Phase Transition and Transport Properties in p-Type Mn-doped β-FeSi₂ Thermoelectric Materials

Iron silicide (β-FeSi₂) is known as a promising thermoelectric (TE) material due to its non-toxicity and low cost. However, pure β-FeSi₂ exhibits relatively low TE performance. The performance of β-FeSi₂ can typically be improved through metal substitution. Adding metal usually causes the formation of secondary metallic phases, which degrade the thermopower, leading to a decrease in TE performance. Therefore, understanding the phase transition and its relationship with transport properties is important for optimizing the material’s performance. The present work aims to investigate the influence of Mn addition on the phase change and properties of β-Fe_1-xMn_xSi₂, where x is varied from 0 to 0.10.

The samples were prepared using arc-melting and a heat treatment process. The phase analysis was performed by Rietveld refinement. The electrical and TE properties, such as carrier density, mobility, electrical resistivity, and Seebeck coefficient, were measured by ResiTest8300 and a home-built apparatus. The thermal conductivity was measured by the power efficiency measurement (PEM-2) system.

The results indicate that the amount of semiconducting β-phase drastically drops at x ≥ 0.09, suggesting that the optimum doping level to improve TE performance should be lower than x < 0.09. Compared to other metals such as Co and Ni, it is found that Mn has a higher solid solution limit in β-FeSi₂. Mn tunes the conduction of β-FeSi₂ from n-type to p-type. The electrical resistivity and the Seebeck coefficient decrease with Mn doping due to the increased carrier density and formation of secondary phases. The thermal conductivity moderately increases with Mn addition. As a result, the highest power factor of 970 μWm⁻¹K⁻² and dimensionless figure of merit of ZT = 0.12 are obtained in the x = 0.03 sample.

This study is useful for understanding the phase transition and its influence on the TE properties of metal-doped β-iron silicide compounds.