Thermoelectric properties of Zn-doped Ca_5In_2Sb_6

The Zintl compound Ca_5Al_2Sb_6 is a promising thermoelectric material with exceptionally low lattice thermal conductivity resulting from its complex crystal structure. In common with the Al analogue, Ca_5In_2Sb_6 is naturally an intrinsic semiconductor with a low p-type carrier concentration. Here, we improve the thermoelectric properties of Ca_5In_2Sb_6 by substituting Zn^(2+) on the In^(3+) site. With increasing Zn substitution, the Ca_5In_(2−x)Zn_xSb_6 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 Ca_5In_2Sb_6 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 Ca_5In_2Sb_6 is 0.7 at 1000 K. While undoped Ca_5In_2Sb_6 has both improved electronic mobility and reduced lattice thermal conductivity relative to Ca_5Al_2Sb_6, these benefits did not dramatically improve the Zn-doped samples, leading to only a modest increase in zT relative to optimally doped Ca_5Al_2Sb_6.