High-temperature defect thermodynamics of nickel substituted double-perovskite cobaltite PrBaCo2−xNixO6−δ (x = 0.2)

The double perovskite praseodymium cobaltites PrBaCo2−xNixO6−δ, where 0.1 ≤ x ≤ 0.3, were obtained by combustion of organo-metallic precursors and shown to have a tetragonal structure with the crystalline lattice parameters a ≈ 3.91 and c ≈ 7.63 Å consistent with electron compensation 2Co3+ = Ni2+ + Co4+ of Ni2+ impurity centers. The oxygen content (6 - δ) vs. log(pO2) dependencies were measured by a coulometric titration technique for x = 0.2 at 650-950 °C and oxygen pressure variations within 10−10 - 0.6 atm. The reductive decomposition of the cobaltite is found to take place at δ = 1.13. The experimental data were utilized in order to derive partial thermodynamic functions of oxygen that govern defect equilibration. The oxygen partial enthalpy ΔH¯O and entropy ΔS¯O are found to exhibit strong changes near (6 - δ) = 5. The observed behavior is explained in frameworks of the defect formation model, which involves reactions of charge disproportionation of Co3+ cations, oxygen exchange with the gas phase and oxygen disordering over O2 and O3 structural positions.