Enhancing the thermoelectric efficiency of n-type half-Heusler compounds via self doping

Ternary intermetallic XYZ half Heusler compounds (ZrNiSn, ZrPdSn, ZrCoSb) are promising thermoelectric compounds. The presence of a large number of tetrahedral voids in their structure as compared to the full Heusler stoichiometry (XY₂Z) offers the possibility of off stoichiometric compositional tailoring. Existing studies have shown that small concentration of self-doping at Y site in 18-electron based half Heuslers leads to anomalous lowering in the k_L while leading to suitable n-type doping of the materials. We explore the change in the electronic and thermal transport behaviour of ZrNi_1+xSn, ZrPd_1+xSn, and ZrCo_1+xSb compounds (for x = 0, 0.03, 0.125, 0.5, 1) from the first principles perspective. Our calculations reveal that ultralow self-doping of the composition by Y site elements lead to the maximum positive impact in the transport coefficients (i.e. k_L reduces by more than 60 %, while the electronic transport coefficients enhance by 10 - 20 %) . These increase the figure of merit of these compounds considerably.