Thermoelectric properties of Zn-doped Ca₅In₂Sb₆

The Zintl compound Ca₅Al₂Sb₆ is a promising thermoelectric material with exceptionally low lattice thermal conductivity resulting from its complex crystal structure. In common with the Al analogue, Ca₅In₂Sb₆ is naturally an intrinsic semiconductor with a low p-type carrier concentration. Here, we improve the thermoelectric properties of Ca₅In₂Sb₆ by substituting Zn²⁺ on the In³⁺ site. With increasing Zn substitution, the Ca₅In_2−xZn_xSb₆ system exhibits increased p-type carrier concentration and a resulting transition from non-degenerate to degenerate semiconducting behavior. A single parabolic band model was used to estimate an effective mass in Ca₅In₂Sb₆ of m* = 2m_e, which is comparable to the Al analogue, in good agreement with density functional calculations. Doping with Zn enables rational optimization of the electronic transport properties and increased zT in accordance with a single parabolic band model. The maximum figure of merit obtained in optimally Zn-doped Ca₅In₂Sb₆ is 0.7 at 1000 K. While undoped Ca₅In₂Sb₆ has both improved electronic mobility and reduced lattice thermal conductivity relative to Ca₅Al₂Sb₆, these benefits did not dramatically improve the Zn-doped samples, leading to only a modest increase in zT relative to optimally doped Ca₅Al₂Sb₆.