Thermoelectric transport in the layered Ca$_3$Co$_{4-x}$Rh$_x$O$_9$ single crystals

TL;DR

This study investigates how isovalent Rh substitution affects the thermoelectric properties of Ca$_3$Co$_{4}$O$_9$ single crystals, revealing decreased correlation effects, increased Seebeck coefficient, and improved power factor with Rh content.

Contribution

It provides new insights into the effects of Rh substitution on transport properties and correlation effects in layered thermoelectric oxides.

Findings

Resistivity and Seebeck coefficient systematically change with Rh content.

Rh substitution weakens electron correlation effects.

Power factor is significantly improved by Rh substitution.

Abstract

We have examined an isovalent Rh substitution effect on the transport properties of the thermoelectric oxide Ca$_3$Co$_{4}$O$_9$ 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 Ca$_3$Co$_{4-x}$Rh$_{x}$O$_9$ samples. In the Fermi-liquid regime where the resistivity behaves as $\rho=\rho_0+AT^2$ 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.