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

Abstract Glasses with the basic composition x Li 2 O · 15Al 2 O 3 · (85− x )SiO 2 ( x =8.5, 11, 13.5, 16 and 18.5) were doped with 0.5 mol% SnO 2 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 Sn 2+ /Sn 4+ -redox pair. Impedance spectra were recorded in a wide frequency range from 10 −2 to 10 6 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 Li 2 O-concentrations of up to 13.5 mol% are equal within the limits of error, further increasing Li 2 O-concentration resulted in more negative peak potentials. Then, the Sn 2+ /Sn 4+ -equilibrium is shifted to the oxidized state.