Evidence of Redox Relaxation during Thermal Treatment of Soda Lime Silica Glasses Doped with Chromium and Manganese

A soda lime silica glass with the basic composition 74SiO 2 ·16Na 2 O·10CaO doped with MnO and Cr 2 O 3 was studied by high-temperature UV−vis−NIR spectroscopy. The measurements were carried out at temperatures in the range from 25 to 800 °C. The spectra were deconvoluted especially with respect to the absorption peaks caused by Cr 6+ and Mn 3+ . Up to a temperature of around 520 °C, the absorptivities of peaks due to Cr 6+ and Mn 3+ decreased with temperature in the same way as in glasses doped with only one of these polyvalent elements. At higher temperature, the absorptivity of peaks caused by Cr 6+ increases while those attributed to Mn 3+ decreases. This is due to the redox reaction Cr 6+ + 3Mn 2+ ⇄ Cr 3+ + 3Mn 3+, which is shifted to the left with increasing temperature. At temperatures < 520 °C, the redox reaction is frozen in. If the glass is heated more slowly, the intensity of the Cr 6+ peaks decreases at temperatures > 520 °C and re-increases at temperatures > 600 °C. If a temperature in the range from 530 to 590 °C is kept, the intensity of the Cr 6+ decreases steadily according to an exponential law. From this dependency, redox relaxation times were calculated. Thermodynamics and kinetics of the redox reaction are illustrated by numerical calculations. This is the first experimental evidence for redox relaxation in glasses.