Structural Characteristics of Novel Ca-Mg Orthosilicate and Suborthosilicate Glasses: Results from 29Si and 17O NMR Spectroscopy

The structural characteristics of novel alkaline-earth suborthosilicate glasses along the compositional join (1 - x) - (Ca0.5Mg0.5O) - - xSiO(2) with 0.28 <= x <= 0.33 are investigated using high resolution Si-29 and O-17 nuclear magnetic resonance spectroscopy. The structures of these glasses consist of isolated Q(0) and Q(1) anionic species and Mg2+ and Ca2+ countercations that are held together by Coulombic interactions. The concentration of the Q(1) species rapidly decreases with decreasing SiO2 content and becomes undetectable in the glass with x = 28 mol %. The compositional variation of the physical properties of these glasses such as glass transition temperature and density can be attributed to the Q-speciation in the structure. The NBOs are associated with a random distribution of the alkaline-earth cations in their nearest neighbor coordination shell. The resulting random packing of dissimilar Ca-NBO and Mg-NBO coordination polyhedra may give rise to structural and topological frustration responsible for the unusual glass-forming ability of these suborthosilicate liquids with extremely low SiO2 contents. Finally, the composition and the formation of Q(1) species necessitate the formation of free O2- ions in the structure of these glasses that are only bonded to Mg2+ and Ca2+ cations. The O-17 NMR results presented in this study allow for direct observation of such oxygen species.