Enhancement of thermoelectric performance of phase pure Zintl compounds Ca_1-xYb_xZn₂Sb₂, Ca_1-xEu_xZn₂Sb₂, and Eu_1-xYb_xZn₂Sb₂ by mechanical alloying and hot pressing

It has been previously shown that Zintl compounds Ca_1-xYb_xZn₂Sb₂ and EuZn₂Sb₂ could be good candidates as thermoelectric materials. However, the conventional synthesis process via melting-solidification-annealing introduces impurities and vacancies, resulting in abnormal high carrier concentration and ultimately low thermoelectric properties. Here we report the enhanced thermoelectric performance of Ca_1-xYb_xZn₂Sb₂ (x=0, 0.25, 0.5, 0.75, and 1) prepared by ball milling and hot pressing. XRD confirms the samples are pure Zintl phase within its limit. Other compounds EuZn₂Sb₂, Eu_0.5Yb_0.5Zn₂Sb₂, and Eu_0.5Ca_0.5Zn₂Sb₂ are also prepared by ball milling and hot pressing to further understand them. The observed changes in effective mass appear to be one of the reasons for the big difference of carrier mobility in Ca and rare earth (Yb, Eu) alloyed compounds. The defects caused by alloying are the dominant phonon scattering source in these materials. The highest figure of merit of ~0.9 is achieved in Ca_0.25Yb_0.75Zn₂Sb₂, ~50% higher than the best reported ZT in similar materials prepared by melting-solidification-annealing method.