Na-site substitution effects on the thermoelectric properties of ${\mathrm{NaCo}}_2{\mathrm{O}}_4$

The resistivity and thermopower of ${\mathrm{Na}}_{1+x}{\mathrm{Co}}_2{\mathrm{O}}_4$ and ${\mathrm{Na}}_{1.1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{O}}_4$ are measured and analyzed. In ${\mathrm{Na}}_{1+x}{\mathrm{Co}}_2{\mathrm{O}}_4,$ 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 ${\mathrm{Na}}_{1.1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{O}}_4,$ the resistivity and the thermopower increase with x, and the ${\mathrm{Ca}}^{2+}$ substitution for ${\mathrm{Na}}^{+}$ reduces the majority carriers in ${\mathrm{NaCo}}_2{\mathrm{O}}_4.$ 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.