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

Raman spectra at 298 K of the SiO4 symmetric stretch region (800-1200 cm(-1)) for low alkali (M) silicate glasses (5 and 10 mol% M2O, for M = Cs, K) yield intense well-resolved Q(3) peaks at similar to 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 Q3 band into two peaks. All linewidths (FWHM) of fitted Q(1), Q(2) and Q(3) species peaks are similar (similar to 35-55 cm(-1)) at 298 K for all the above glasses. The Q(3) species FWHM of 5 and 10 mol% Cs2O 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% K2O and 50 mol% Na2O 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% Na2O spectrum shows that Q(3) > Q(1), indicating similar to 4.5 mol% free oxygen in the glass, a value in good agreement with previous Si-29 NMR and O is XPS results. Lorentzian/Gaussian lineshapes now need to be considered for many more silicate glass/melt Raman spectra.