Distinct impact of order degree on thermoelectric power factor of p-type full-Heusler Mn₂VAl compounds

Abstract

Full-Heusler compounds have three crystal structures with a different order degree; the highly ordered L2 1 , partially disordered B2, and completely disordered A2 structures. To reveal the effects of the order degree on thermoelectric (TE) properties of the full-Heusler compounds, Mn 2 VAl samples with varied L2 1 and B2 order degrees are prepared by changing preparation conditions, and their Seebeck coefficients and electrical conductivities are measured in a wide temperature range. As the B2 order degree becomes higher, the Seebeck coefficient increases, leading to the increase of the power factor (PF). The maximum PF is 2.84×10 −4 Wm −1 K −2 at 767 K for the Mn 2 VAl sample with the highest B2 order degree. This study demonstrates that the TE properties of Mn 2 VAl can be enhanced by increasing the fraction of the B2 phase. A relation between the Seebeck coefficient and crystal structures is also discussed based on the calculation of the electronic density of states of Mn 2 VAl with the L2 1 and B2 structures.

Magnetic Full-Heusler Compounds for Thermoelectric Applications

“…Thus, to develop a TE device using full-Heusler compounds, a p-type counterpart to Co 2 MnSi is needed. For this purpose, Li et al [60,78] prepared a half-metallic Mn 2 VAl compound and measured its TE properties. Although Mn 2 VAl is a p-type material showing positive S, its highest PF (2.84 Â 10 À4 W/K 2 m at 767 K [78]) is lower than that of Co 2 MnSi.…”

“…For this purpose, Li et al [60,78] prepared a half-metallic Mn 2 VAl compound and measured its TE properties. Although Mn 2 VAl is a p-type material showing positive S, its highest PF (2.84 Â 10 À4 W/K 2 m at 767 K [78]) is lower than that of Co 2 MnSi. Thus, there is a need to explore more p-type half-metallic full-Heusler compounds with high PF.…”

“…This result implies that the B2 and/or A2 structures exhibit higher S than the L2 1 structure. Recently, Li et al [78] investigated the effect of structural disorder on the value of S for half-metallic Mn 2 VAl compounds by varying the B2 order degree. Figure 9(a) shows the measured S values for Mn 2 VAl with the B2 order degree of 27 and 66%.…”

“…Among the full-Heusler compounds, nonmagnetic Fe 2 VAl-based compounds have been intensively investigated as one of the potential TE semiconductors [93]. Despite the long historical investigation, Hinterleitner et al [94] discovered quite recently that a metastable Fe 2 V 0.8 W 0.2 Al thin film exhibits extremely high zT of $6 at 350 K as a result of its high S. The crystal structure of the thin film is reported to be the disordered A2 structure, which could be the reason for its high S, as in the cases of several half-metallic Co-based and Mn-based full-Heusler compounds [53,78]. If the disorder in structure contributes to the high S, then the strategy of enhancing zT by controlling structural disorder would be applicable to the other full-Heusler compounds.…”

“…(b) Measured S values of Mn 2 VAl at 767 K plotted against the B2 order degree. (Reprinted from[78]. Copyright 2020, with permission from IOP Publishing).…”