Abstract: Element doping is a widely employed strategy to enhance the thermoelectric (TE) properties of various materials. β-FeSi₂ is a promising low-cost high-temperature TE material with exceptional thermal stability; however, the doping limit of β-FeSi₂ is usually very low, which limits the tunability of electrical and thermal properties. Recently, a high doping content of 0.16 in β-FeSi₂ has been achieved by the introduction of iridium (Ir), leading to the highest reported figure of merit (zT) of 0.6 in β-FeSi₂. Motivated by the successful heavy doping with Ir, this work aims to explore element heavy doping in β-FeSi₂ with cobalt (Co), a cheaper, more readily available dopant with a smaller atomic radius and closer electronegativity to iron (Fe). In this study, we successfully obtained a record-high doping content of 0.24 in Co-doped β-FeSi₂ through a prolonged annealing process. Despite the absence of a substantial enhancement in the zT of Co-doped β-FeSi₂ at high doping levels, with a maximum zT of 0.3 at 900 K in Fe_0.92Co_0.08Si₂, we observed a transition in the carrier transport mechanism as a function of Co doping content, attributed to changes in the band structure. At a low Co doping content (x ≤ 0.12), Fe_1-xCo_xSi₂ demonstrates dominant carrier transport via impurity levels within the band gap, exhibiting hopping conduction. As the Co doping content increases (x > 0.16), the impurity levels overlap and form an impurity band, and the carrier transport turns into the impurity band conduction. The observed band conduction behavior of Fe_1-xCo_xSi₂ (x > 0.16) mirrors that of Ir-doped β-FeSi₂, but Fe_1-xCo_xSi₂ shows much lower mobility, which can be attributed to the localized feature of the impurity band introduced by the Co doping. Overall, this study provides insights into the heavy Co doping and its influence on the TE properties and carrier conduction mechanisms in β-FeSi₂, helpful for the further development of this TE system.

Key words: β-FeSi₂, Heavy doping, Hopping, Band conduction