Structure and thermoelectric property evolution of TiFe1.1−xNixSb with the filling of Ni

Abstract

TiFeSb is a multiphase material with the half-Heusler structure as the main phase. In this work, we discover that TiFeSb can be stabilized by filling Ni to the vacant 4d site of the half-Heusler structure, and a single-phase TiFeNi0.1Sb alloy is obtained. To improve the thermoelectric properties, TiFe1.1−xNixSb (x = 0.1–1.0) samples with more valence electrons are designed and synthesized. Structural analysis reveals that the samples with x = 0–0.3 are single-phase alloys with the half-Heusler-like structure, the samples with x = 0.4–0.7 are two-phase materials with the double half-Heusler as the main phase, and the samples with x = 0.8–1.0 are single-phase alloys with the full-Heusler-like structure. Meanwhile, both promising p-type (x = 0–0.3) and n-type (x = 0.8–1.0) electrical properties are realized in TiFe1.1−xNixSb, which can be well explained by the Slater-Pauling rule. In combination with greatly reduced lattice thermal conductivity owing to the disordered crystal structure, the p-type TiFe0.8Ni0.3Sb and n-type TiFe0.3Ni0.8Sb achieve dimensionless thermoelectric figure of merits zT about 0.6 and 0.4 at 973 K, respectively. We also demonstrate that the thermoelectric properties of these samples can be further optimized by conventional doping or alloying methods. By replacing Ni (or Fe) with Cu, the carrier concentration of p-type TiFe0.8Ni0.2Cu0.1Sb is decreased, and the zT value reaches 0.8 at 973 K, which is the record of vacancy-filled Heusler alloys.