Selective Scatterings of Phonons and Electrons in Defective Half-Heusler Nb1-CoSb for the Figure of Merit Zt > 1

The recently developed defective 19-electron half-Heusler (HH) compounds, represented by Nb1-delta CoSb, possess massive intrinsic vacancies at the cation site and thus intrinsically low lattice thermal conductivity that is desirable for thermoelectric (TE) applications. Yet the TE performance of defective HHs with a maximum figure of merit (zT) <1.0 is still inferior to that of the conventional 18-electron ones. Here, a peak zT exceeding unity is obtained at 1123 K for both Nb0.7Ta0.13CoSb and Nb0.6Ta0.23CoSb, a benchmark value for defective 19-electron HHs. The improved zT results from the achievement of selective scatterings of phonons and electrons in defective Nb0.83CoSb, using lanthanide contraction as a design factor to select alloying elements that can strongly impede the phonon propagation but weakly disturb the periodic potential. Despite the massive vacancies induced strong point defect scattering of phonons in Nb0.83CoSb, Ta alloying is still found effective in suppressing lattice thermal conductivity while maintaining the carrier mobility almost unchanged. In comparison, V alloying significantly deteriorates the carrier transport and thus the TE performance. These results enlarge the category of high-performance HH TE materials beyond the conventional 18-electron ones and highlight the effectiveness of selective scatterings of phonons and electrons in developing TE materials even with massive vacancies.