Theoretical prediction of electronic and thermal transport properties of the double half-Heusler compounds X2FeNiSn2 (X=Ta,Nb,V)

Driven by the growing interest in double-half-Heusler structures known for their lower thermal conductivity compared to traditional half-Heusler materials, this study employs first-principles computational calculations to explore the thermoelectric properties of ${X}_{2}{\mathrm{FeNiSn}}_{2}$ ($X=\mathrm{Ta},\mathrm{Nb} \text{and} \mathrm{V}$) compounds. We analyzed the structural, mechanical, electronic, and vibrational properties of these structures and calculated their thermoelectric properties using the Boltzmann transport theory for electron and phonon transport. The electronic transport properties are also comparatively studied with constant relaxation time approximation, full-band relaxation time approximation, and energy-dependent phonon- and impurity-limited carrier scattering time approximation. Our findings indicate that considering electron scattering from all phonon branches is critical for accurately assessing the transport properties of these materials, resulting in significantly different $ZT$ values compared to those obtained using the constant and full-band relaxation time approximations.