Structure and Thermoelectric Properties of New Layered Compounds in the Quaternary System Cs-Pb-Bi-Te
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Structure and Thermoelectric Properties of New Layered Compounds in the Quaternary System Cs-Pb-Bi-Te Kuei-Fang Hsua , Duck-Young Chunga , Sangeeta Lalb, Tim Hoganb and Mercouri G. Kanatzidisa a
Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, MI 48824, USA b Department of Electrical and Computer Engineering, Michigan State University, East Lansing, MI 48824, USA ABSTRACT By introducing of various equivalents of PbTe into the layered framework of CsBi4 Te6 , the four new compounds CsPbBi3 Te6 (1), CsPb2 Bi3 Te7 (2), CsPb3 Bi3 Te8 (3) and CsPb4 Bi3 Te9 (4), were discovered. The compounds adopt layered structures built up of anionic slabs of progressively increasing thickness. The [PbmBi3 Te5+m]- (m = 1, 2, 3, 4) slabs in the four structures can be viewed as fragments excised from the PbTe-type structures with 4-, 5-, 6- and 7-monolayers, respectively. As prepared, these materials are off-stoichiometric and n-type conductors. We present preliminary results of the crystal structures and thermoelectric properties of these materials and classify them as members of the new homologous series CsPbmBi3 Te5+m (m = 1 to 4). INTRODUCTION The incorporation of alkaline or alkaline earth cations into the Bi2 Q3 (Q= Se, Te) lattice led to many new phases such as α-, β-K2 Bi8 Se13 [1,2], K2.5 Bi8.5 Se14 [2], BaBiTe3 [3], CsBi4 Te6 [4], A2 Bi8 Se13 (A = Rb, Cs) [5], CsBi3.67 Se6 [5] and BaBi2 Se4 [5]. We continued to improve the thermoelectric properties of these materials by composition doping, as in the case of β-K2 Bi8 Se13 and CsBi4 Te6 . In an effort to explore the ultimate performance of CsBi4 Te6 (a low temperature thermoelectric material), we introduced PbTe into its layered framework. Consequently, the four new compounds of CsPbBi3 Te6 (1), CsPb2 Bi3 Te7 (2), CsPb3 Bi3 Te8 (3) and CsPb4 Bi3 Te9 (4) were discovered by the reactions of CsBi4 Te6 with increasing content of PbTe. The Pb/Bi/Te system was extensively studied for thermoelectric applications in past work [6-10]. The Pb and Bi atoms are well known to disorder on the same crystallographic positions due to their similar size and coordination preferences. In general, this results in low thermal conductivity by increasing phonon scattering. The four new compounds adopt novel layered structures built up of anionic slabs with progressively increasing thickness. These anionic slabs can be viewed as fragments excised from {PbTe}-type structure with the thickness of 4-, 5-, 6- and 7-{PbTe} monolayers. This family offers a brand-new homologous series CsPbmBi3 Te5+m (m = 1 to 4) available for structural and thermoelectric investigations. G8.25.1
Table 1. Summary of Crystallographic Data for the CsPbmBi3Te5+m Compounds Compounds 1 2 3 4 Formula CsPbBi3 Te6 CsPb2 Bi3 Te7 CsPb3 Bi3 Te8 CsPb4 Bi3 Te9 Crystal Habit needle needle needle needle a, Å 6.3326(6) 4.3456(6) 6.3736(8) 4.4524(6) b, Å 28.667(3) 32.476(5) 37.731(5) 42.132(6) c, Å 4.3637(4) 12.508(2) 4.4416(6) 12.742(2) V, Å3 792.1(2) 1765.2(7) 1068.1(4) 2390.3(9
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