Mechanical and thermoelectric response of 18-valence electron half-Heusler tellurides XFeTe (X=Ti, Hf): A theoretical perspective

Abstract

To address growing global energy needs, half Heusler compounds offer a cost-effective and efficient solution for power generation applications. We present a study on the structural, electronic, magnetic, phonon, mechanical and thermoelectric (TE) properties of two hH tellurides, XFeTe (X= Ti, Hf) with 18 valence electrons. The investigation employs Density functional theory (DFT), Semi-classical Boltzmann transport equations (BTE), Quasi-harmonic approximation (QHA), Density functional perturbation theory (DFPT) and Deformation potential theory (DPT). The equilibrium lattice constants are determined to be 5.882 Å and 6.041 Å for TiFeTe and HfFeTe respectively. The electronic structures of TiFeTe and HfFeTe are modeled with generalised gradient approximation (GGA), revealing indirect band gaps of 0.93 eV and 0.79 eV, respectively. The conduction bands in TiFeTe and HfFeTe exhibit remarkably low effective masses of 0.72 me and 0.61 me, respectively, resulting in higher carrier relaxation times. The compounds are dynamically and mechanically stable. TiFeTe is ductile whereas HfFeTe is brittle. The elastic constants analysis suggests that these compounds exhibit stiffness, better hardness, elastic anisotropy and high melting point. TE parameters are investigated across a temperature range from 400 K to 1200 K. At room temperature, TiFeTe and HfFeTe exhibit lattice thermal conductivities of 25.43 Wm−1K−1 and 29.29 Wm−1K−1, respectively. At 1200 K, zT for the n-type, p-type compositions are 1.79, 1.11 for TiFeTe and 1.35, 1.06 for HfFeTe, respectively. The n-type composition not only exhibits superior TE performance compared to the p-type but also shows the peak zT at more feasible doping levels.