Abstract
Temperature dependent electrical resistivity, crystal structure and heat capacity measurements reveal a resistivity drop and electrical transport behavior change corresponding to a structural change near 400 K in Ca3Co4O9. The lattice parameter c varies smoothly with increasing temperature while anomalies in a, b1 and b2 lattice parameters occur near 400 K. The Ca site in the Ca2CoO3 block becomes distorted and a change in electrical transport behavior is found above 400 K. Resistivity and heat capacity measurements as a function of temperature under magnetic field combined with Co L-edge x-ray absorption spectra reveal only a weak spin contribution to this change. Reduced resistivity associated with the structural change enhances the thermoelectric properties at moderately high temperatures and points to the electrical transport behavior change as a mechanism for improved ZT in this thermoelectric oxide.
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