The hexagonal perovskite Ba₅In₂Al₂ZrO₁₃ and In³⁺-doped phase Ba₅In_2.1Al₂Zr_0.9O_12.95 were prepared by the solid-state synthesis method. The introduction of indium in the Zr-sublattice was accompanied by an increase in the unit cell parameters: a = 5.967 Å, c = 24.006 Å vs. a = 5.970 Å, c = 24.011 Å for doped phase (space group of P6₃/mmc). Both phases were capable of incorporating water from the gas phase. The ability of water incorporation was due to the presence of oxygen deficient blocks in the structure, and due to the introduction of oxygen vacancies during doping. According to thermogravimetric (TG) measurements the compositions of the hydrated samples corresponded to Ba₅In₂Al₂ZrO_12.7(OH)_0.6 and Ba₅In_2.1Al₂Zr_0.9O_12.54(OH)_0.82. The presence of different types of OH⁻-groups in the structure, which participate in different hydrogen bonds, was confirmed by infrared (IR) investigations. The measurements of bulk conductivity by the impedance spectroscopy method showed that In³⁺-doping led to an increase in conductivity by 0.5 order of magnitude in wet air (pH₂O = 1.92·10⁻² atm); in this case, the activation energies decreased from 0.27 to 0.19 eV. The conductivity−pO₂ measurements showed that both the phases were dominant proton conductors at T < 500 °C in wet conditions. The composition Ba₅In_2.1Al₂Zr_0.9O_12.95 exhibited a proton conductivity ~10⁻⁴ S·cm⁻¹ at 500 °C. The analysis of partial (O²⁻, H⁺, h^•) conductivities of the investigated phases has been carried out. Both phases in dry air (pH₂O = 3.5·10⁻⁵ atm) showed a mixed (oxygen-ion and hole) type of conductivity. The obtained results indicated that the investigated phases of Ba₅In₂Al₂ZrO₁₃ and Ba₅In_2.1Al₂Zr_0.9O_12.95 might be promising proton-conducting oxides in the future applications in electrochemical devices, such as solid oxide fuel cells. Further modification of the composition and search for the optimal dopant concentrations can improve the H⁺-conductivity.

The development of solid oxide fuel cells operating at medium temperatures (500–700 °C and even lower) requires the search for proton conductors based on complex oxides that would have a wide range of required properties. This task stimulates the search for new promising phases with proton conductivity. The new hexagonal perovskite-related compound Ba₇In₆Al₂O₁₉ was synthesized by the solid-state method. The phase was characterized by powder X-ray diffraction, thermogravimetric analysis, FT-IR spectroscopy, and impedance spectroscopy (in a wide range of temperatures, and partial pressures of oxygen at various atmospheric humidities). The investigated phase had a hexagonal structure with a space group of P6₃/mmc; the lattice parameters for Ba₇In₆Al₂O₁₉ are a = 5.921(2) Å, c = 37.717(4) Å. The phase is capable of reversible hydration and incorporates up to 0.15 mol H₂O. IR-data confirmed that protons in the hydrated compound are presented in the form of OH^–-groups. Electrical conductivity data showed that the sample exhibited dominant oxygen-ion conductivity below 500 °C in dry air and dominant proton conductivity below 600 °C in wet air.