A Combined Variable-Temperature Neutron Diffraction and Thermogravimetric Analysis Study on a Promising Oxygen Electrode, SrCo_0.9Nb_0.1O_3−δ, for Reversible Solid Oxide Fuel Cells

The present study investigates the temperature–structure–stoichiometry relationship of a promising oxygen electrode SrCo0.9Nb0.1O3−δ over a temperature (T) range from room temperature (RT) to 900 °C. The techniques employed are variable-temperature neutron diffraction (VTND) and thermogravimetric analysis (TGA). At T < 75 °C, VTND reveals a tetragonal (P4/mmm) structure with a G-type magnetic ordering. Above 75 °C, the nucleus structure remains the same, while the magnetic ordering disappears. A phase transition from tetragonal (P4/mmm) to cubic (Pm3̅m) is observed at 412 °C, where the two Co sites and three O sites in the P4/mmm phase converge to one equivalent site, respectively. The phase transition temperature coincides with the peak temperature of oxygen uptake obtained by TGA. It is also observed that the Nb dopant has no preferred Co site to occupy. The oxygen vacancies are mostly located at the O3 site surrounding the Co2 site in the P4/mmm structure. The intermediate-spin state of Co3+ at the Co2 site is responsible for the observed distortions of CoO6 octahedra, i.e., elongation of Co2O6 octahedra and shortening of Co1O6 octahedra along the c-axis, which is a phenomenon known as Jahn–Teller distortion. At high temperatures, large thermal displacement factor for O2– is observed with high concentration of oxygen vacancies, providing a structural environment favorable to high O2– conductivity in Nb-doped SrCoO3-based oxygen electrode materials.