Anisotropy in Thermoelectric Properties of CsBi 4 Te 6
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Anisotropy in Thermoelectric Properties of CsBi4Te6 Duck-Young Chung1, S. D. Mahanti2, Wei Chen3, Citrad Uher3, Mercouri G. Kanatzidis1* Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, MI 48824, USA 2 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA 3 Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA. 1
ABSTRACT CsBi4Te6 (ZT ~ 0.8 at 225 K) shows highly anisotropic features in its crystal morphology and structure as expressed by the parallel infinite [Bi4Te6] rods which are linked via Bi-Bi bonds. Band calculations also point to a significant anisotropy in the carrier effective masses, and for this reason we examined the anisotropic thermoelectric properties of CsBi4Te6. The electrical conductivity, thermopower and thermal conductivity were measured along the three different crystallographic directions of the monoclinic structure of CsBi4Te6. These measurements were performed on samples with different degrees of doping. The strong charge transport anisotropy of these samples was confirmed and also observed that the thermopower values along the c-axis direction (which is perpendicular to the layer of Cs atoms) was negative (-80 µV/K) while those along the needle direction (b-axis) and parallel to the [Bi4Te6] layers (a-axis) were p-type (50 – 100 µV/K at room temperature. Other anisotropic features in the crystal growth habit, electronic band structure, and electrical and thermal conductivities are also presented. INTRODUCTION Since CsBi4Te6 suggested as a promising thermoelectric material (ZT ~ 0.8 at 225 K) [1], for low temperature applications, extensive investigations have been performed to understand the material and to improve the thermoelectric properties [2]. The compound crystallizes in a long needle-type morphology reflecting monoclinic unit cell parameters of a = 51.9205(8) Å, b = 4.4025(1) Å, c = 14.5118(3) Å, β = 101.480(1)o. The structure features NaCl-type [Bi4Te6] slabs that are infinitely extended along the needle axis (b-axis) and Bi-Bi bonds interconnecting the slabs along the a-axis forming layers which are separated by Cs atoms. These morphology and structural characteristics imply highly anisotropic features in its thermoelectric (TE) properties. The TE properties previously reported on CsBi4Te6 have been performed along the needle (b-) axis since effective charge transport is generally expected to be along this axis. In order to better understand the properties of this material further investigations of the anisotropy of its physical and thermoelectric properties are necessary. Here, we reports charge transport as well as thermal transport measurements for samples of CsBi4Te6 along all primary crystallographic directions. The observed results are surprising and are discussed and compared with those predicted from the band structure calculations.
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EXPERIMENTAL DETAILS Three different samples of CsBi4Te6 were prepared (a) a so-called “as-prepared”, (b)
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