The Zintl Compound Ca₅Al₂Sb₆ for Low‐Cost Thermoelectric Power Generation

TLDR

Zintl compounds are of interest for thermoelectricity because their unusual chemistry and structural complexity lead to low thermal conductivity and promising transport properties. The authors use a single parabolic band model to predict that Ca₄.₇₅Na₀.₂₅Al₂Sb₆ has an optimal carrier concentration for thermoelectric power generation. Ca₅Al₂Sb₆ is a charge‑balanced semiconductor with a 0.5 eV bandgap, but Na doping creates holes that turn it metallic; it exhibits a very low lattice thermal conductivity (0.6 W m⁻¹ K⁻¹ at 850 K) and a hole mass of ~2 mₑ, leading to a predicted near‑optimal carrier concentration and a maximum zT > 0.6 at 1000 K, while its 1D covalent structure may enable interesting magnetic behavior.

Abstract

Abstract Understanding transport in Zintl compounds is important due to their unusual chemistry, structural complexity, and potential for good thermoelectric performance. Resistivity measurements indicate that undoped Ca 5 Al 2 Sb 6 is a charge‐balanced semiconductor with a bandgap of 0.5 eV, consistent with Zintl–Klemm charge counting rules. Substituting divalent calcium with monovalent sodium leads to the formation of free holes, and a transition from insulating to metallic electronic behavior is observed. Seebeck measurements yield a hole mass of ∼2 m e , consistent with a structure containing both ionic and covalent bonding. The structural complexity of Zintl compounds is implicated in their unusually low thermal conductivity values. Indeed, Ca 5 Al 2 Sb 6 possesses an extremely low lattice thermal conductivity (0.6 W mK −1 at 850 K), which approaches the minimum thermal conductivity limit at high temperature. A single parabolic band model is developed and predicts that Ca 4.75 Na 0.25 Al 2 Sb 6 possesses a near‐optimal carrier concentration for thermoelectric power generation. A maximum zT > 0.6 is obtained at 1000 K.Beyond thermoelectric applications, the semiconductor Ca 5 Al 2 Sb 6 possesses a 1D covalent structure which should be amenable to interesting magnetic interactions when appropriately doped.