Electronic structure and its contribution to the thermoelectric power of Ca/sub 3/Co/sub 4/O/sub 9/ and Na/sub x/CoO/sub 2/ layered cobalt oxides

Takeuchi, Tsunehiro; Kitao, T.; Kondo, Takeshi; Mikami, Masashi

doi:10.1109/ict.2005.1519989

Cited by 2 publications

(2 citation statements)

References 12 publications

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“…(4) are shown in Table 1. The Fermi energy values are in good agreement with the values found from band structure calculations [35][36][37]. However, the E F and n values decrease in the Yb-susbtituted samples, compared to the unsubstituted ones.…”

Section: Resultssupporting

confidence: 88%

Decrease of Ca3Co4O9+δ thermal conductivity by Yb-doping

KIirat

Aksan

Rasekh

et al. 2015

Ceramics International

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In this study, the effect of Yb-substitution on the structural, electrical and thermal transport properties of the Ca 3 Co 4 O 9 þ δ system has been investigated in the low temperature region (between 10 and 300 K). The resistivity of samples increases with raising the Yb-concentration in the system. All the samples show a metal-semiconductor transition below 85 K. In the analysis based on Strongly Correlated Fermi Liquid Model, an increased bandwidth and a reduced electronic correlation are found. It has also been found that the energy gap, E 0 , value also decreases with Ybsubstitution. The samples show positive thermoelectric power, indicating that dominant charge carriers are holes in all the samples. The thermoelectric power value decreases with Yb-substitution. From Mott equation, it is determined that the Fermi energy and hole concentration decrease in the Yb-susbtituted samples, compared to the undoped ones. Thermal conductivity, κ, is decreased in about 50% of the measured in undoped samples for the 0.01 and 0.03 Yb-doped samples at 300 K. Highest figure of merit, ZT, value is found to be 5.4 10 À 3 at 300 K for the unsubstituted sample and the ZT value decreases by the substitution.

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Section: Resultssupporting

confidence: 88%

Decrease of Ca3Co4O9+δ thermal conductivity by Yb-doping

KIirat

Aksan

Rasekh

et al. 2015

Ceramics International

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“…However this unconventional contribution ceases above a cut-off energy (typically that for phonons or magnons) in materials with single band at Fermi level [41]. Since the ARPES studies [30,42] show that Na x CoO 2 and Ca 3 Co 4 O 9 are single-band materials, the Drude-like excitation is more or less similar to those of the copper oxides. We examined that the optical conductivity above 0.5 eV is essentially independent of the details of the low-energy extrapolation.…”

Section: Rtmentioning

confidence: 99%

Optical conductivity of layered calcium cobaltate Ca₃Co₄O₉

Tanabe

Okazaki

Taniguchi

et al. 2016

J. Phys.: Condens. Matter

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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.

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Electronic structure and its contribution to the thermoelectric power of Ca/sub 3/Co/sub 4/O/sub 9/ and Na/sub x/CoO/sub 2/ layered cobalt oxides

Cited by 2 publications

References 12 publications

Decrease of Ca3Co4O9+δ thermal conductivity by Yb-doping

Decrease of Ca3Co4O9+δ thermal conductivity by Yb-doping

Optical conductivity of layered calcium cobaltate Ca₃Co₄O₉

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Electronic structure and its contribution to the thermoelectric power of Ca/sub 3/Co/sub 4/O/sub 9/ and Na/sub x/CoO/sub 2/ layered cobalt oxides

Cited by 2 publications

References 12 publications

Decrease of Ca3Co4O9+δ thermal conductivity by Yb-doping

Decrease of Ca3Co4O9+δ thermal conductivity by Yb-doping

Optical conductivity of layered calcium cobaltate Ca3Co4O9

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Product

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Optical conductivity of layered calcium cobaltate Ca₃Co₄O₉