Electronic structure and transport in thermoelectric compounds AZn_2Sb_2 (A = Sr, Ca, Yb, Eu)
The AZn_2Sb_2 (P¯3m1, A = Ca, Sr, Eu, Yb) class of Zintl compounds has shown high thermoelectric efficiency (zT ~ 1) and is an appealing system for the development of Zintl structure–property relationships. High temperature transport measurements have previously been conducted for all known compositions except for SrZn_2Sb_2; here we characterize polycrystalline SrZn_2Sb_2 to 723 K and review the transport behavior of the other compounds in this class. Consistent with the known AZn_2Sb_2 compounds, SrZn_2Sb_2 is found to be a hole-doped semiconductor with a thermal band gap ~ 0.27 eV. The Seebeck coefficients of the AZn2Sb2 compounds are found to be described by similar effective mass (m* ~ 0.6 m_e). Electronic structure calculations reveal similar m* is due to antimony p states at the valence band edge which are largely unaffected by the choice of A-site species. However, the choice of A-site element has a dramatic effect on the hole mobility, with the room temperature mobility of the rare earth-based compositions approximately double that found for Ca and Sr on the A site. This difference in mobility is examined in the context of electronic structure calculations.
Additional Information© 2010 Royal Society of Chemistry. Received 15th July 2009; accepted 16th September 2009; first published as an Advance Article on the web 23rd October 2009. EST thanks the Beckman Foundation for support. We thank the DARPA-Nano Materials Program and NASA-JPL for funding and facilities. BCM gratefully thanks J. M. Rondinelli and K. T. Delaney for fruitful discussions regarding the band structure calculations as well as the NSF for support through a Career Award (DMR 0449354) to his research advisor Ram Seshadri at UCSB.
Published - Toberer2010p6904Dalton_T.pdf