Lorentzian dominated lineshapes and linewidths for Raman symmetric stretch peaks (800–1200 cm−1) in Qn (n = 1–3) species of alkali silicate glasses/melts

Abstract Raman spectra at 298 K of the SiO 4 symmetric stretch region (800–1200 cm −1 ) for low alkali (M) silicate glasses (5 and 10 mol% M 2 O, for M = Cs, K) yield intense well-resolved Q 3 peaks at ~1100 cm −1 with mostly Lorentzian character, in contrast to previous Gaussian fits for silicate glasses. The spectra of 5 mol% Na and Li glasses show an additional Q 3 peak. It results from close approach of alkalis (M) to bridging oxygen (BO) which splits the Q 3 band into two peaks. All linewidths (FWHM) of fitted Q 1 , Q 2 and Q 3 species peaks are similar (~35–55 cm −1 ) at 298 K for all the above glasses. The Q 3 species FWHM of 5 and 10 mol% Cs 2 O silicate glasses show a T dependence similar to those of crystal silicate spectra: both increase by 35–45 cm −1 from 298 K to 1200 K. The T dependence of the FWHM of these Lorentzian dominated peaks are consistent with theory. The more complex spectra of the 30 mol% K 2 O and 50 mol% Na 2 O glass spectra are then readily fit with two or three Q 2 or Q 3 peaks, constraining the % Lorentzian character of all peaks from the fit to the resolved peak in those spectra. The 50 mol% Na 2 O spectrum shows that Q 3  > Q 1 , indicating ~4.5 mol% free oxygen in the glass, a value in good agreement with previous 29 Si NMR and O 1s XPS results. Lorentzian/Gaussian lineshapes now need to be considered for many more silicate glass/melt Raman spectra.