Convergence of multi-valley bands as the electronic origin of high thermoelectric performance in CoSb3 skutterudites

Abstract

Filled skutterudites RxCo4Sb12 are excellent n-type thermoelectric materials owing to their high electronic mobility and high effective mass, combined with low thermal conductivity associated with the addition of filler atoms into the void site. The favourable electronic band structure in n-type CoSb3 is typically attributed to threefold degeneracy at the conduction band minimum accompanied by linear band behaviour at higher carrier concentrations, which is thought to be related to the increase in effective mass as the doping level increases. Using combined experimental and computational studies, we show instead that a secondary conduction band with 12 conducting carrier pockets (which converges with the primary band at high temperatures) is responsible for the extraordinary thermoelectric performance of n-type CoSb3 skutterudites. A theoretical explanation is also provided as to why the linear (or Kane-type) band feature is not beneficial for thermoelectrics.

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Figure 1: Experimental and theoretical evidence showing multiple conduction bands in n-type CoSb3.
Figure 2: Band non-parabolicity and its effect on the Seebeck coefficient and energy-dependent Seebeck effective mass, mS(E).
Figure 3: High-temperature band convergence in CoSb3 as shown from optical absorption and thermoelectric figure of merit.

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Acknowledgements

We acknowledge the funding support of the Materials Project by Department of Energy Basic Energy Sciences Program under Grant No. EDCBEE, DOE Contract DE-AC02-05CH11231 (DFT band structure calculation, Fermi surface plot, optical measurements, modelling); DOE-Gentherm (sample synthesis, structural characterization and thermoelectric property measurements); Solid-State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0001299 (modelling, preparation of the manuscript). L.A., M.B.N. and S.C. acknowledge the support of the DOD (ONR-MURI under Contract N00014-13-1-0635). The band structure analysis for this project was mainly performed at Texas Advanced Computing Center (TACC) at the University of Texas Austin. The authors thank the Molecular Materials Research Center (MMRC) at the Beckman Institute at Caltech for use of their optical equipment for measurements performed in this work. We thank Y. Li, X. Shi and L. Chen of the Shanghai Institute of Ceramics, Chinese Academy of Sciences for ZEM-3 measurements as part of the International S&T Cooperation Program of China (2015DFA51050). We also thank H. Xiao and T. Chapasis for helpful discussions regarding this paper.

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This paper was written collaboratively by Y.T., Z.M.G. and G.J.S. with input from all other authors. Sample synthesis, structural characterization and thermoelectric transport property measurements were performed by Y.T. Optical measurements were performed by Z.M.G. Development of the Kane band model effective mass relation was performed by Z.M.G. and confirmed by Y.T. Band modelling was done collaboratively by Y.T. and Z.M.G. with assistance from H.-S.K. Electronic band structure calculations and Fermi surface plotting were performed by L.A.A. G.L. validated L.A.A.’s DFT calculations and provided additional results and insight. M.B.N. and S.C. contributed to helpful discussions.

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Correspondence to G. Jeffrey Snyder.

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Tang, Y., Gibbs, Z., Agapito, L. et al. Convergence of multi-valley bands as the electronic origin of high thermoelectric performance in CoSb3 skutterudites. Nature Mater 14, 1223–1228 (2015). https://doi.org/10.1038/nmat4430

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