Phase transition in the oxygen and proton conductor Ba2In2O5 in humid atmospheres below 300°C

Abstract The well known oxygen ion and proton conductor Ba 2 In 2 O 5 transforms to a new hydrate-like structure of composition Ba 2 In 2 O 5 ·1H 2 O (labelled β here) when exposed to humidity below 300°C. The water vapor pressure curve over the coexisting α- and β-phases is described by the following thermodynamic parameters: Δ H 0 =−0.89 eV and Δ S 0 =−1.27×10 −3 eV/K. X-ray powder diffraction also provided evidence for the above phase transition. DTA experiments showed a large endothermic peak at the transition temperature. A tentative quasi-binary phase diagram of the system Ba 2 In 2 O 5 –water vapor is presented. The new proton-rich phase Ba 2 In 2 O 5 ·1H 2 O is compared with recent high-temperature proton conductors.