Improved Thermoelectric Characteristics of Si-Doped Misfit-Layered Cobaltite

The cobaltite Ca₃Co₄O_9+δ has shown large thermopower and is considered to be a good candidate for use as a thermoelectric material. The composition of Ca₃Co₄O_9+δ is better expressed as Ca₂CoO₃]CoO₂] _b1/b2 with the misfit-layered structure featuring different periodicities along the b axis, with b ₁ referring to the b-axis length of the NaCl-type Ca₂CoO₃] sublattice and b ₂ referring to the b-axis length of the CoO₂] sublattice. The crystal structure of Ca₃Co₄O_9+δ can be viewed as being of two subsystems, i.e., the distorted NaCl-type Ca₂CoO₃] sublattice and the CdI₂-type CoO₂] sublattice, alternately stacked along the c-axis. In this paper, we report measurements of the electrical resistivity and Seebeck coefficient for a series of misfit-layered oxides Ca₃Co_4−x Si _x O_9+δ prepared by solid-state reaction. Structural parameters are refined with the superspace group X2/m(0β0)s0 using powder x-ray diffraction data. With partial substitution of Si⁴⁺ for Co³⁺, the resistivity decreases, while the thermopower increases simultaneously. These results indicate that partial substitution of Si⁴⁺ improves the thermoelectric characteristics of Ca₃Co₄O_9+δ .