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

Abstract

The A-site disordered perovskite manganite SOFC electrode material, Nd0.5Ba0.5MnO3, 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 Nd0.5Ba0.5MnO3 has a pseudotetragonal unit cell with orthorhombic symmetry (space group Imma, √2 ap × 2 ap × √2 ap) 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 Pm-3m or Cmcm that have been previously stated from X-ay diffraction studies. The in situ neutron diffraction study carried out on Nd0.5Ba0.5MnO3 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 form Nd0.5Ba0.5MnO3 proceeds with retention of A-site disorder until the oxygen content reaches the Nd0.5Ba0.5MnO2.5 composition at 600 °C. The phase transition to layered NdBaMn2O5 with localization of the oxygen vacancies in the Nd layer proceeds at 800 °C with retention of the oxygen content. Impedance spectroscopy measurements for the A-site ordered electrode material, NdBaMn2O6, screen printed on a Ce0.9Gd0.1O2-δ (CGO) electrolyte showed promising electrochemical performance with polarization resistance of 1.09 Ω cm2 at 700 °C in air without any optimization.