The Zintl Compound Ca_5Al_2Sb_6 for Low-Cost Thermoelectric Power Generation
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_5Al_2Sb_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 ∼2m_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_5Al_2Sb_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_2Sb_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_5Al_2Sb_6 possesses a 1D covalent structure which should be amenable to interesting magnetic interactions when appropriately doped.