Conductivity behavior of composites in the La_0.6Sr _0.4CoO_3±δ-CeO₂ system: Function of connectivity and interfacial interactions

The electrical properties of a series of composites formed by reactive sintering have been investigated. Six composite samples with the same composition 43 mol % La_0.6Sr_0.4CoO _3±δ·57 mol % CeO₂ (LSCC57) were fabricated through mixing of the initial LSC and CeO₂ followed by milling of the mixture for different periods of time (up to 261 h) and fired at 1350 °C. According to X-ray diffraction (XRD), at room temperature all the LSCC57 composites are composed of the modified perovskite with rhombohedrally distorted perovskite structure (R3̄c, no. 167) and modified ceria with fluorite structure (Fm3̄m, no. 225). The modification of the initial phases takes place because of cross-dissolution of La, Sr, and Co from the initial LSC into the fluorite structure and Ce from the CeO₂ into the perovskite structure. Depending upon fabrication history these composites showed unusual conductivity behavior with strong reversible hysteresis between heating and cooling stages. The transport properties of the LSCC57 composites are determined by several factors: (i) good three-dimensional (3D) connectivity between grains of the most conductive phase (modified perovskite); (ii) recombination of holes (from the modified perovskite) and electrons (from the modified ceria) at phase interfaces could decrease electronic conductivity in the composite materials; (iii) existence of epitaxial coherence between oxygen sublattices at the {modified perovskite/modified ceria} interface could facilitate oxygen transport across the phase interfaces; and (iv) mechanochemical interactions during fabrication (mixing followed by milling) and during high temperature treatment. The transport properties of nanoscale phases at interfacial regions are thought to have strong influence on the conductivity of the LSCC57 composites, in particular, where the grains of the modified perovskite do not form a robust 3D continuous network through the composite material and lead to the appearance of the conductivity hysteresis with the temperature variation.