Thermodynamics of the Sn2+/Sn4+-equilibrium in Li2O/Al2O3/SiO2-glass melts studied by square-wave voltammetry and impedance spectroscopy

Glasses with the basic composition xLi2O · 15Al2O3 · (85−x)SiO2 (x=8.5, 11, 13.5, 16 and 18.5) were doped with 0.5 mol% SnO2 and studied by square-wave voltammetry and impedance spectroscopy in the temperature range from 1400 to 1600 °C. All square-wave voltammograms recorded exhibited a well pronounced peak attributed to the Sn2+/Sn4+-redox pair. Impedance spectra were recorded in a wide frequency range from 10−2 to 106 s−1 as a function of the dc-potential superimposed. The spectra were simulated using an equivalent circuit taking into account an electron transfer reaction from an adsorbed state. The Warburg parameter obtained from spectra simulation exhibited a minimum which within the limits of error occurred at the same potentials as the voltammetric peak. This is evidence for a electrode reaction solely controlled by diffusion. The peak potentials in melts with Li2O-concentrations of up to 13.5 mol% are equal within the limits of error, further increasing Li2O-concentration resulted in more negative peak potentials. Then, the Sn2+/Sn4+-equilibrium is shifted to the oxidized state.