Thermoelectric transport in the layered Ca₃Co_{4- x}Rh_xO₉ single crystals

We have examined an isovalent Rh substitution effect on the transport properties of the thermoelectric oxide Ca₃Co₄O₉ using single-crystalline form. With increasing Rh content x, both the electrical resistivity and the Seebeck coefficient change systematically up to x = 0.6 for Ca₃Co_{4- x}Rh_xO₉ samples. In the Fermi-liquid regime where the resistivity behaves as ρ = ρ 0 + A T 2 around 120 K, the A value decreases with increasing Rh content, indicating that the correlation effect is weakened by Rh 4d electrons with extended orbitals. We find that, in contrast to such a weak correlation effect observed in the resistivity of Rh-substituted samples, the low-temperature Seebeck coefficient is increased with increasing Rh content, which is explained with a possible enhancement of a pseudogap associated with the short-range order of spin density wave. In high-temperature range above room temperature, we show that the resistivity is largely suppressed by Rh substitution while the Seebeck coefficient becomes almost temperature-independent, leading to a significant improvement of the power factor in Rh-substituted samples. This result is also discussed in terms of the differences in the orbital size and the associated spin state between Co 3d and Rh 4d electrons.