Enhancement of thermoelectric properties at room temperature through isoelectronic ruthenium-substitution in n-type half-Heusler VFeSb compounds

The effects of ruthenium (Ru) substitution on the thermoelectric properties of half-Heusler (HH) V(Fe_1−xRu_x)Sb compounds have been theoretically and experimentally investigated. All V(Fe_1−xRu_x)Sb samples with deficient HH structures reveal deficiencies in V 4a and Fe 4c sites, interstitial Fe occupied 4d sites, and Ru substituted for Fe 4c sites, by Rietveld analysis. Based on the refined crystal structures and electronic structure analysis, the Ru content x with a maximum power factor (PF) is predicted to be around x=0.02. A small amount of Ru-substitution improves the PF by 26% at room temperature for the V(Fe_0.98Ru_0.02)Sb sample, which is coincident with the predicted range of Ru content for maximum PF. The lattice thermal conductivity κ_L of the V(Fe_1−xRu_x)Sb samples decreases with an increase in x. The atomic mass and radius of Ru are substantially larger than those of Fe; therefore, κ_L of the V(Fe_1−xRu_x)Sb samples decreases mainly due to phonon scattering strengthened by the introduction of Ru at the Fe 4c sites. A significantly reduced κ_L of 5.8 W/Km at room temperature is obtained for the V(Fe_0.92Ru_0.08)Sb sample, which is a ca. 40% decrease compared with the pristine HH-VFeSb sample. The dimensionless thermoelectric figure-of-merit reaches close to 0.2 at 300 K for the V(Fe_0.98Ru_0.02)Sb sample.