Redox equilibria in a tin doped melt with the basic composition 16 Li_2O centre dot 10CaO centre dot 74SiO_2 studied by square wave voltammetry and impedance spectroscopy

摘要

Glasses with the basic composition 16Li_2O centre dot 10CaO centre dot 74SiO_2 were doped with 0.25 mol percent SnO_2 and studied by square wave voltammetry and impedance spectroscopy at temperatures in the range 1000-1600 deg C. The square wave voltammograms recorded at temperatures >1300 deg C and step times >5 ms show one pronounced peak, while a further peak a less negative potentials is observed at temperatures <= 1300 deg C or small step times. Impedance spectra were recorded in a wide frequency range from 10~(-2) to 10~6 s~(-1) as a function of the superimposed dc potential. The obtained spectra were simulated by a simple equivalent circuit taking into account a diffusion controlled electron transfer reaction. At temperatures <=1300 deg C, a further branch was necessary in the equivalent circuit to fit the data measured in sufficient accuracy. The Warburg impedance obtained from simulation exhibits a minimum which is in agreement with the potential of the peak observed in square wave voltammetry at more negative potentials. The peak is attributed to the Sn~(4+)/Sn~(2+) redox pair. The peak at less negative potentials is attributed to the reduction of Sn~(4+) from or to an adsorbed state.