Steel & Sumitomo Metal Corporation, 20-1 Shintomi, Futtsu, Chiba 293-8511, Japan.
authors.forename: Koji
authors.initials: K
authors.lastname: Kanehashi
keywords: (17)O NMR; (27)Al NMR; (43)Ca NMR; Aluminosilicate glass; Charge compensator; MQMAS NMR; Network modifier; Solid-state NMR