Optical conductivity of layered calcium cobaltate Ca₃Co₄O₉

Abstract

We report the optical properties of layered calcium cobaltate, Ca3Co4O9, which is regarded as a promising candidate for use as a thermoelectric material. The optical conductivity shows three broad peaks related to the inter-band transition below 4 eV, which are quite similar to those in the spectra of Na x CoO2. This similarity implies that the CoO2 layer, which is an essential unit for both Ca3Co4O9 and Na x CoO2, is dominant in the energy band structure below 4 eV. In addition, we estimate the effective carrier number per Co site and find similarity between the CoO2 layers of Ca3Co4O9 and Na0.75CoO2, which is consistent with the similarity in their Seebeck coefficients. To discuss the contribution of the rocksalt-type Ca2CoO3 layer in Ca3Co4O9, we propose the concept of optical sheet conductivity in the layered materials and estimate its value in the Ca2CoO3 layer. A comparison with the spin-polarized band calculation of the LDA + Hubbard U formalism with U = 5 eV suggests that the Ca2CoO3 layer has the inter-band transition of 2.6 eV in the spin-down band structure. Evaluation of the valences of Co 3d orbitals indicates the existence of charge transfer from the Ca2CoO3 layer to the CoO2 layer and mixing of Co(3+) and Co(4+) in the CoO2 layer, which may be the origin of the large thermoelectric effect.

Atomic layer deposition of thermoelectric layered cobalt oxides

“…13,48 Absorption peaks at similar wavelengths as reported here have been attributed to charge transfer transition between the O 2− and Co 3+ ions, 48 and the transition between the crystal-field splitted t 2g and e g Co 3d bands. 49 This is consistent with the observations made by Chambrier et al, 50 who observed an absorption onset at similar wavelengths for their Ca 3 Co 4 O 9 films fabricated with PLD.…”

Thermoelectric transport in the layered Ca3Co4–xRhxO9 single crystals

“…Ca 3 Co 4 O 9 consists of alternate stacking of two types of layered subsystems with different b-axis parameter ( Fig. 1) [7]: The CdI 2 -type CoO 2 layer responsible for the charge transport and the rocksalt-type Ca 2 CoO 3 layer which behaves as the charge reservoir to supply holes into the CoO 2 layer [8][9][10][11]. The anomalous charge transport of this compound is clearly seen in the non-monotonic temperature variations of the electrical resistivity and the Seebeck coefficient [7,[12][13][14][15], but an underlying origin for these behaviors remains unclear.…”

Nonlinear transport associated with spin density wave dynamics in Ca3Co4O9

“…This compound consists of two subsystems of CdI 2 -type CoO 2 and rocksalt-type Ca 2 CoO 3 layers, alternately stacked along the c axis [1,2]. The former is responsible for the charge transport and the latter behaves as the charge reservoir to supply holes into the conduction layer [3][4][5][6]. As is distinct from other layered systems such as the high-T c cuprates, these subsystems have different b axis parameters (b 1 being for the rocksalt and b 2 for the CoO 2 layers).…”