The Fe₂VAl, a Heusler-type intermetallic compound, is a promising thermoelectric material because of its semimetallic nature and favorable Seebeck coefficient. Under normal conditions, Fe₂VAl exhibits a positive Seebeck coefficient, indicating hole-dominated conduction. However, recent experiments have shown that rapid quenching at high temperatures can reverse the Seebeck coefficient sign, suggesting a shift towards electron-dominated conduction. This reversal occurs without any change in material stoichiometry, indicating a more subtle mechanism that involves atomic-scale disorder.

The key is the formation of antisite defects when atoms occupy incorrect sites. Specifically, vanadium atoms replace iron sites (V_Fe) and vice versa (Fe_V). These defects introduce magnetic impurities into the system, significantly affecting carrier scattering near the Fermi level. A research group at Tokyo University of Science proposed a theoretical model to explain this behavior, introducing a bipolar random Anderson model, which is an extension of the traditional impurity Anderson model used to describe magnetic impurities in metals.

In this model, Fe₂VAl is treated as a semimetal with separated valence and conduction bands, Fe-rich and V-rich, respectively. Antisite defects act as randomly distributed magnetic impurities: V_Fe in the valence band and Fe_V in the conduction band. These impurities generate resonance states near the Fermi level, which are split into spin-up and spin-down states due to Coulomb interactions. These resonance states enhance the scattering of hole carriers in the valence band more than electron carriers do in the conduction band.

To analyze this effect quantitatively, researchers have employed the self-consistent T-matrix approximation, a method suitable for handling multiple scattering events from randomly distributed impurities. This approach allows the calculation of carrier-dependent scattering probabilities and temperature dependence of the Seebeck coefficient. Their results showed that the increased scattering of holes suppresses their contribution to thermoelectric voltage, allowing the electrons to dominate and reverse the Seebeck coefficient from positive to negative.

Notably, this mechanism does not rely on doping or altering the carrier concentration but instead leverages the disorder-induced scattering asymmetry. The model successfully reproduced the experimental observations and provides a new framework for understanding and controlling the thermoelectric properties of magnetic semimetals.

This discovery opens up a novel strategy for thermoelectric material design. By engineering antisite defects and magnetic disorder, the direction and magnitude of the thermoelectric voltage can be tuned. Such control is crucial for optimizing energy conversion efficiency and can lead to the development of next-generation thermoelectric devices with enhanced performance and stability.

(Written by Takami Tohyama)

Share this topic

Fields

Related Articles