An ab initio study of novel vanadium-based half Heusler alloys VRhZ (Z = Ge, Si and Sn) as high temperature thermoelectric materials

We investigated three novel 18 valence electron count (VEC) vanadium based half-Heusler compounds, VRhZ (Z = Ge, Si and Sn), for their structural, mechanical, electronic, lattice dynamics and thermoelectric properties. The Perdew–Burke–Ernzerhof generalized gradient approximation (GGA-PBE) is used for exchange–correlation functional within first principle Density Functional Theory (DFT). Spin–orbit coupling (SOC) is included in the calculations and all the three alloys have exhibited structural, mechanical, and dynamical stability. The influence of SOC interaction in the systems generated a significant shift in the bands. We obtained the lattice constants of 5.496, 5.807, and 5.795 Å for VRhGe, VRhSi, and VRhSn respectively. The elastic parameters predict a ductile and hard nature of the compounds, and they exhibit anisotropic behaviour. The calculated electronic band structure demonstrated that all three alloys are semiconductors with indirect band gap values of 0.28, 0.69, and 0.27 eV for VRhGe, VRhSi and VRhSn respectively. The non-existence of imaginary frequency in the phonon dispersion affirms the dynamic stability of the investigated compounds. The highest values of Electronic Fitness Function (EFF) at 800K with SOC are calculated for VRhGe, VRhSi, and VRhSn respectively. The figure of merit (ZT) obtained at 800 K with SOC are 0.89, 0.92 and 0.95 for VRhGe, VRhSi, and VRhSn respectively. These values of ZT and EFF suggest that VRhZ (Z = Ge, Si and Sn) compounds are promising high performance thermoelectric materials.