Bi-substitution effects on crystal structure and thermoelectric properties of Ca₃Co₄O₉ single crystals

We grew single crystals of partially Bi-substituted Ca₃Co ₄O₉ phase in a solution consisting of K₂CO ₃-KCl solvent. All the X-ray diffraction patterns of the single crystals with different Bi contents were attributable to the Ca ₃Co₄O₉ structure, although weak diffraction peaks from a secondary phase of Bi₂Ca₂Co₂O _x 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 Co₃O ₄O₉ phase.