Reaching the ionic limit in the (1 − X)[Ca0.5–Mg0.5]O–XSiO2 pseudo binary glass system with 0.5 < X < 0.27: Glass formation and structure

Container-less levitation techniques and CO2-laser heating were used to prepare a series of silicate glasses along the compositional join (1 − X)(0.5Ca–0.5 Mg)O–XSiO2 with X ranging between 0.50 and 0.27; thus reaching compositions beyond the orthosilicate limit (X = 0.33). Polarized and depolarized Raman spectra measured at ambient conditions for all these glasses show systematic and smooth band intensity changes with composition. Qi-species analysis of the isotropic Raman spectra shows the presence a) of bridging oxygens in all glasses studied with silica mole fractions down to X = 0.33 and b) of free oxygen at all compositions. Beyond the X = 0.33 limit, the connectivity between the SiO4 tetrahedra decreases drastically and the corresponding glass structures are characterized by isolated negatively charged “tetrahedral” SiO44 − and O2 − anions with M2 + (M = Mg, Ca) as counter cations, held together by pure Coulombic (ionic) interactions. The Boson peak appears in the low-frequency range 70–90 cm− 1 and its polarization characteristics and frequency shifts with glass composition are pointed out and discussed.