Glass formation and Raman spectra of CaO–SiO2 glasses towards the orthosilicate limit

A series of silicate glasses formed in the binary system (1-X)CaO-XSiO₂with silica mole fractions X ranging from 0.61to 0.38have been prepared using container-less aerodynamic levitation techniques and CO₂-laser heating. Glasses with X<0.45were prepared for the first time but, no glass formation was possible at compositions X<0.38. Ambient temperature polarized and depolarized Raman spectra were measured for all these glasses. Qⁱ-speciation analysis of the isotropic Raman spectra shows that near X～0.38the predominant structures present are the SiO₄^4? tetrahedra and the single bridged Q¹species. Oxygen bridging was present at all compositions studied while at X<0.45 small amounts of free oxygen anions was present. The data are compared with the resent NMR measurements obtained with the same glass samples used in the present study. Stokes and anti-Stokes Raman spectra were measured in low frequencies revealing the Boson peak (BP) at ～50 and ～70 cm^?1 for the corresponding polarized and depolarized configurations. On the Stokes side the BP frequencies exhibit a fictional shift due to contributions from the low frequency vibrational modes of the glass.