Ca2Cr0.5Ga1.5O5—An extremely redox-stable brownmillerite phase

• Anion deficient oxide stable to both oxidation and reduction up to 800 °C.
• Cation-ordered brownmillerite structure determined by powder neutron diffraction.
• Low solubility of Cr3+ in framework due to spherical d3 electron configuration.

Investigation of the Ca2CrxGa2−xO5 compositional series reveals a maximum chromium solubility of 25%. The most chromium rich composition, Ca2Cr0.5Ga1.5O5, adopts a brownmillerite-type anion deficient perovskite structure described in space group Pnma (a=5.368 Å, b=14.547 Å, c=5.593 Å). Neutron powder diffraction data reveals rigorous B-site cation order, with all of the tetrahedral coordination sites occupied exclusively by gallium and the octahedral coordination sites occupied by gallium or chromium. Annealing studies reveals Ca2Cr0.5Ga1.5O5 is stable in both oxidizing (100% O2) and reducing (5% H2 in N2) conditions up to 800 °C, suggesting it could find application as a stable host lattice for fuel cell electrodes or electrolytes with suitable doping to enhance catalytic behaviour and/or anionic conductivity.

Ca2Cr0.5Ga1.5O5, adopts a brownmillerite-type anion deficient perovskite structure yet it is stable in both oxidizing (100% O2) and reducing (5% H2 in N2) conditions up to 800 °C. Figure optionsDownload as PowerPoint slide