The influence of modifier cations on the Raman stretching modes of Qn species in alkali silicate glasses

Abstract

AbstractThe Raman spectra of alkali silicate glasses containing 5 to 30 mol % M2O (M = Li, Na, K, Rb, and Cs) have been fit successfully with pseudo‐Voigt lineshapes of dominantly Lorentzian character in order to quantify the Qn species distributions. This differs from the more popular Gaussian lineshapes which have been used for the past four decades. There is an increase in asymmetry in the Q3 band, with increasing M2O content which appears to result from the weakened Si‐O force constants of some Q3 bands due to charge transfer via M‐BO bonds. With charge transfer to the tetrahedra, the negative charge accumulates preferentially on Si atoms thus decreasing Si‐O Coulombic interactions, weakening Si‐O force constants, and shifting the Qn A1 symmetric stretch vibrational frequencies to lower values (eg, from ∼1100 cm−1 to ∼1050 cm−1). The fraction of affected Q3 species increases with alkali content, as does the Q3 peak asymmetry. We propose that this extends through to all the Qn species and postulate that there are multiple vibrational modes for each Qn species which are dictated by their proximity to network modifier cations.