Na-site substitution effects of the thermoelectric properties of NaCo_2O_4

TL;DR

This study investigates how sodium and calcium substitutions affect the electrical resistivity and thermopower of NaCo₂O₄, revealing insights into carrier behavior and strong electronic correlations in this thermoelectric material.

Contribution

It provides new experimental data on Na-site substitution effects in NaCo₂O₄ and analyzes their impact on thermoelectric properties and carrier dynamics.

Findings

Excess Na does not supply carriers but reduces conduction paths.

Ca substitution decreases hole carriers, increasing resistivity and thermopower.

Strong electronic correlations are indicated by temperature-dependent resistivity.

Abstract

The resistivity and thermopower of Na$_{1+x}$Co$_2$O$_4$ and Na$_{1.1-x}$Ca$_x$Co$_2$O$_4$ are measured and analyzed. In Na$_{1+x}$Co$_2$O$_4$, whereas the resistivity increases with $x$, the thermopower is nearly independent of $x$. This suggests that the excess Na is unlikely to supply carriers, and decreases effective conduction paths in the sample. In Na$_{1.1-x}$Ca$_x$Co$_2$O$_4$, the resistivity and the thermopower increase with $x$, and the Ca$^{2+}$ substitution for Na$^+$ reduces the majority carriers in NaCo$_2$O$_4$. This means that they are holes, which is consistent with the positive sign of the thermopower. Strong correlation in this compound is evidenced by the peculiar temperature dependence of the resistivity.