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Double half‐Heuslers comprising two aliovalent half‐Heuslers are promising candidates for thermoelectric materials because of their intrinsically low lattice thermal conductivity; however, poor electronic transport properties need to be overcome. Herein, the effects of Sn doping on the electronic and thermal transport properties of p‐type Ti2FeNiSb2 to enhance the thermoelectric performance via the compositional tuning route are investigated. The power factor is significantly improved owing to the synergetic effect of the increase in the density‐of‐states effective mass and the carrier concentration. In addition, the lattice thermal conductivity is slightly reduced, benefitted from intensified phonon scattering due to lattice disordering by Sn substitution at the Sb‐site. A peak figure of merit (zT) of ≈0.28 is obtained at 973 K in Ti2FeNiSb1.8Sn0.2, which is almost twice higher than that of the pristine Ti2FeNiSb2.

Ti 2 FeNiSb 2 is a promising double half-Heusler thermoelectric compound with intrinsically low thermal conductivity due to low phonon group velocity. Since it is introduced in 2019, many efforts have been focused on further reducing its thermal conductivity via doping. However, the effects of doping on its electronic transport properties have been neglected. Here, we investigate the effects of doping Co and Bi at the Fe- and Sb-sites, respectively, in Ti 2 FeNiSb 2 for the first time. Changes in band parameters due to atomic site-targeted doping are estimated by the Single Parabolic Band model. Bypassing the trade-off relation between the Seebeck coefficient and electrical conductivity is observed when doping Co at Fe-sites. The physics behind the bypass is explained in terms of temperature-dependent reduced chemical potential and non-degenerate mobility. As a result, peak figure of merit zT values of ∼0.69 in Ti 2 Fe 0.9 Co 0.1 NiSb 2 is achieved, which is approximately six times higher than that of the pristine Ti 2 FeNiSb 2. The thermoelectric performance of Ti 2 FeNiSb 2 can be improved by selective band engineering to bypass the Seebeck coefficient-electrical conductivity trade-off relation from atomic-site targeted doping. • Atomic site-targeted doping can fine-tune band parameters of double half-Heusler. • Doping at different atomic sites has a different impact on the phonon scattering. • Doping Co at Fe-sites of Ti 2 FeNiSb 2 improves zT of pristine Ti 2 FeNiSb 2 by six times. [ABSTRACT FROM AUTHOR]