Thermoelectric property enhancement by merging bands in NbFeSb-based half-Heusler mixtures

Abstract

Half-Heusler (HH) materials with 18 electrons exhibit semiconducting behavior and are potential candidates for thermoelectric materials. Based on the well-studied 18-e NbFeSb compound, we mix the corresponding 17-e and 19-e systems to construct the novel 18-e HH compounds: replacing the Fe or Nb elements with their corresponding left and right elements in the periodic table (Mn and Co for Fe; Zr and Mo for Nb). Two new thermodynamically stable 18-e HH compounds (Nb₄Mn₂Co₂Sb₄ and Zr₂Mo₂Fe₄Sb₄) are predicted using the cluster expansion methodology. Owing to the merging valence band maximum in 17-e NbMnSb and 19-e NbCoSb, the Nb₄Mn₂Co₂Sb₄ mixture possesses high band degeneracy (N^L_v = 16) and significantly boosts the Seebeck coefficient and the power factor, much higher than those of NbFeSb. Additionally, the complex geometries and soft acoustic branches in Nb₄Mn₂Co₂Sb₄ and Zr₂Mo₂Fe₄Sb₄ induce lower thermal conductivities than NbFeSb. Finally, the calculated p-type (n-type) zT value of Nb₄Mn₂Co₂Sb₄ can reach 0.72 (0.62) at 1000 K, higher than that of pristine NbFeSb. Our results not only provide a new HH research system for experiments, but also provide a band engineering methodology for the preparation of HH thermoelectric materials with excellent performance.