Roles of Defects and Sb-Doping in the Thermoelectric Properties of Full-Heusler Fe 2 TiSn.

The potential of Fe ₂ TiSn full-Heusler compounds for thermoelectric applications has been suggested theoretically, but not yet proven experimentally, due to the difficulty in obtaining reproducible, homogeneous, phase-pure and defect-free samples. In this work, we studied Fe ₂ TiSn _{1- x} Sb _x polycrystals ( x from 0 to 0.6), fabricated by high-frequency melting and long-time high-temperature annealing. We obtained fairly good phase purity, a homogeneous microstructure, and good matrix stoichiometry. Although the intrinsic p-type transport behavior is dominant, n-type charge compensation by Sb-doping is demonstrated. Calculations of the formation energy of defects and electronic properties carried out using the density functional theory formalism reveal that charged iron vacancies V _Fe ^2- are the dominant defects responsible for the intrinsic p-type doping of Fe ₂ TiSn under all types of (except Fe-rich) growing conditions. In addition, Sb substitutions at the Sn site give rise either to Sb _Sn , Sb _Sn ¹⁺ , which are responsible for n-type doping and magnetism (Sb _Sn ) or to magnetic Sb _Sn ^1- , which act as additional p-type dopants. Our experimental data highlight good thermoelectric properties close to room temperature, with Seebeck coefficients up to 56 μV/K in the x = 0.2 sample and power factors up to 4.8 × 10 ^-4 W m ^-1 K ^-2 in the x = 0.1 sample. Our calculations indicate the appearance of a pseudogap under Ti-rich conditions and a large Sb-doping level, possibly improving further the thermoelectric properties.