Unraveling the Role of Structural Order in the Transformation of Electrical Conductivity in CaFeCoO, CaSrFeCoO, and SrFeCoO.

The ability to control the electrical conductivity of solid-state oxides using structural parameters has been demonstrated. A correlation has been established between the electrical conductivity and structural order in a series of oxygen-deficient perovskites using X-ray and neutron diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and electrical conductivity studies at a wide temperature range, 25-800 °C. The crystal structure of CaSrFeCoO has been determined, and its stark contrast to CaFeCoO and SrFeCoO has been demonstrated. The Fe/Co distribution over tetrahedral and octahedral sites has been determined using neutron diffraction. There is a systematic increase in the structural order in progression from SrFeCoO (δ = 0.5) to CaSrFeCoO (δ = 0.8) and CaFeCoO (δ = 0.9) . The oxygen contents of these materials were determined using iodometric titration and TGA. At room temperature, there is an inverse correlation between the electrical conductivity and structural order. The ordered Ca and CaSr compounds are semiconductors, while the disordered Sr compund shows metallic behavior. The metallic nature of the Sr material persists up to 1073 K (800 °C), while the Ca and CaSr compounds undergo a semiconductor-to-metal transition above 500 and 300 °C, respectively, highlighting another important impact of the structural order. At high temperature, the CaSr compound has the highest conductivity compared to the Ca and Sr materials. There appears to be an optimum degree of structural order that leads to the highest conductivity at high temperature. Another consequence of the structural order is the observation of mixed ionic-electronic conductivity in CaSr and Ca compounds, as is evident from the hysteresis in the conductivity data obtained during heating and cooling cycles. The average ionic radius required for each structural transition was determined through the synthesis of 21 different materials by systematic variation of the Ca/Sr ratio. In addition, SEM and XPS were employed to gain insight into the crystallite morphology and oxidation states of transition metals, revealing an interesting redox process between Fe and Co.