Optical conductivity of layered calcium cobaltate Ca3Co4O9
Optical conductivity of layered calcium cobaltate Ca3Co4O9
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DOI：
10.1088/0953-8984/28/8/085601
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发表时间：
2016-01
期刊：
影响因子：
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通讯作者：
K. Tanabe;R. Okazaki;H. Taniguchi;I. Terasaki
中科院分区：
文献类型：
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作者：
K. Tanabe;R. Okazaki;H. Taniguchi;I. Terasaki
我们报告的光学性能的层状钴酸钙，Ca3Co4O9，这被认为是一个有前途的候选人用作热电材料。的光学电导率显示出三个宽峰相关的带间跃迁低于4 eV，这是非常相似的光谱NaxCoO2。这种相似性意味着CoO 2层是Ca 3Co 4 O 9和NaxCoO 2的基本单元，在4 eV以下的能带结构中占主导地位。此外，我们估计每个钴网站的有效载流子数，并找到Ca3Co4O9和Na0.75CoO2的CoO2层之间的相似性，这是一致的，在他们的塞贝克系数的相似性。为了讨论Ca_3Co_4O_9中岩盐型Ca_2CoO_3层的贡献，提出了层状材料中光学片电导率的概念，并估算了其在Ca_2CoO_3层中的值。与LDA + Hubbard U公式（U = 5 eV）的自旋极化能带计算结果的比较表明，Ca2CoO3层在自旋向下的能带结构中具有2.6 eV的带间跃迁. Co3d轨道的价态分析表明，在CoO2层中存在着从Ca2CoO3层到CoO2层的电荷转移以及Co3+和Co4+的混合，这可能是产生大热电效应的原因.
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 NaxCoO2. This similarity implies that the CoO2 layer, which is an essential unit for both Ca3Co4O9 and NaxCoO2, 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 Co3+ and Co4+ in the CoO2 layer, which may be the origin of the large thermoelectric effect.