Phase Boundary Mapping to Obtain n-type Mg₃Sb₂-Based Thermoelectrics
Zintl compounds make excellent thermoelectrics with many opportunities for chemically tuning their electronic and thermal transport properties. However, the majority of Zintl compounds are persistently p-type even though computation predicts superior properties when n-type. Surprisingly, n-type Mg₃Sb₂-based thermoelectrics have been recently found with exceptionally high figure of merit. Excess Mg is required to make the material n-type, prompting the suspicion that interstitial Mg is responsible. Here we explore the defect chemistry of Mg₃Sb₂ both theoretically and experimentally to explain why there are two distinct thermodynamic states for Mg₃Sb₂ (Mg-excess and Sb-excess) and why only one can become n-type. This work emphasizes the importance of exploring all of the multiple thermodynamic states in a nominally single-phase semiconductor. This understanding of the existence of multiple inherently distinct different thermodynamic states of the same nominal compound will vastly multiply the number of new complex semiconductors to be discovered for high zT thermoelectrics or other applications.
Additional Information© 2017 Published by Elsevier Inc. Received 19 July 2017, Revised 4 October 2017, Accepted 8 November 2017, Available online 1 December 2017, Published: December 1, 2017. The authors would like to thank Prof. Vladan Stevanović of Colorado School of Mines for fruitful discussions regarding the defect formation energy calculation. This work was supported by the NASA Science Mission Directorate's Radioisotope Power Systems Thermoelectric Technology Development. This work made use of the IMSERC and EPIC facilities at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the State of Illinois and International Institute for Nanotechnology (IIN). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract no. DE-AC02-06CH11357. P.G. and E.S.T. acknowledge NSF DMREF 1729594. G.J.S. acknowledges NSF DMREF 1729487. Author Contributions: S.O., S.D.K., and G.J.S. conceived and designed this research project; S.O. and K.I. carried out synthesis and characterizations at Northwestern University; H.T., H.K.S., and T.K. executed experiments at Panasonic; S.O. analyzed all the experimental data; S.A. performed phase diagram calculation; P.G. conducted DFT defect calculations; S.O., S.D.K., S.A., H.T., P.G., E.S.T., and G.J.S. prepared and edited the manuscript.
Supplemental Material - mmc1.pdf