Crystal structure, diffusion path, and oxygen permeability of a Pr ₂NiO₄-based mixed conductor (Pr_0.9la _0.1)₂(Ni_0.74Cu_0.21Ga _0.05)O₄+δ

We have investigated in situ the crystal structure, oxygen diffusion path, oxygen permeation rate, and electrical conductivity of a doped praseodymium nickel oxide, Pr₂NiO₄-based mixed conductor, (Pr _0.9La_0.1)₂(Ni_0.74Cu _0.21Ga_0.05)O_4+δ (PLNCG) in air between 27 °C and 1015.6 _°C. The PLNCG has a tetragonal 14/mmm K ₂NiF₄-type structure which consists of a (Pr _0.9La_0.1)(Ni_0.74Cu_0.21Ga _0.05)O₃ perovskite unit and a (Pr_0.9La _0.1)O rock salt unit in the whole temperature range. Both experimental and theoretical electron density maps indicated two-dimensional networks of (Ni_0.74Cu_0.21Ga_0.05)-O covalent bonds in PLNCG. Highest occupied molecular orbitais (HOMO) in PLNCG demonstrate that the electron-hole conduction occurs via Ni and Cu atoms in the (Ni _0.74Cu_0.21 Ga_0.05)-O layer. The bulk oxygen permeation rate was high (137/μmol cm^-2 min^-1 at 1000 °C), and its activation energy was low (51 kJ mol^-1 at 950 °C). The Rietveld method, maximum-entropy method (MEM), and MEM-based pattern fitting analyses of neutron and synchrotron diffraction data indicate a large anisotropic thermal motion of the apical O₂ oxygen at the 4e site (0, 0, z; z≈ 0.2) in the (Pr_0.9La_0.1)(Ni _0.74Cu_0.21Ga_0.05S)O₃ perovskite unit. Neutron and synchrotron diffraction data and theoretical structural optimization show the interstitial oxygen (O3) atom at (x, 0, z) (x ap; 0.6 and z ap; 0.2). The nuclear density analysis indicates that the bulk oxide-ion diffusion, which is responsible for the high oxygen permeation rate, occurs through the interstitial O3 and anisotropic apical O2 sites. The nuclear density at the bottleneck on the oxygen diffusion path increases with temperature and with the oxygen permeation rate. The activation energy from the nuclear density at the bottleneck decreases with temperature, which is consistent with the decrease of the activation energy from oxygen permeation rate. The extremely low activation energy (12 kJ mol^-1at 900 °C) from the nuclear density at the bottleneck indicates possible higher bulk oxygen permeation rates in quality single crystals and epitaxial thin films.