Investigations of Solid Solutions of CsBi 4 Te 6
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INVESTIGATIONS OF SOLID SOLUTIONS OF CsBi4Te6 Duck-Young Chung1, Tim P. Hogan2, Nishant Ghelani2, Paul W. Brazis3, Melissa A. Lane3, Carl R. Kannewurf3 and Mercouri G. Kanatzidis1 1 Department of Chemistry, Michigan State University, East Lansing, MI 48824. 2 Dept of Electrical Engineering, Michigan State University, East Lansing, MI 48824. 2 Dept of Electrical and Computer Engineering, Northwestern University, Evanston, IL 60208.
ABSTRACT Results on the synthesis and characterization of the solid solutions CsBi4-xSbxTe6, CsBi4Te6-ySey, as well as doping experiments on CsBi4Te6 are reported. We report X-ray structural investigations showing that the Sb or Se atoms in these compounds are not uniformly distributed in the lattice but show preferential occupation of specific crystallographic sites. Thermoelectric properties of selected systems are presented. INTRODUCTION Recently, we described the synthesis, structure and thermoelectric (TE) properties of CsBi4Te6, which, when doped with SbI3, achieves a maximum ZT of ~0.8 at 225 K [1]. We believe additional improvements in TE performance of this material are possible. Thus we are pursuing further exploration of doping agents and the investigation of solid solutions such as CsBi4xSbxTe6, CsBi4Te6-ySey and Cs1-zRbzBi4Te6. The latter could result in substantially lower thermal conductivities. In addition, a complete TE cooling device needs both a p-type and n-type versions of a material to operate. Therefore we explored whether n-type doping is possible with various doping agents for CsBi4Te6. Here we report initial results on the synthesis of the solid solutions CsBi4-xSbxTe6, CsBi4Te6-ySey, and their characterization and doping studies of CsBi4Te6 with In2Te3 and BiI3. RESULTS AND DISCUSSION CsBi4Te6 is composed of anionic [Bi4Te6] slabs alternating with layers of Cs+ ions, Figure 1A. The reaction of cesium with two equivalents of Bi2Te3 does not produce a formal intercalation compound, but causes a complete reorganization of the bismuth telluride framework to produce a new structure type. The added electrons localize on the Bi atoms and form Bi-Bi bonds that are 3.238(1) Å long. The presence of these bonds seems to play a role in the charge transport properties of the material. The [Bi4Te6] layers are strongly anisotropic as they consist of onedimensional [Bi4Te6] laths running parallel to the crystallographic b-axis. The width and height of these laths is 23 Å by 12 Å. This structure type is not stable when Bi is substituted with Sb or when Te is substituted with Se. Therefore the entire range of solid solutions CsBi4-xSbxTe6 and CsBi4Te6-ySey is not possible. In this work we probed the extent of x and y, and report the crystallographic refinement of CsBi4xSbxTe6 and CsBi4Te6-ySey (x = 0.3; y = 0.3). Interestingly, we found the Sb or Se atoms in these compounds to be distributed in a preferential manner as opposed to uniformly. This probably
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