Silicate Glasses at the Ionic Limit: Alkaline-Earth Sub-Orthosilicates

Container-less levitation techniques and CO 2 laser heating were used to prepare a series of silicate glasses along the compositional join (1– x )(Ca,Mg)O– x SiO 2 beyond the orthosilicate limit for the first time with the silica mole fraction x ranging between 0.33 and 0.27. Raman spectroscopic measurements in the temperature range 25−700 °C show complete absence of connectivity between the SiO 4 tetrahedra in glasses with x < 0.3. The corresponding glass structures are characterized by isolated negatively charged “tetrahedral” SiO 4 4– and O 2– anions with M 2+ (M = Mg,Ca) as the counter cations, held together by pure Coulombic (ionic) interactions. This structural view is supported by semiempirical MO simulations which were used to derive the structure of the Ca,Mg suborthosilicate glass 0.5(1– x )CaO–0.5(1– x )MgO– x SiO 2 with x = 0.286. The Raman spectra permit the experimental determination of all the vibrational modes of the isolated SiO 4 4– tetrahedra in silicate glasses.