VIBRATIONAL SPECTROSCOPY OF THE AMORPHOUS SILICATES

It has been demonstrated that the Si2O unit instead of the SiO4 unit approach would much better explain the IR spectra of crystalline and vitreous silica and framework alumino-silicates. In this approach all modes are lattice vibrations other than Si2O unit vibrations. Within the lattice vibrations it is suggested to distinguish: high-wavenumber pseudo-lattice vibrations in the 700-400 cm-1 range and low-wavenumber lattice vibrations below 300 cm-1. The first range is responsible for some kind of order larger than a single unit, while the second one covers the long range order in the structure. Similarities between the spectra of crystalline and vitreous forms of respective silicates indicate a structural relationship between the medium range structural order of glass and the relevant crystal structure. The detailed discussion and conclusions are based on the far-infrared, mid-infrared and Raman spectra measured for vitreous and crystalline potassium silicates and potassium alumino-silicates. The spectra of silica polymorphs measured at different temperatures are also presented and discussed.