Atomic site-targeted doping in Ti₂FeNiSb₂ double half-Heusler alloys : zT improvement via selective band engineering and point defect scattering

Ti₂FeNiSb₂ 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 Ti₂FeNiSb₂ 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 Ti₂Fe_0.9Co_0.1NiSb₂ is achieved, which is approximately six times higher than that of the pristine Ti₂FeNiSb₂. The thermoelectric performance of Ti₂FeNiSb₂ can be improved by selective band engineering to bypass the Seebeck coefficient-electrical conductivity trade-off relation from atomic-site targeted doping.