Enhancement of thermoelectric power factor by inducing octahedral ordering in La2-xSrxCoFe O6 double perovskites

Abstract

Among the various oxide thermoelectric materials, double perovskites provide more flexibility to maneuver interdependent thermoelectric parameters to achieve enhanced thermoelectric figure of merit (ZT), as octahedral ordering, i.e., arrangement of B′O6 and B″O6 octahedra, present in the A2B′B″O6 structure is impacted by cation doping. In this work, we investigated the role of octahedral distortion on thermoelectric properties of La2-xSrxCoFeO6 (LSCF) double perovskites with 0.0≤x≤1.0, synthesized by the autocombustion route. Rietveld refinement of x-ray diffraction data revealed the phase transition with increasing Sr concentration (x) in LSCF from rhombohedral crystal structure with R-3c space group (x≤0.6) to monoclinic P21/n (0.8≤x≤1.0) space group. X-ray photoemission spectroscopy analysis further confirmed the presence of multiple oxidation states of Co and Fe, and shifts in oxidation states population driven by Sr content. These multivalent cations participated in the charge transport mechanism, which was explained by the small polaron hopping conduction model in these double perovskites. The electrical conductivity at room temperature was found to be increased by more than 107 times in LSCF due to Sr doping, causing a large enhancement in the thermoelectric power factor. Gradual decrease in the octahedral tilt angle with increasing Sr content in LSCF, leading toward the change of crystal structure from disordered (R-3c) to rock-salt-ordered (P21/n) double perovskites, was found to be responsible for the large decrease in activation energy barrier for small polaron hopping conduction in the LSCF system, resulting in the phenomenal increase in electrical conductivity. Maximum thermoelectric figure of merit, ZT=0.11 was obtained at 723 K for La2-xSrxCoFeO6 with x=0.2 composition. © 2019 American Physical Society.