Improved Thermoelectric Characteristics of Si-Doped Misfit-Layered Cobaltite

The cobaltite Ca3Co4O9+δ has shown large thermopower and is considered to be a good candidate for use as a thermoelectric material. The composition of Ca3Co4O9+δ is better expressed as [Ca2CoO3][CoO2]b1/b2 with the misfit-layered structure featuring different periodicities along the b axis, with b1 referring to the b-axis length of the NaCl-type [Ca2CoO3] sublattice and b2 referring to the b-axis length of the [CoO2] sublattice. The crystal structure of Ca3Co4O9+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 Ca3Co4−xSixO9+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 Ca3Co4O9+δ.