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

The AZn₂Sb₂ (Pm1, 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 SrZn₂Sb₂; here we characterize polycrystalline SrZn₂Sb₂ to 723 K and review the transport behavior of the other compounds in this class. Consistent with the known AZn₂Sb₂ compounds, SrZn₂Sb₂ 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.