Enhanced thermopower by double-site substitution of Ti in Fe2(VAl)1-xTi2x

Thermoelectric (TE) materials, which directly convert heat into electricity, hold promise for sustainable energy applications. For widespread adoption of this technology, the development of efficient, cost-effective, and non-toxic TE materials is crucial. Here, we attempt to improve the thermoelectric properties of Fe₂VAl-based full-Heusler compounds through the targeted substitution of (VAl) by Ti₂ in

. Our study reveals a miscibility gap between $0.4<x<0.9$, yet significant enhancement of the thermoelectric performance for both $p$- and $n$-type compounds was achieved for smaller substitutions. While Fe₂VAl and Fe₂Ti₂ are semimetals, a band gap opening occurs in the solid solution series, yielding a substantial enhancement of the Seebeck coefficient up to 130 μV/K in $p$-type materials. By additional optimization via isovalent V/Ta heavy-element substitution, we further optimize the TE performance, achieving one of the best $zT$ values in $p$- and $n$-type full-Heusler compounds and revealing the (VAl)/Ti${}_2$ substitution as a promising pathway for improving the TE efficiency of full-Heusler compounds.