Improved thermoelectric characteristics of misfit-layered cobaltites Ca$_{3}$Co$_{4-x}$Fe$_{x}$O$_{9+\delta }$.

We have measured the electrical resistivity, Seebeck coefficients and thermal conductivity as a function temperature for a new series of oxides Ca$_{3}$Co$_{4-x}$Fe$_{x}$O$_{9}$ (x=0, 0.05, 0.1, 0.15, 0.2) prepared by the conventional solid state reaction. Structural parameters were refined with a superspace group of X2/m(0b0)s0 using powder X-ray diffraction data. With the substitution of Fe$^{+2 }$ for Co$^{+3}$, the resistivity ($\rho )$ decreases, while the thermoelectric power (S) also increases simultaneously. In the low temperature regime from 15 K to 60 K, the electrical conductivity follows the Mott's law of the form exp[T$_{0}$/T$^{-1/4}$], suggesting the variable-range-hopping transport. The thermoelectric power also shows the same transport mechanism in the same temperature regime. The x = 0.05 sample exhibits a higher power factor value (3.3 $\mu $W/K$^{2}$-cm) than that of undoped Ca$_{3}$Co$_{4}$O$_{9}$ (1.2 $\mu $W/K$^{2}$-cm) at 300K, indicating the improvement of the thermoelectric characteristics upon Fe substitution for Co.