Improved Thermoelectric Characteristics of Si-Doped Misfit-Layered Cobaltite

The cobaltite Ca 3 Co 4 O 9+ δ has shown large thermopower and is considered to be a good candidate for use as a thermoelectric material. The composition of Ca 3 Co 4 O 9+ δ is better expressed as [Ca 2 CoO 3 ][CoO 2 ] b 1/ b 2 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 [Ca 2 CoO 3 ] sublattice and b 2 referring to the b -axis length of the [CoO 2 ] sublattice. The crystal structure of Ca 3 Co 4 O 9+ δ can be viewed as being of two subsystems, i.e., the distorted NaCl-type [Ca 2 CoO 3 ] sublattice and the CdI 2 -type [CoO 2 ] 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 Ca 3 Co 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 Si 4+ for Co 3+ , the resistivity decreases, while the thermopower increases simultaneously. These results indicate that partial substitution of Si 4+ improves the thermoelectric characteristics of Ca 3 Co 4 O 9+ δ .