Oxygen-Ion and Proton Transport in Y³⁺-Doped Hexagonal Perovskite Ba₇In₆Al₂O₁₉

The thermal and electric properties of the Y³⁺-doped Ba₇In_5.9Y_0.1Al₂O₁₉ phase with the hexagonal perovskite structure (a = 5.935(7) Å, c = 37.736(8) Å) are studied. It is shown that this phase can incorporate protons and exhibit protonic conduction. Upon addition of an isovalent dopant, yttrium, the concentration of protons increases (up to the limiting value for Ba₇In_5.9Y_0.1Al₂O₁₉·0.55H₂O), as a result of the increase in the unit cell volume and, correspondingly, in the free space for accommodating OH^– groups in the oxygen-deficient block containing coordination-unsaturated polyhedrons [BaO₉]. The isovalent doping increases the oxygen-ionic conductivity due to an increase in interatomic distances and a decrease in the activation energy of migration. In a humid atmosphere (pH₂O = 1.92 × 10⁻² atm), the Ba₇In_5.9Y_0.1Al₂O₁₉ phase exhibits the higher values of protonic conductivity as compared with the matrix compound Ba₇In₆Al₂O₁₉ and below 500°C is characterized by the predominant proton transport both in air and in a wide pO₂ region (10^–18–0.21 atm).