Redox-Active Impurity Ions in Solid Electrolytes and their Influence on Transport Properties

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Abstract
The importance of redox-active impurities in solid electrolytes is discussed. As long as the content of these impurities is negligible with respect to the aliovalent oxide content (e.g., the yttria concentration in stabilized-ZrO2), the equilibrium concentrations of electrons and holes are not affected by the impurities. However, they may have a considerable influence on the chemical diffusion of oxygen by the internal source/sink effects as well as on the electronic conductivity directly under conditions where the interaction with the gas phase is frozen in. Defect chemistry and transport equations for solid electrolytes are given including the ionization equilibrium reactions of ions of variable charges. The valence state and its change of doped transition metal ions (Ti, V, Cr, Mn, Fe, Co, Ni, Cu) and selected rare earth ions (Ce, Pr, Gd) in 9.5 mol%Y2O3-stabilized ZrO2 single crystals as a model solid electrolyte are identified by electron paramagnetic resonance (EPR7ESR) and optical absorption spectroscopy (NIR, VIS, UV). The single crystals, annealed at 800°C in a po2 range from 1.0 down to 10−20 bar, are used in this study. The redox-active impurity ions in the solid electrolyte are specified by both methods.

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