Defect structure analysis of proton-oxide ion mixed conductor BaCe _0.9Nd_0.1O_{3 - δ}

The defect chemical relationship in Nd doped BaCeO₃ was discussed based on the defect concentration determined by thermogravimetry and electrical conductivity measured by D.C. 4-terminal method as a function of oxygen partial pressure, P(O₂), water vapor partial pressure, P(H₂O), and temperature. In BaCe_0.9Nd_0.1O _{3 - δ}, the defect concentration strongly depended on P(O ₂), P(H₂O) and temperature. This is different from the results of other acceptor-doped BaCeO₃ such as BaCe_0.9 M_0.1O_{3 - δ} (M = Y and Yb), which the concentration of protonic defects is independent of P(O₂). The electrical conductivity of BaCe_0.9Nd_0.1O_{3 - δ}, showed unique P(O₂) dependence at high P(O₂) atmospheres compared to other acceptor-doped BaCeO₃. At temperature above 973 K, σ increased with increasing P(O₂). However, at temperature below 873 K, σ decreased with increasing P(O₂). This is also different from the general conductivity scheme in other acceptor-doped BaCeO₃ that the σ values increase with increasing P(O ₂) in high P(O₂) atmospheres. A different behavior of Nd doped BaCeO₃ was explained by considering the changes of oxygen vacancy concentration accompanied by the valence changes of Nd ions between trivalent and tetravalent states.