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Discovered more than 200 years ago in 1821, thermoelectricity is nowadays of global interest as it enables direct interconversion of thermal and electrical energy via the Seebeck/Peltier effect. In their seminal work, Mahan and Sofo mathematically derived the conditions for ’the best thermoelectric’—a delta-distribution-shaped electronic transport function, where charge carriers contribute to transport only in an infinitely narrow energy interval. So far, however, only approximations to this concept were expected to exist in nature. Here, we propose the Anderson transition in a narrow impurity band as a physical realisation of this seemingly unrealisable scenario. An innovative approach of continuous disorder tuning allows us to drive the Anderson transition within a single sample: variable amounts of antisite defects are introduced in a controlled fashion by thermal quenching from high temperatures. Consequently, we obtain a significant enhancement and dramatic change of the thermoelectric properties from p-type to n-type in stoichiometric Fe2VAl, which we assign to a narrow region of delocalised electrons in the energy spectrum near the Fermi energy. Based on our electronic transport and magnetisation experiments, supported by Monte-Carlo and density functional theory calculations, we present a novel strategy to enhance the performance of thermoelectric materials.

Recently, n-type Fe2VAl-based full-Heusler systems, exhibiting high thermoelectric power factors, have sparked new interest in this material class for thermoelectric applications. In this paper, the aluminum-rich region of the L21 phasespace in Fe–V–Al is explored by a step-by-step increase in the Al content in Fe2VAlx. We reveal a promising route to improve the thermoelectric properties of p-type Heusler alloys. First, we find an ultrahigh solubility of Al in Fe2VAlx and confirm the presence of a single Heusler phase up to x = 2 using x-ray diffraction analysis and scanning electron microscopy. Second, thermoelectric transport properties, measured in a wide temperature range from 4 to 800 K, show a substantial increase in the thermopower by over 100% and a significant decrease in the thermal conductivity by up to 80% for the Al-rich samples. Detailed analysis of the carrier-concentration-dependent thermopower as well as Hall measurements indicate the formation of a resonant state at the valence band edge as a likely origin for this enhancement. This is further corroborated by density-functional-theory calculations of the electronic density of states. Our work sets the stage for p-type full-Heusler materials with enhanced thermoelectric performance, applying the principle of resonant states to this material class.

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