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

• A set of ten SixOyFz clusters is designed to model the different silicate classes.
• Normal modes of SixOyFz clusters are computed by Density Functional Theory.
• The effect of the number of bridging O per Si on Raman intensities is examined.
• The Ip polymerization index evolves similarly in SixOyFz clusters and in silicates.
• SixOyFz clusters can be used to rationalize Raman intensities of silicates.

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.