Substitution versus full-Heusler segregation in TiCoSb

Half-Heuslers (HHs) are promising thermoelectric materials with great compositional flexibility. Here, we extend work on the p-type doping of TiCoSb using abundant elements. Ti_0.7V_0.3Co_0.85Fe_0.15Sb_0.7Sn_0.3 samples with nominal 17.85 p-type electron count were investigated. Samples prepared using powder metallurgy have negative Seebeck values, S ≤ -120 µV K^-1, while arc-melted compositions are compensated semiconductors with S = -45 to +30 µV K^-1. The difference in thermoelectric response is caused by variations in the degree of segregation of V(Co_0.6Fe_0.4)₂Sn full-Heusler and Sn phases, which selectively absorb V, Fe, and Sn. The segregated microstructure leads to reduced lattice thermal conductivities, κ_lat = 4.5-7 W m^-1 K^-1 near room temperature. The largest power factor, S²/ρ = 0.4 mW m^-1 K^-2 and ZT = 0.06, is observed for the n-type samples at 800 K. This works extends knowledge regarding suitable p-type dopants for TiCoSb.