Structural and electrical characterization of the novel SrCo0.9Sb0.1O3-δ perovskite:: Evaluation as a solid oxide fuel cell cathode material

A novel perovskite oxide with the title composition has been prepared by soft-chemistry procedures followed by thermal treatments at 1000 degrees C. This polycrystalline sample has been characterized by temperature-dependent neutron powder diffraction (NPD), thermal analysis, electrical conductivity, and thermal expansion measurements, in order to evaluate its potential use as a mixed electronic-ionic conductor in intermediate-temperature solid oxide fuel cells (IT-SOFCs). At room temperature (RT), the sample adopts a tetragonal superstructure of perovskite with a = a(0), c = 2a(0) (a(0) approximate to 3.9 angstrom) defined in the P4/mmm space group. Co and Sb are distributed at random over the octahedral positions of the perovskite; flattened and elongated (Co,Sb)O-6 octahedra alternate along the c axis, sharing corners in a three-dimensional array (3C-like structure). The refinement of the oxygen occupancy factors yields the crystallographic formula SrCo0.9Sb0.1O2.73(4)(4); the oxygen vacancies are located at the equatorial O2 and O3 atoms, in alternating layers with different occupancy. O3 atoms exhibit, at RT, large thermal factors of 5.3 angstrom(2), suggesting a considerable mobility. This structure is stable up to 500 degrees C; between 500 and 700 degrees C, an order-disorder phase transition takes place to give a fully disordered simple-cubic perovskite with a = a(0) (space group Pm (3) over barm); this structure is shown to be stable up to 940 degrees C from NPD data. This is a second-order nonreconstructive transition, which is not observed at the differential thermal analysis curves, although it is probably responsible for a subtle change of slope at 650 degrees C in the thermal expansion curve. The thermal evolution of the electrical conductivity exhibits a maximum of 300 S . cm(-1) at 400 degrees C; above this electronic transition, the conductivity regularly decreases, but it is still well above the required 100 S.cm(-1) in the temperature region 650-850 degrees C corresponding to the working regime of a IT-SOFC.