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

The thermal and electric properties of the Y3+-doped Ba7In5.9Y0.1Al2O19 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 Ba7In5.9Y0.1Al2O19·0.55H2O), 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 Ba7In5.9Y0.1Al2O19 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).