Relaxation dispersion of ionic conductivity in a Zr_0·85Ca_0·15O_1·85 single crystal

The dynamic behavior of the oxygen ion conductivity of a cubic Zr_0·85Ca_0·15O_1·85 single crystal has been investigated with AC impedance spectroscopy and a dynamic pulse method as a function of both temperature and frequency between 450 and 1200 K and 20 and 10⁸ Hz. This is the frequency-temperature range where the relaxation dispersion of the ionic conductivity can be observed. From the temperature dependence of the relaxation frequency, the diffusion coefficient and the mobility of oxygen vacancies were determined. In the entire temperature range investigated, the temperature dependence of the ionic conductivity of a Zr_0·85Ca_0·15O_1·85 single crystal arises exclusively from the temperature dependence of the mobility of oxygen vacancies, the concentration of which remains constant with temperature and is equal to the concentration of all extrinsic oxygen vacancies created by calcia stabilizing. No transition in the Arrhenius plot of the ionic conductivity due to a gradual dissociation of oxygen vacancy-defect cation associates, as proposed in the literature, has been observed. A simple model for the temperature dependence of the ionic conductivity of solid electrolytes in terms of the parallel and serial combination of RC-elements is given.