A-site order–disorder in the NdBaMn2O5+δ SOFC electrode material monitored in situ by neutron diffraction under hydrogen flow

Abstract

The A-site disordered perovskite manganite, Nd_0.5Ba_0.5MnO₃, has been obtained by heating the A-site-ordered and vacancy ordered layered double perovskite, NdBaMn₂O₅, in air at 1300 °C for 5 h. Combined transmission electron microscopy (TEM) images and neutron powder diffraction (NPD) analysis at 25 °C revealed that Nd_0.5Ba_0.5MnO₃ has a pseudotetragonal unit cell with orthorhombic symmetry (space group Imma, √2a_p × 2a_p × √2a_p) at 20 °C with the cell dimensions a = 5.503(1) Å, b = 7.7962(4) Å, c = 5.502(1) Å, in contrast to Pmm or Cmcm that have been previously stated from X-ray diffraction studies. The in situ neutron diffraction study carried out on Nd_0.5Ba_0.5MnO₃ in hydrogen flow up to T ∼ 900 °C, allows monitoring the A-site cation disorder–order structural phase transition of this representative member of potential SOFC anode materials between air sintering conditions and hydrogen working conditions. Oxygen loss from Nd_0.5Ba_0.5MnO₃ proceeds with retention of A-site disorder until the oxygen content reaches the Nd_0.5Ba_0.5MnO_2.5 composition at 600 °C. The phase transition to layered NdBaMn₂O₅ and localization of the oxygen vacancies in the Nd layer proceeds at 800 °C with retention of the oxygen content. Impedance spectroscopy measurements for the oxidized A-site ordered electrode material, NdBaMn₂O₆, screen printed on a Ce_0.9Gd_0.1O_2−δ (CGO) electrolyte showed promising electrochemical performance in air at 700 °C with a polarization resistance of 1.09 Ω cm² without any optimization.