Acceptor-doped BaPrO₃ bulk ceramic is a proton conducting material in the intermediate-temperature (IMT) region from 300 to 700 °C. In particular, the electrical conductivity at 500 °C of Gd³⁺- or Yb³⁺-doped BaPrO₃ is ~1.0×10¹ Scm⁻¹ in wet atmosphere. Furthermore, the occurrence of electron (or hole)–ion mixed conduction, which is expected as the electrode material of a solid state fuel cell (SOFC) using an oxide proton conductor, has also been proposed in the IMT region of Yb³⁺-doped BaPrO₃.

In this study, in-plane-oriented BaPrO_3−δ thin film with mixed valence states has been prepared on an Al₂O₃(0001) substrate by RF magnetron sputtering. With increasing crystallization temperature (T_sub), the lattice constant decreases and the orientation changes from the a-axis to the b-axis. The thin film prepared above T_sub = 800 °C exhibits a higher proton conductivity than bulk ceramics. The conductivity below 400 °C decreases with oxygen gas partial pressure, indicating the existence of hole-ion mixed conduction. The valence band consists of O 2p states hybridized with the Pr⁴⁺ (4f⁰) and Pr³⁺ (4f¹L) states, which are closely related to the mixed conduction. The energy difference between the top of the valence band and the Fermi level corresponds to the activation energy of holes for the total conductivity below 400 °C.