Effects of electronegativities and charge delocalization on Q2 Raman shifts of alkaline- and alkaline earth-bearing glasses and metasilicate crystals

Abstract

Abstract Raman shifts of the symmetric stretch of silicate Q2 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 Q2 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 Q2 tetrahedron. The charge transferred to NBO is redistributed (delocalized) over all atoms of the tetrahedron by the four equivalent Si sp3 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 Q2 species. Density functional theoretical (DFT) calculations on six staggered and eclipsed M6Si2O7 (M = Li, Na, K) molecules corroborate the findings. Charge is transferred from the metal atoms to NBO and delocalized over tetrahedra in accordance with Li, Na, and K electronegativities. Calculated Si-O force constants and Raman shifts decrease with decreasing electronegativity of the cation but surprisingly, calculated Si-NBO bond lengths are largely unaffected, with all being similar at 1.665 ± 0.003 Å.