Correlation between reconstructive phase transitions and transport properties from SrCoO2.5 brownmillerite: A neutron diffraction study

Temperature dependent X-ray and neutron powder diffraction have been exploited to revisit the structural evolution of SrCoO2.5 brownmillerite-like phase from room temperature to 1200 degrees C, in complement with DTA-TG, conductivity and thermal expansion measurements. Three different polymorphs have been identified: the orthorhombic "O" brownmillerite phase between room temperature and 653 degrees C, the hexagonal "H" phase between 653 degrees C and 920 degrees C, and the cubic perovskite "C" above 920 degrees C, which is transformed again, by cooling, into the "H" phase at 774 degrees C. The metastable "O" brownmillerite phase can only be obtained by quenching in liquid N-2 from 1000 degrees C. The refinement of the crystal and magnetic structures of SrCoO2.5 from NPD data collected "in situ" as a function of temperature in an air atmosphere led to define structural details of the very distinct coordination polyhedra present in the different polymorphs, which have been correlated with the transport properties. The conductivity shows little dependence up to 250 degrees C, and it abruptly increases above this temperature, which is concomitant with a contraction of the Co2-O3 bonds of the tetrahedral CoO4 units of the "O" brownmillerite structure at 350 degrees C, suggesting the presence of an insulator-to-metal transition in this structural polymorph. The dramatic reduction of conductivity above 500 degrees C is connected with the transformation to the "H" polymorph, with a complete oxygen sublattice and a face-sharing octahedral framework with a poor 1D electronic conduction. Further heating above 900 degrees C boosts again the conductivity when the sample is entering the cubic perovskite region, characterized by a 3D vertex-sharing network of CoO6 octahedra. The total conductivity displays a maximum value of 150 S cm(-1) at 900 degrees C and it increases during the cooling run, thus displaying a characteristic metallic behaviour for the "C" phase. This polymorph presents conductivity values substantially above the required 100 S cm(-1) in solid oxide fuel cells, suggesting that it is a good candidate for cathode materials in energy conversion devices. (C) 2008 Elsevier Masson SAS. All rights reserved.