Stability of X-IV-IV half Heusler semiconductor alloys: a DFT study

Abstract

We present reports for the structural, electronic, mechanical (elastic), and thermodynamic properties of NiHfX (X = Si, Sn, Ge) half Heusler compounds. We performed the calculation based on the density functional theory as implemented in the quantum espresso computational suite. We also predict the structural stability of the alloys, and the mechanical and thermodynamic properties using the linear response density functional perturbation theory. The compounds are non-magnetic semiconductors with narrow bandgaps. Results for the equilibrium lattice parameter for NiHfSn is in reasonable agreement with reports in the existing literature. The elastic parameters obey Born’s stability criteria, and there are no negative phonon dispersions, hence, establishing the compounds’ mechanical stability. NiHfGe is a brittle and hard material with directional covalent bonding, while NiHfSi and NiHfSn alloys are ductile and malleable. The alloys obey the Dulong-Petit law at a heat capacity of 72 and 74 J/Kmol and temperatures of 720 K and 800 K. The phonon frequencies shows that the wavelength of the materials is the far-infrared region. NiHfSi has the highest of 411.68 K at zero pressure and will have the highest thermal conductivity implying less viability as a thermoelectric material than NiHfSn with a of 340.19 K.