Effect of A-site defects in Sc-doped CaTiO3 oxides on proton-oxide ion mixed conduction properties

In this study, A-site defective Ca_yTi_0.93Sc_0.07O_3-α oxides were prepared to examine their ionic conduction properties. The electrical conductivities of two typical compositions were measured as functions of oxygen partial pressure P_O2, temperature, and humidity. Additionally, phase transition, chemical expansion, and CO₂ tolerance were examined in Ca_0.985Ti_0.93Sc_0.07O_3-α using an atmosphere-controlled high-temperature X-ray diffraction. In Ca_0.947Ti_0.93Sc_0.07O_3-α, the ionic conduction domain over a wide range of P_O2 was observed at 500–800 °C, even though the humidity dependence of conductivities was confirmed only at 500 °C. Conversely, in Ca_0.985Ti_0.93Sc_0.07O_3-α, the conductivities were enhanced in humidified atmospheres at 500–800 °C, while the ionic conductivities in dry atmospheres were higher than those of 8YSZ. As protonic and oxide ionic conductivities are comparable, the proton-oxide ion mixed conduction can be considered to occur in Ca_0.985Ti_0.93Sc_0.07O_3-α. Therefore, a small percentage of Ca defect in Ca_yTi_0.93Sc_0.07O_3-α affects not only conductivity but also conductive ionic species. Furthermore, Ca_0.985Ti_0.93Sc_0.07O_3-α did not show any phase transition and chemical expansion with hydration up to 900 °C. The crystal phase of Ca_0.985Ti_0.93Sc_0.07O_3-α during the CO₂ tolerance test was observed to be stable. Therefore, the material properties of Ca_yTi_0.93Sc_0.07O_3-α suggest its high potential as electrolytes in high temperature electrochemical devices.