Correlation between reconstructive phase transitions and transport properties from SrCoO_(2.5) brownmillerite: A neutron diffraction study

摘要

Temperature dependent X-ray and neutron powder diffraction have been exploited to revisit the structural evolution of SrCoO_(2.5) brownmil-lerite-like phase from room temperature to 1200 deg 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 deg C, the hexagonal "H" phase between 653 deg C and 920 deg C, and the cubic perovskite "C" above 920 deg C, which is transformed again, by cooling, into the "H" phase at 774 deg C. The metastable "O" brownmillerite phase can only be obtained by quenching in liquid N_2 from 1000 deg C. The refinement of the crystal and magnetic structures of SrCoO_(2.5) 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 deg C, and it abruptly increases above this temperature, which is concomitant with a contraction of the Co2-O3 bonds of the tetrahedral CoO_4 units of the "O" brownmillerite structure at 350 deg C, suggesting the presence of an insulator-to-metal transition in this structural polymorph. The dramatic reduction of conductivity above 500 deg C is connected with the transformation to the "H" polymorph, with a complete oxygen sublattice and a face-sharing octahedral framework with a poor ID electronic conduction. Further heating above 900 deg C boosts again the conductivity when the sample is entering the cubic perovskite region, characterized by a 3D vertex-sharing network of CoO_6 octahedra. The total conductivity displays a maximum value of 150 S cm~(-1) at 900 deg 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.