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

摘要

Container-less levitation techniques and CO2 laser heating were used to prepare a series of silicate glasses along the compositional join (l—x)(Ca,Mg)O—xSiO2 beyond the orthosili-cate 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 SiO4 tetrahedra in glasses with x < 0.3. The corresponding glass structures are characterized by isolated negatively charged "tetrahedral" SiO4~(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 semiempi-rical MO simulations which were used to derive the structure of the Ca,Mg suborthosilicate glass 0.5(l-x)CaO-0.5(l-x)MgO-xSiO2 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.