Enhanced thermoelectric properties of Sr_5In_2Sb_6 via Zn-doping
Zintl phases exhibit inherently low thermal conductivity and adjustable electronic properties, which are integral to designing high-efficiency thermoelectric materials. Inspired by the promising thermoelectric figure of merit of optimized A_5M_2Sb_6 Zintl phases (A = Ca or Sr, M = Al, Ga, In), Zn-doped Sr_5In_(2−x)Zn_xSb_6 (x = 0, 0.025, 0.05, 0.1) compounds were investigated. Optical absorption measurements combined with band structure calculations indicate two distinct energy transitions for Sr_5In_2Sb_6, one direct (E_g ~0.3 eV) and the other from a lower valence band manifold to the conduction band edge (E_g ~0.55 eV). Sr_5In_2Sb_6 exhibits nondegenerate p-type semiconducting behavior with low carrier concentration (~4 × 10^(18) h+ cm^(−3) at 300 K). Charge carrier tuning was achieved by Zn^(2+) substitution on the In^(3+) site, increasing carrier concentrations to up to 10^(20) h+ cm^(−3). All samples displayed relatively low thermal conductivities (~0.7 W m^(−1) K^(−1) at 700 K). The Zn-doped samples exhibited significantly higher zT values compared to the undoped sample, reaching a value of ~0.4 at 750 K for Sr_5In_(1.9)Zn_(0.1)Sb_6.
© 2015 The Royal Society of Chemistry. Received 17th March 2015; Accepted 9th April 2015; First published online 10 Apr 2015. This article is part of themed collection: 2015 Journal of Materials Chemistry A Hot Papers. This research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration and was supported by the NASA Science Missions Directorate's Radioisotope Power Systems Technology Advancement Program. S. C. would like to thank Jan-Hendrik Poehls, Stephen Dongmin Kang, and Saneyuki Ohno for their helpful discussions. U. A. acknowledges the financial assistance of The Scientific and Technological Research Council of Turkey. We would like to acknowledge the Molecular Materials Research Center (MMRC) at Caltech for allowing use of their instruments for the optical measurements obtained in this work.
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