Bi-Substitution Effects on Crystal Structure and Thermoelectric Properties of Ca3Co4O9 Single Crystals

We grew single crystals of partially Bi-substituted Ca3Co4O9 phase in a solution consisting of K2CO3–KCl solvent. All the X-ray diffraction patterns of the single crystals with different Bi contents were attributable to the Ca3Co4O9 structure, although weak diffraction peaks from a secondary phase of Bi2Ca2Co2Ox were observed in the crystals grown from a starting composition molar ratio of Ca:Bi:Co=2.5:0.5:4.0 (BC-0.5 crystal). Thermoelectric properties of the crystals in the ab-plane were measured at various temperatures. Seebeck coefficient (S) was increased by the partial Bi-substitution at room temperature, whereas electrical resistivity (ρ) was decreased at room temperature except for BC-0.5 crystals. The simultaneous increase in S and decrease in ρ suggest an increase in carrier mobility. Although Bi atoms are heavier than the replaced Ca or Co atoms, the phonon part of thermal conductivity is increased by the Bi-substitution. We suggest that these effects of the Bi-substitution on thermoelectric properties are largely governed by changes in the peculiar crystal structure, such as the misfit relationship between the CoO2 and Ca2CoO3 layers, which constitute the layered structure of the Co3O4O9 phase.