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

The Raman spectra of glasses containing 0 to 30 mol% Cs2O have been fit successfully with line shapes of dominantly Lorentzian character for the Q(3) species, allowing quantification of Q(3) and Q(4) 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 Q(4) species A(1) 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 Q(4) species, promotes the polarizability of Q(4) tetrahedra and these primed Q(4) species (Q(4)-p) produce a strong Q(4) signal. There are, therefore, two variants of the Q(4) species, a Q(4)-p species which produces a strong signal, and an unprimed species (Q(4)-u) which yields a very weak signal. The increase in the abundance of the primed Q(4) species (Q(4)-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 Q(4) species, k is a constant and x is the mol% counter oxide in the glass (here Cs2O). Comparison of calculated XQ(4-p) values with experimental results indicates that its cross-section is similar to that of the Q(3) species. There is no evidence for a similar to 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 Q(3) band, which increases with Cs2O content of the glass. The asymmetry results from weakened Si-O force constants of some Q(3) tetrahedra due to charge transfer via Cs-BO 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 Q(4) species increases with Cs2O content. With charge transfer to tetrahedra, the negative charge accumulates preferentially on Si atoms thus decreasing Si-O coulombic interactions, weakening Si-O force constants, and shifting the Q(3) A(1) symmetric stretch vibrational frequencies to lower values (e.g., from similar to 1100 cm(-1) to similar to 1050 cm(-1)). The fraction of affected Q(3) species increases with alkali content, as does the Q(3) peak asymmetry. The Raman shifts of the Q(4) species are also affected by increased Cs2O contents.