Vibrational properties of silicates: A cluster model able to reproduce the effect of “SiO4” polymerization on Raman intensities

An ensemble of ten silicate clusters is examined using quantum chemical calculations (Density Functional Theory) as a potential model to study the effect of polymerization of the SiO4 units on Raman intensities of silicates (crystalline and amorphous). Quite originally, instead of saturating non-bridging oxygen (NBO) with hydrogen atoms as generally found in the literature for similar approaches, NBOs, which hold a negative charge if not saturated, have been substituted by isoelectronic fluorines whose mass is corrected in normal mode calculations to be equal to that of an oxygen. By adjusting the number of fluorines per silicon, the different Qn coordination types characterizing the different classes of silicates are modeled. The relevance of this ensemble of clusters as a model to study the effect of polymerization on Raman intensities is established in several steps, the most important one being the qualitative reproduction of evolution of the Ip polymerization index with the number of bridging oxygen per silicon.