Phase transformations in Sr0.8Ba0.2CoO2.5 brownmillerite: correlation between structure and transport properties

Abstract

An oxygen-defective perovskite oxide with the title composition has been prepared by soft-chemistry procedures followed by quenching in liquid N₂ from 900 °C. This polycrystalline sample has been characterized by temperature-dependent X-ray (XRPD) and neutron powder diffraction (NPD), thermal analysis, electrical conductivity and thermal expansion measurements, in order to correlate the physico-chemical properties and the structural features. At room temperature (RT), the sample adopts an orthorhombic brownmillerite-like structure defined in the Ibm2 space group, containing layers of CoO₆ octahedra alternating with layers of CoO₄ tetrahedra along the b axis. This phase is stable between room temperature and 350 °C, where a topotactic intake of oxygen increases the coordination of the tetrahedra to octahedral, with change of the space group to Pnma, as unveiled by the in-situNPD study. This intermediate phase has been identified for the first time. At 653 °C, this phase irreversibly transforms to a hexagonal “H” phase. At 920 °C, a cubic perovskite phase “C” is identified, which is transformed again, upon cooling, into the “H” phase at 774 °C. The features of the very distinct coordination polyhedra present in the different polymorphs have been correlated with the transport properties. There is a substantial increment of the conductivity at 350 °C, upon the oxygen insertion process, concomitant with a contraction of the axial Co–O bonds of the octahedral CoO₆ units and the transformation of the tetrahedra into octahedra, also characterized by dilatometry measurements. The dramatic reduction of the conductivity above 700 °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. In Sr_0.8Ba_0.2CoO_2.5, the plateau of stability of the 3C-like structure, with useful transport properties in the range of σ = 50–60 S cm⁻¹, is extended up to 650 °C with respect to the pristine SrCoO_2.5. By heating above 900 °C, the conductivity abruptly rises when the sample is entering the cubic perovskite region, characterized by a three-dimensional vertex-sharing network of CoO₆ octahedra. The total conductivity displays a maximum value of 75 S cm⁻¹ at 900 °C, which increases during the cooling run, exhibiting a typical metallic behaviour. Moreover, in this cubic phase, the oxygen atoms show large thermal factors of 5.5 Ų, suggesting a considerable mobility and a mixed conductor behaviour.