The Mechanism behind the High Thermoelectric Performance in YbCd_2-xMg_xSb₂

Abstract

YbCd₂Sb₂ is a promising Zintl compound for waste heat recovery applications due to its low thermal conductivity, originating from its complex crystal structure. Many strategies such as alloying or doping have been suggested to further reduce the thermal conductivity of YbCd₂Sb₂ to improve its thermoelectric performance. However, the effects of alloying or doping on the electronic transport properties of YbCd₂Sb₂ have not been evaluated in detail. Here, previously reported thermoelectric properties of YbCd_2-xMg_xSb₂ (<i>x</i> = 0, 0.2, 0.4) with drastic thermal conductivity suppression were evaluated using the Single Parabolic Band (SPB) model and Callaway von Bayer (CvB) model. The SPB and CvB models evaluate any changes in electronic band parameters and phonon scattering strength, respectively, due to Mg alloying. Based on the SPB model, Mg alloying deteriorated the weighted mobility, mostly due to non-degenerate mobility reduction. However, the magnitude of point-defect phonon scattering significantly increased with Mg alloying, as evaluated by the CvB model. As a result, the maximum <i>zT</i> is achieved when x = 0.4 at 700 K despite the decreased electronic transport properties from Mg alloying. Our work suggests that carefully designed alloying can improve the thermoelectric performance of the Zintl compound even when it changes its electronic and thermal transport properties in opposite directions.