Factors affecting line shapes and intensities of Q3 and Q4 Raman bands of Cs silicate glasses

Abstract

The Raman spectra of glasses containing 0 to 30 mol% Cs2O have been fit successfully with line shapes of dominantly Lorentzian character for the Q3 species, allowing quantification of Q3 and Q4 species intensities (Q represents a Si tetrahedron and the superscript indicates the number of bridging oxygen atoms, BOs, bonded to Si.) The intensity of the Q4 species A1 symmetric stretch is exceptionally weak in vitreous silica (v-SiO2) but it increases dramatically with addition of small amounts of Cs2O to the glass. We propose that Cs, where in close proximity to BO of Q4 species, promotes the polarizability of Q4 tetrahedra and these primed Q4 species (Q4-p) produce a strong Q4 signal. There are, therefore, two variants of the Q4 species, a Q4-p species which produces a strong signal, and an unprimed species (Q4-u) which yields a very weak signal. The increase in the abundance of the primed Q4 species (Q4-p) can be modelled as a function of alkali content using a simple, upper-bounded growth model:XQ4-p = (1 − e−kx)where XQ4-p is the fraction of polarizable Q4 species, k is a constant and x is the mol% counter oxide in the glass (here Cs2O). Comparison of calculated XQ4-p values with experimental results indicates that its cross-section is similar to that of the Q3 species.There is no evidence for a ~1050 cm−1 band in the 5 mol% Cs2O glass spectrum but in the 30 mol% Cs2O glass spectrum about 11% of spectral intensity is observed at about this frequency. The intensity likely results from development of asymmetry on the Q3 band, which increases with Cs2O content of the glass. The asymmetry results from weakened SiO force constants of some Q3 tetrahedra due to charge transfer via CsBO bonds. As evidence, Si 2p and O 1s X-ray Photoelectron Spectroscopic (XPS) studies demonstrate that the electron density over Si and BO atoms of Q4 species increases with Cs2O content. With charge transfer to tetrahedra, the negative charge accumulates preferentially on Si atoms thus decreasing SiO coulombic interactions, weakening SiO force constants, and shifting the Q3 A1 symmetric stretch vibrational frequencies to lower values (e.g., from ~1100 cm−1 to ~1050 cm−1). The fraction of affected Q3 species increases with alkali content, as does the Q3 peak asymmetry. The Raman shifts of the Q4 species are also affected by increased Cs2O contents.