Role of Spin–Orbit Coupling in Modulating the Scattering Mechanisms and Transport Properties of NbFe-Based Half-Heusler Thermoelectric Materials

We investigate the electronic and thermal transport properties of promising p-type thermoelectric half-Heusler compounds NbFeZ (Z = As, Sb, Bi) using density functional theory (DFT) combined with Boltzmann transport calculations. Till date, most theoretical studies have relied on the constant relaxation time approximation (CRTA), often neglecting detailed scattering processes. In this study, comprehensive carrier scattering mechanisms, including acoustic deformation potential (ADP), ionized impurity (IMP), polar optical phonon (POP), and mean free path (MFP) scattering, are explicitly incorporated to provide a more realistic description of transport behavior. The influence of spin–orbit coupling (SOC), which is often neglected in theoretical studies is taken into consideration and its effect on the transport coefficients is highlighted. Our results show that POP scattering dominates across all compounds due to the intrinsic polar nature of NbFeZ, severely limiting carrier lifetimes and suppressing electrical conductivity. Additionally, we calculate the lattice thermal conductivity (κL) using anharmonic phonon interactions and find that NbFeBi exhibits the lowest κL among the series. This reduction arises from a pronounced phonon frequency gap and strong anharmonic scattering, resulting in a thermoelectric figure of merit (zT) of 1.47 at 1200 K, which is significantly higher than the earlier reported zT values. Although heavy elements can introduce adverse effects on electronic transport through band structure modifications, they simultaneously enhance phonon scattering, creating a beneficial trade-off. Overall, this study highlights the critical role of specific scattering mechanisms and the effect of SOC in determining the thermoelectric performance of these materials.