Unexpected band gap increase in the Fe2VAl Heusler compound

Abstract

Knowing the electronic structure of a material is essential in energy applications to rationalize its performance and propose alternatives. Materials for thermoelectric applications are generally smallgap semiconductors and should have a high figure of merit ZT. Even if the Fe2VAl Heusler compound has a decent ZT, its conductive nature (semimetal or semiconductor) is not yet clarified especially at low temperature. In this paper, we focus our DFT calculations on the effect of temperature on the bandgap of Fe2VAl. In contrast to what is usually observed, we show that both the temperature increase and the formation of thermally-activated Al/V inversion defects (observed experimentally), open the bandgap. Such an unusual behavior is the key for reconciling all bandgap measurements performed on the Fe2VAl compound using a standard GGA functional and could be an efficient way for improving the thermoelectric properties of this family of materials.

Anderson transition: a novel route to high thermoelectric performance

“…This picture of Anderson-localised states close to the Fermi level also reconciles many other peculiar properties of this compound, e.g. metallic thermodynamic and photoemission data in spite of semiconductorlike transport properties 20 , negative magnetoresistance 29 , anomalous Hall effect 30 etc., which have been an ongoing discussion over the past three decades 14,20,31 .…”

The electronic pseudo band gap states and electronic transport of the full-Heusler compound Fe2VAl

“…8 Quite recently, a further remarkable observation was made from DFT calculations: an opening of the band gap, driven by Al/V antisite occupations, which increases with temperature. 18 Antisite defects in the crystal structure of Fe 2 VAl will have an effect on physical properties, involving transport properties as well. From DFT calculations, Bandaru and Jund 19 demonstrated that the antisite defects Al V , Al Fe and V Al have the lowest formation enthalpies, thus being the most probable ones.…”

Numerical and experimental determination of the thermal conductivity of pristine and substituted Fe2VAl

“…They can also be seen as two combined Al V and V Al antisite defects. Since their formation energy is equal to 0.34 eV [16], they are the most likely thermally activated defects as shown in experiments [17] and simulations [16] and thus can be formed during annealing at high temperature T > 800 °C. Moreover, in our previous study [16], we showed that by taking Al/V inversions and temperature effects into account, the disagreement between the experimental and calculated band gap of Fe 2 VAl at high temperature (400 -800 K) can be reduced, indicating that inversions are probably present in experimental samples.…”

“…To answer the first question, the present work considers the role of Al/V inversions in calculating the lattice thermal conductivity of Fe 2 VAl. We have shown in a previous theoretical study [16] that Al/V inversions (a combination of Al V and V Al antisite defects) are the most probable defects to form at high temperature. Experimentally, even strictly stoichiometric Fe 2 VAl samples most likely contain a sizable concentration of Al V and V Al antisite defects, which depends on their thermal history (for instance their annealing temperature when T > 800 °C) since at T = 1080 °C, Fe 2 VAl undergoes an order-disorder transition (second order) where 50 % of V and 50 % of Al atoms mix on their respective crystallographic sites [17].…”