Proton conduction of BaCe_0.90Y_0.10O_{3 -δ} Thin film with lattice distortion

The proton conduction of BaCe_0.9Y_0.1O_{3 -δ} (BCY) film on Al₂O₃ (0001) substrate grown by RF magnetron sputtering has been studied as a function of film thickness (20-600 nm) in the temperature region of 300-700 °C. The lattice constants and grain size of the as-deposited film increase with increasing film thickness. The film thickness above ~ 200 nm, which close to the lattice constants of the bulk, shows high proton conductivity with small activation energy (E_A) of ~ 0.3 eV below 500 °C. The BCY thin film has the mixed valent states of Ce^{4 +} and Ce^{3 +} created by the charge transfer from O 2p to Ce 4f states. The E_A corresponds to the energy difference between the top of valence band and the Fermi level. These results originate to the changes of O-O bond length and oxygen vacancies in the a-c plane of the BCY thin film.