Electronic structure and transport in thermoelectric compounds AZn2Sb2 (A = Sr, Ca, Yb, Eu)

The AZn(2)Sb(2) (P3m1, A = Ca, Sr, Eu, Yb) class of Zintl compounds has shown high thermoelectric efficiency (zT approximately 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(2)Sb(2); here we characterize polycrystalline SrZn(2)Sb(2) to 723 K and review the transport behavior of the other compounds in this class. Consistent with the known AZn(2)Sb(2) compounds, SrZn(2)Sb(2) is found to be a hole-doped semiconductor with a thermal band gap approximately 0.27 eV. The Seebeck coefficients of the AZn(2)Sb(2) compounds are found to be described by similar effective mass (m* approximately 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.