  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.
