Temperature dependent electrical resistivity, crystal structure and heat capacity measurements reveal a resistivity drop and metal to semiconductor transition corresponding to first order structural phase transition near 400 K in Ca3Co4O9. The lattice parameter c varies smoothly with increasing temperature, while anomalies in the a, b1 and b2 lattice parameters occur at ~ 400 K. Both Ca2CoO3 and CoO2 layers become distorted above ~ 400 K associated with the metal to semiconductor transport behavior change. Resistivity and heat capacity measurements as a function of temperature under magnetic field indicates low spin contribution to this transition. Reduced resistivity associated with this first order phase transition from metallic to semiconducting behavior enhances the thermoelectric properties at high temperatures and points to the metal to semiconductor transition as a mechanism for improved ZT in high temperature thermoelectric oxides.
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