Cu-substitution-driven orbital and phonon engineering breaks the thermoelectric trade-off in n-type Fe₂VAl-based...

Narrow-bandgap full-Heusler Fe₂VAl-based alloys have received increasing attention for thermoelectric applications due to their favorable electrical transport behavior, high mechanical strength, and Earth-abundant constituent elements. However, the significant enhancement of the figure of merit is severely hindered by the low Seebeck coefficient due to the near-linear band structure and intrinsically high lattice thermal conductivity caused by the strong bonding. Here, employing first-principles calculations, we find that substituting 1/16 Fe atoms with Cu atoms can induce a strong localization of the 3d e_g orbital electrons of Fe and V atoms around the impurity atoms. This leads to the coexistence of dispersive bands and flat bands near the Fermi level, significantly boosting the Seebeck coefficient while maintaining high electrical conductivity, yielding a power factor of 5.8 mW m^-1 K^-2 at room temperature. Meanwhile, the Cu atoms with large atomic displacement form weak bonds with surrounding V atoms, strengthening low-frequency phonon scattering. Furthermore, the introduction of Cu atoms enhances the crystal anharmonicity, triggering a further decrease in the lattice thermal conductivity to 4.57 W m^-1 K^-1 at 300 K. Consequently, promising zT values of 0.36 and 2.3 are achieved at 300 K and 700 K, respectively. Our work demonstrates that Cu substitution in Fe sites may successfully decouple electrical and thermal transport of Fe₂VAl, offering an effective and scalable strategy for designing high-performance Fe₂VAl-based thermoelectric materials.