On the origin of enhanced thermoelectricity in Fe doped Ca3Co4O9

Abstract

Resistivity and Seebeck coefficient measurements on Ca₃Co_4−xFe_xO₉ (x = 0, 0.05, 0.1, 0.2 and 0.25) reveal enhanced thermoelectric performance with an optimal x value of 0.2. X-ray diffraction measurements show continuous Fe doping into the host lattice, while X-ray absorption experiments reveal that Fe substitutes for Co in the Ca₂CoO₃ (rock salt) block. The Fe substitution for Co produces electron doping. The local structure around Fe in the Ca₂CoO₃ block becomes disordered, while the structure in the conducting CoO₂ layer becomes more ordered. The structural change in the CoO₂ layer plays the key role to enhance the electron transport. The highest ordered structure is achieved at x = 0.2 with the lowest resistivity. Soft X-ray absorption measurements find no Co site spin-state change with Fe doping. Thermoelectric property enhancement associated with doping induced structural change points to a new approach for creating materials with improved ZT in complex oxide systems.