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
Résumé
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 Q3 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 Q3 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 Q1, Q2 and Q3 species peaks are similar ( 35-55 cm-1) at 298 K for all the above glasses. The Q3 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 Q2 or Q3 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 Q3 > Q1, indicating 4.5 mol% free oxygen in the glass, a value in good agreement with previous 29Si NMR and O 1s XPS results. Lorentzian/Gaussian lineshapes now need to be considered for many more silicate glass/melt Raman spectra.