Crystal and magnetic structure of the Bi2RuMnO7 pyrochlore: A potential new cathode for solid oxide fuel cells

Abstract

A new pyrochlore-like phase of composition Bi2RuMnO7 has been prepared as polycrystalline powder, structurally characterized from X-ray diffraction (XRD) and neutron powder diffraction (NPD) data, in complement with magnetic and transport measurements, and finally tested as cathode material for solid-oxide fuel cells (SOFCs). Bi2RuMnO7 pyrochlore is defined in a cubic unit cell with space group Fd-3m; the structural analysis from NPD data unveils a singular feature consisting of a cation disorder of Mn and Bi atoms between A and B positions. This disorder leads to the reduction of Mn4+ at B position to Mn2+ at A position and the oxidation of Bi3+ at A position to Bi5+ at B position. The low-temperature NPD data indicates an antiferromagnetic coupling of two subsets of Mn4+/Ru4+ spins, demonstrating that the magnetic frustration is partially relieved by the random distribution of Mn and Ru over the 16c sites. The obtained compound displays a metallic-like behavior with conductivity values at the SOFCs working temperatures (650–850 °C) that span between 39 and 28 Scm−1. Bi2RuMnO7 shows good performance working as a cathode with LSGM electrolyte, yielding output power densities of 360 mW cm−2 at 850 °C with pure H2 as a fuel.