Improved thermoelectric properties in Zn-doped Ca5Ga2Sb6
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 Ca5Al2−xNaxSb6). 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 Ca5Ga2−xZnxSb6 and thus optimize its thermoelectric behavior. A single parabolic band model was used to estimate an effective mass of m* = 1.6me, 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).