Compositional And Structural Modifications In Ternary Bismuth Chalcogenides And Their Thermoelectric Properties
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COMPOSITIONAL AND STRUCTURAL MODIFICATIONS IN TERNARY BISMUTH CHALCOGENIDES AND THEIR THERMOELECTRIC PROPERTIES Duck-Young Chung1, Melissa A. Lane2, John R. Ireland2, Paul W. Brazis2, Carl R. Kannewurf2, Mercouri G. Kanatzidis1* 1 Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, MI 48824. 2Department of Electrical and Computer Engineering, Northwestern University, Evanston, IL 60208
ABSTRACT Based on the versatile combination of PbQ- and Bi2Q3-type (Q = S, Se, Te) fragments, we explored new compounds in the Pb/Bi/Se ternary system. The new class of compounds, Pb5Bi6Se14, Pb5Pb12Se23, and PbBi8Se13 are homologues with different combination of alternating Bi2Se3- and PbSe-type layers. α- and β -Pb6Bi2Se9 were obtained in different synthetic conditions and the former is isostructural to heyrovskyite (Pb6Bi2S9) while the latter is a NaCl-type cubic phase. Pb5Bi6Se14 shows a power factor of 11.2 µW/cm·K2 with electrical conductivity of 657 S/cm and thermopower of -131 µV/K at 271 K. The most significant characteristic of this material is the extremely low thermal conductivity of less than 1.0 W/m·K at room temperature. On the basis of these properties, a preliminary doping study for Pb5Bi6Se14 with Sn, Sb, and SbBr3 as dopants was undertaken and the results are presented in this report. INTRODUCTION The need of high performance thermoelectric materials is now recognized and, in addition to further optimization of the Bi2-xSbxTe3-ySe3 alloys, exploration of new materials is necessary to meet technological and societal demands. We have initiated exploratory research to identify new multinary systems, particularly, with Bi, Pb, S, Se and Te for thermoelectric applications. This class of compounds exhibit various combinations[1] of fragments which are essentially “excised” out of the structures of PbQ and Bi2Q3, as observed in the widely known mineral sulfo and selenosalts; PbBi2Q4(Q = S, Se, Te)[2, 3, 4], PbBi4Q7(Q = S, Se, Te)[4, 5, 6], PbBi4S8[7], PbBi6S10[8], Pb3Bi2S6[9], Pb6Bi2S9[10], Pb2Bi2Se5[5], and Pb3Bi4Se9[11]. This structural and compositional versatility of Pb/Bi/Q system motivated us to find new compounds with these elements. In addition, Pb/Bi/Q is a very interesting chemical system from the thermoelectric point of view. Binary bismuth and lead chalcogenides such as Bi2Te3, PbTe, and their solid solutions have been intensively studied for thermoelectric applications, however, the physical origin of their high thermoelectric figure of merit[12] of these compounds remains under investigation. Physical and chemical similarity of heavy Pb and Bi atoms creates widely disordered structures that may lead to low thermal conductivity by increasing phonon scattering. Therefore, this system should be explored for new thermoelectric materials. We have synthesized candidate materials such as Pb5Bi6Se14, PbBi8Se13, Pb5Bi12Se23, α- and β -Pb6Bi2Se9 which are narrow gap semiconductors. Along with their thermoelectric and physicochemical properties, we present the
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