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Raman shifts of the symmetric stretch of silicate Q 2  species vary over a range of ~90 cm −1  in crystals and glasses containing alkali and alkaline earth oxides. The shifts display a striking, sympathetic relationship with the electronegativity of the alkali and alkaline earth metals (M), with the highest frequency observed for Mg-silicate glasses and crystals and the lowest frequency for Cs-bearing glasses. Frequencies are determined primarily by the electron density on constituent Si and O atoms of the Q 2  tetrahedra, as measured by Si 2p and O 1s X‑ray photoelectron spectra (XPS). The electron density is, in turn, determined by the extent to which electronic charge is transferred from the modifier metal “M” to the NBO of the Q 2  tetrahedron. The charge transferred to NBO is redistributed (delocalized) over all atoms of the tetrahedron by the four equivalent Si sp 3  orbitals. Although negative charge accumulates on all atoms of the tetrahedron, it accumulates preferentially on Si. Coulombic interactions among Si and all O atoms are thus weakened, resulting in decreased force constants and lowered symmetric stretch frequencies of Q 2  species.

american mineralogist

Effects of electronegativities and charge delocalization on Q 2  Raman shifts of alkaline- and alkaline earth-bearing glasses and metasilicate crystals