High temperature UV-vis-NIR spectroscopy of glasses doped with polyvalent elements
The redox behaviour of polyvalent elements (Cu, Sn, Sb, As, Cr, Mn) in glasses with the base composition 16 Na2O - 10 CaO - 74 SiO2 in temperature range between 25 and 800 °C was studied by means of high temperature UV-vis-NIR spectroscopy in the spectral range from 300 to 1100 nm. Glasses simultaneously doped with CuO and SnO, Sb2O3 or As2O3, respectively, show no influence on the structure of Cu2+O6, i.e., the position and the half bandwidth (FWHM) of the Cu2+ absorption band remain constant. Increasing temperatures led to a shift of the Cu2+ absorption band towards longer wavelengths. Simultaneously, the band is becoming broader. Glasses doped with both CuO and Sb2O3 or As2O3 showed an increase in the intensity of the absorption band at temperature > 600 °C. This was explained by the redox reaction: 2Cu+ + Sb5+ = 2 Cu2+ + Sb3+ (similar for the As5+/3+). Thermodynamic calculations based on standard enthalpy and standard entropy values measured by square-wave voltammetry qualitatively and quantitatively explain the shift in the redox ratio observed during heating (or cooling). In glasses doped with both manganese and chromium, the redox reaction, Cr3+ + 3 Mn3+ = Cr6+ + 3 Mn2+, is frozen in below 520 °C, while it is in equilibrium at temperatures > 650 °C. Though the variation of the heating rate from 20 down to 1 K/min, it is possible to get new important knowledge about the kinetics of the reaction between chromium and manganese in glasses. The relaxation times are evaluated. Thermodynamics and kinetics of the redox reaction are illustrated by numerical calculations. This is the first experimental evidence for chemical redox relaxation in glasses.