First-principles electronic structure, phonon properties, lattice thermal conductivity and prediction of figure of merit of FeVSb half-Heusler

Abstract: In this work, we have studied the electronic structure of a promising thermoelectric half-Heusler FeVSb using FP-LAPW method and SCAN meta-GGA including spin-orbit coupling. Using the obtained electronic structure and transport calculations we try to address the experimental Seebeck coefficient $S$ of FeVSb samples. The good agreement between the experimental and calculated $S$ suggests the band gap could be $\sim$0.7 eV. This is supported by the obtained mBJ band gap of $\sim$0.7 eV. Further, we study and report the phonon dispersion, density of states and thermodynamic properties. The effect of long range Coulomb interactions on phonon frequencies are also included by non-analytical term correction. Under quasi-harmonic approximation, the thermal expansion behaviour upto 1200 K is calculated. Using the first-principles anharmonic phonon calculations, the lattice thermal conductivity $\kappa_{ph}$ of FeVSb is obtained under single-mode relaxation time approximation considering the phonon-phonon interaction. At 300 K, the calculated $\kappa_{ph}$ is $\sim$18.6 W$m^{-1}K^{-1}$ which is higher compared to experimental value. But, above 500 K the calculated $\kappa_{ph}$ is in good agreement with experiment. A prediction of figure of merit $ZT$ and efficiency for p-type and n-type FeVSb is made by finding out optimal carrier concentration. At 1200 K, a maximum $ZT$ of $\sim$0.66 and $\sim$0.44 is expected for p-type and n-type FeVSb, respectively. For p-type and n-type materials, maximum efficiency of $\sim$12.2 \% and $\sim$6.0 \% are estimated for hot and cold temperature of 1200 K and 300 K, respectively. A possibility of achieving n-type and p-type FeVSb by various elemental doping/vacancy is also discussed. Our study is expected to help in further exploring the thermoelectric material FeVSb.