Complex IR spectra of OH–groups in silicate glasses: Implications for the use of the 4500 cm–1IR peak as a marker of OH–groups concentration

Previous studies of hydrous glasses and melts with infrared spectroscopy have led to the conclusion that the IR combination peaks near 4500 and 5200 cm-1reflect the existence of OH- (hydroxyl) groups and H2Omolwater molecules in those materials. Here, we show that the glass chemical composition can impact profoundly the intensities and frequencies of the fundamental O-H stretching signal and, therefore, potentially those of the 4500 and 5200 cm-1combination peaks. In alkali silicate glasses, compositional effects can give rise to peaks assigned to fundamental O-H stretching at frequencies as low as 2300 cm-1. This expanded range of Raman intensity assigned to O-H stretch is increasingly important as the ionic radius of the alkali metal increases. As a result, the combination of the fundamental O-H stretch in OH-groups with the Si-O-H stretch located near 910 cm-1gives rise to a complex combination signal that can extend to frequencies much lower than 4200 cm-1. This combination signal then becomes unresolvable from the high-frequency limb of the band assigned to fundamental O-H stretch vibration in the infrared spectra. It follows that, when O-H stretch signals from OH- groups extend to below 3000 cm-1, the 4500 cm-1peak does not represent the total OH-signal. Under such circumstances, this infrared peak may not be a good proxy for determining the concentration of OH-hydroxyl groups for glassy silicate materials.