Valence band study of thermoelectric Zintl-phase SrZn₂Sb ₂ and YbZn₂Sb₂: X-ray photoelectron spectroscopy and density functional theory

The electronic structure of SrZn₂Sb₂ and YbZn ₂Sb₂ is investigated using density functional theory and high-resolution x-ray photoemission spectroscopy. Both traditional Perdew-Burke-Ernzerhof and state-of-the-art hybrid Heyd-Scuseria-Ernzerhof functionals have been employed to highlight the importance of proper treatment of exchange-dependent Zn3d states, Yb4f states, and band gaps. The role of spin-orbit corrections in light of first-principles transport calculations are discussed and previous claims of Yb³⁺ valence are investigated with the assistance of photoelectron as well as scanning and transmission electron microscopy.