Electronic structure and thermoelectric properties of pnictogen-substituted ASn_(1.5)Te_(1.5) (A = Co, Rh, Ir) skutterudites

Abstract

Substituting group 14 and 16 elements on the pnictogen site in the skutterudite structure yields a class of valence-precise ternary AX_(1.5)Y_(1.5) compounds (A = Co, Rh, Ir, X = Sn, Ge, and Y = S, Se, Te), in which X and Y form an ordered sub-structure. Compared with unfilled binary skutterudites, pnictogen-substituted phases exhibit extremely low lattice thermal conductivity due to increased structural complexity. Here, we investigate the role of the transition metal species in determining the electronic structure and transport properties of Asn_(1.5)Te_(1.5) compounds with A = Co, Rh, Ir. Density functional calculations using fully ordered structures reveal semiconducting behavior in all three compounds, with the band gap varying from 0.2 to 0.45 eV. In CoSn_(1.5)Te_(1.5), the electronic density of states near the gap is significantly higher than for A = Ir or Rh, leading to higher effective masses and higher Seebeck coefficients. Experimentally, Ir and Rh samples exhibit relatively large p-type carrier concentrations and degenerate semiconducting behavior. In contrast, CoSn_(1.5)Te_(1.5) shows mixed conduction, with n-type carriers dominating the Seebeck coefficient and light, high mobility holes dominating the Hall coefficient. zT values of up to 0.35 were obtained, and further improvement is expected upon optimization of the carrier concentration or with n-type doping.

Additional Information

© 2015 AIP Publishing LLC. Received 7 April 2015; accepted 27 June 2015; published online 21 July 2015. 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. We gratefully acknowledge Gregory Gerig for performing Seebeck measurements. Financial assistance of the Scientific and Technological Research Council of Turkey and NASA Postdoctoral Fellowship Program are acknowledged by A.U. and A. Z., respectively.

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