Improved thermoelectric properties in Zn-doped Ca5Ga2Sb6

Abstract

Zintl compounds with the chemical formula Ca₅M₂Sb₆ have attracted attention as candidates for use in thermoelectric applications due to their low thermal conductivity and promising high temperature performance (i.e., zT = 0.6 at 1000 K in Ca₅Al_2−xNa_xSb₆). We have shown previously that, relative to Ca₅Al₂Sb₆, both Ca₅Ga₂Sb₆ and Ca₅In₂Sb₆ have reduced phonon velocities and improved carrier mobility, suggesting that improved zT can be achieved in these materials. Here we further investigate Ca₅Ga₂Sb₆, which is an intrinsic semiconductor with a small concentration of p-type carriers. By substituting Zn²⁺ on the Ga³⁺ site, we show that it is possible to increase and control the carrier concentration in Ca₅Ga_2−xZn_xSb₆ and thus optimize its thermoelectric behavior. A single parabolic band model was used to estimate an effective mass of m* = 1.6m_e, which is slightly lower than Al-based compounds. Though the reduced m* leads to a lower Seebeck coefficient, it also leads to a much higher electronic mobility. The high mobility leads to increased thermoelectric figure of merit (zT) at low and intermediate temperatures relative to Zn-doped Ca₅Al₂Sb₆. However, due to the decreased band gap in Ca₅Ga₂Sb₆ relative to Ca₅Al₂Sb₆, the maximum zT in optimally doped Ca₅Ga₂Sb₆ is reduced (peak zT ∼ 0.35 at T = 775 K).