IR peak shift due to phase separation of Na2O–SiO2 system glasses

The IR reflection peak shift of a silica structural band was found to be a sensitive measure of phase separation in glasses. In order to examine the details of the origin of the peak shift, a model system glass, Na2O–SiO2, with a well-established immiscibility boundary was examined. Selected glasses in the system were heat-treated for phase separation and the IR reflection peak wavenumber was measured as a function of heat-treatment time and temperature. The IR peak wavenumber approached constant values, which varied systematically with the heat-treatment temperature. The observed constant values of the IR peak wavenumber were compared with the IR peak positions of the spectra synthesized by summing spectra of separated phase compositions at appropriate molar ratios. The peak wavenumbers obtained by the synthesis agreed reasonably well with the observed values. It appears that the main cause of the phase separation-related IR peak shift is the compositional shift weighted by the fractional contribution of the separated phases.