A structural change in Ca ₃Co ₄O ₉ associated with enhanced thermoelectric properties

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 400K in Ca ₃Co ₄O ₉. The lattice parameter c varies smoothly with increasing temperature while anomalies in a, b ₁ and b ₂ lattice parameters occur near 400K. The Ca site in the Ca ₂CoO ₃ block becomes distorted and a change in electrical transport behavior is found above 400K. Resistivity and heat capacity measurements as a function of temperature under magnetic field combined with CoL-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.