Determination of electronic and ionic conductivity in mixed ionic conductors: HiTEC and in-situ impedance spectroscopy analysis of isovalent and aliovalent doped BaTiO3

The ionic and electronic conductivities of nonstoichiometric BaTiO3 (undoped, Ca-doped, and Zr-doped BaTiO3–δ) ceramics were investigated through high temperature equilibrium conductivity (HiTEC) and in-situ impedance measurements at various equilibrium conditions with different oxygen partial pressures over a temperature range of 950–1050 °C. Contribution of mobile oxygen vacancies on the electrical conductivity has been determined by HiTEC measurement as a function of oxygen partial pressure; the electrical conductivity with mobile oxygen vacancies shows a broad transition from p-type to n-type, and thereby there is an increase of the minimum conductivity at the n–p transition point. Through combining in-situ impedance spectroscopy measurements with the HiTEC measurements, it was confirmed clearly that the mobile oxygen vacancy contributes to the total conductivity, and the oxides become mixed conductors around the n–p transition regime (minimum electronic conductivity regime). It was found that Warburg impedance can be observed at the condition of tion/telectronic ≳ 0.05 in the temperature range of 950–1050 °C and pO2 range of 0.95–10− 16 atm. The ionic conductivity varied with the concentration of extrinsic oxygen vacancies and dopants, and the activation energy for mobility of oxygen vacancy in Ca-doped BaTiO3–δ was found to be 1.04 ± 0.05 eV using the two techniques in a very good agreement.