Unexpected band gap increase in the Fe2VAl Heusler compound

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 small-gap semiconductors and should have a high figure of merit ZT. Even if the Fe2VAl Heusler compound has a decent ZT, its conductive nature (semi-metal 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.Comment: 10 pages, 5 figure

First-principles investigation on the bulk properties of americium dioxide and sesquioxides

International audienceWe report an investigation of the ground state properties of americium dioxide and its sesquioxides using first-principles calculations. In order to take into account strong 5f electronic correlations, we apply the generalized gradient approximation (GGA)+U as well as the hybrid functionals to these compounds. We present a systematic study of several bulk properties such as structural, elastic, energetic, electronic and magnetic properties as a function of the U and J GGA+U parameters in the range of 0.0-8.0 eV and 0.00-0.75 eV respectively. The values (U,J) = (6.00 eV,0.75 eV) for onsite Coulomb interaction U and exchange J parameters are those providing the best description of bulk properties by comparison to the available experimental data. We discuss the effect of loss of symmetry caused by DFT+U on several properties. We also discuss the effect of the spin-orbit coupling on these properties. Furthermore, we highlight that in order to reach the ground state of americium dioxide and sesquioxides using GGA+U, the monitoring of the occupation matrices of 5f correlated orbitals is crucial. Our computational scheme provides reliable and accurate results concerning several bulk properties that had not been studied experimentally yet. For instance, our elastic constants calculated for AmO2 follow the same trend as those of other actinide oxides (UO2, NpO2 and PuO2) and can be used as reference. We have also computed the dielectric properties of americium dioxide. Finally, we find the A-type hexagonal structure to be the most stable structure at low temperature for americium sesquioxides. For this phase, we predict the internal structural parameters as well as the bulk modulus. We provide the first data concerning the formation enthalpy of the Am2O3 bcc C-type

Coupled first-principles and experimental positron annihilation study of defects in actinide mixed oxides

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