Metal Distributions, Efficient n-Type Doping, and Evidence for in-Gap States in TiNiM_ySn (M = Co, Ni, Cu) half-Heusler Nanocomposites

XNi_1+ySn nanocomposites consisting of a XNiSn half-Heusler (HH) matrix with segregated XNi₂Sn Full Heusler (FH) inclusions promise improvements in thermoelectric efficiencies. We extend recent research by reporting on TiNiM_ySn (0 ≤ y ≤ 1) nanocomposites with M = Co (3d⁹), Ni (3d¹⁰) and Cu (3d¹⁰4s¹). Neutron powder diffraction reveals that the Ni and Cu series produce a matrix of TiNiSn with nanosegregated TiNi₂Sn and TiNi_1+dCu_1–dSn, respectively. For the Co series, the Co inserts into both phases to obtain a TiNi_1–yCo_ySn matrix with nanosegregated TiNi_2–yCo_ySn. Systematic changes in Seebeck coefficient (S) and electrical resistivity (ρ) are observed in all three series. For M = Ni, changes in S and ρ are attributed to in-gap states arising from the nanosegregation. The M = Co composites show a complex interplay between the hole doped TiNi_1–yCo_ySn matrix and similar in-gap states, where the p- to n-type transition temperature increases but the maximum S remains unchanged at +30 μV K^–1. The 4s¹ electron for M = Cu is delocalized in the HH matrix, leading to metal-like ρ(T) and up to 100% improved thermoelectric power factors compared to TiNiSn (S²/ρ = 2 mW m^–1 K^–2 at 600–700 K for y = 0.025). These results broaden the range of segregated FH phases that could be used to enhance HH thermoelectric performance.