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

Abstract

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.

Thermoelectric transport in the layered Ca3Co4–xRhxO9 single crystals

“…All the peaks are indexed as (00l) reflections of the parent compound, meaning that the measured crystal surface is the ab plane. In Bi-substituted Ca 3 Co 4 O 9 , it is reported that an impurity phase is grown even in single crystals [32]. The present crystals do not show such impurity phase even for x = 0.6 sample.…”

“…The experiments were performed using Ca 3 Co 4−x Rh x O 9 single crystals (x = 0, 0.2, 0.4, 0.6) with typical sample dimensions of 1 × 1 × 0.02 mm 3 grown by a flux method [32]. was set to be 1.5% in molar ratio.…”

Thermoelectric Properties of the Layered Cobaltite Ca<SUB>3</SUB>Co<SUB>4</SUB>O<SUB>9</SUB> Epitaxial Films Fabricated by Topotactic Ion-Exchange Method

“…The Ca 3 Co 4 O 9 epitaxial film exhibits a low value of 4:0 Â 10 À3 cm, which is much lower than those of the polycrystalline materials ($ 1 Â 10 À2 cm) 12,13) and comparable to those of single crystals, [6][7][8]14) and a large S value of $þ200 mV K À1 , reasonably agreeing with the reported values in bulk samples, [6][7][8][12][13][14] at 1000 K. The high TE properties may be attributed to the larger carrier mobility resulting most likely from the high crystal quality of the film.…”

“…Therefore, intensive studies have been made in TE properties of Ca 3 Co 4 O 9 to date with bulk single crystalline and polycrystalline forms. [6][7][8][12][13][14] The film is more favorable for practical TE applications. In particular, a high-quality epitaxial film can provide high-performance as TE device because high-quality epitaxial films can exhibit intrinsic electrical properties as bulk single crystal.…”

Thermoelectric properties of Ca3Co4O9 and Ca2.8Bi0.2Co4O9 thin films in their island formation mode

“…In this study, we used the PLD technique to deposit Ca 3 Co 4 O 9 and Ca 2.8 Bi 0.2 Co 4 O 9 thin films on a LaAlO 3 (LAO) substrate. Bismuth was chosen as a dopant because Bi substitution has been proven to be effective in increasing thermopower (a) while maintaining electrical conductivity (r) in the bulk counterpart of this system, [3][4][5] as well as in single crystals 14 and thin films. The phase purity of the final target samples as well as that of the thin films was detected by x-ray diffraction (XRD) analysis by means of a GBC MMA diffractometer with a Cu K a radiation source (k 5 0.154 nm).…”