Cubic full-Heusler, X2YZ, and half-Heusler XYZ systems have been proven to be very promising active thermoelectric materials because i) of their thermoelectric performance and ii) because of outstanding thermal and mechanical properties, important for applications in different domains. X and Y are (in general) transition metal elements and Z is a main group element. Although X, Y and Z elements are metallic in solid form, many of the full-Heusler and half-Heusler systems turn out to be semiconductors or semimetals, or, at least, the respective Fermi energy is located very near to a gap in the electronic density of states. Such a scenario is a pre-requisite for capable thermoelectric materials.
In this Colloquium, we report on i) tuning of full-Heusler systems based on Fe2VAl by substituting appropriate elements on all lattice sites of the Cu2MnAl type structure and ii) on a deeper understanding of the resulting relevant electronic and thermal properties by combining experiments, carried out in a broad temperature range, and first principles calculations in terms of the density functional theory (DFT) using the package VASP.
Depending whether these materials are manufactured as bulk or as thin films, a variety of unexpected features, like meta-stable states, behaviours reminiscent of high entropy alloys, huge logarithmic derivatives of the electronic density of states or certain types of protected states are observed (both latter from DFT calculations), which might positively influence the thermoelectric behaviour of this family of compounds. Since thermal conductivity is further suppressed for the films - in comparison to the bulk material - the figure of merit, ZT, for such films turns out to be larger than values reported in literature .
Research supported by the “Christian Doppler Laboratory for Thermoelectricity” and the JST, program “MIRAI”.
…B. Hinterleitner, et al., “Thermoelectric performance of a metastable thin-film Heusler alloy," Nature, vol. 576, no. 7785, pp. 85-90, 2019