Vibrational properties of silica species in MgO-SiO2 glasses obtained from ab initio molecular dynamics

We report the vibrational properties of silica species in magnesio-silicate glasses, obtained from ab initio molecular dynamics. The mode-projection method employed in this study decomposes the vibrational density of states of silica species into subspectra resulting from normal-mode-like vibrations of mainly two fundamental structural subunits: the SiO44- tetrahedron and the Si- O-Si bridging oxygen (BO). This decomposition reveals the mode-specific frequency shifts as a function of tetrahedral polymerization. The method is validated by application to crystalline Mg2SiO4 at 300 K, and by comparison to results obtained from normal mode analysis (NMA). Our main findings are: (1) The frequency of the spectroscopically important tetrahedral symmetric stretching mode v(1)(TET) of Q(1)- to Q(4)-species is in general lower than commonly assumed. The Q(2)-species shows a double instead of a single peak. (2) The BO asymmetric stretching mode v(3)(BO) contributes to the vibrational density in the region 900-1200 cm(-1). If this contribution is not considered in the fitting of Raman intensity in the high-frequency region between 800 cm(-1) and 1200 cm(-1) and spectra are explained by tetrahedral contributions of Q(n)-species only, then the degree of polymerization of a glass is likely to be overestimated. (3) The Si2O76- dimer, which is an important structural unit in silica-poor MgO-SiO2 glasses, possesses a specific ethane-like symmetric stretching vibration at about 935 cm(-1).(C) 2012 Elsevier B.V. All rights reserved