Crystal Structure and Thermal Conductivity of Al-Doped β-FeSi₂

Iron silicide (β-FeSi₂) is an important semiconductor that has attracted significant attention for optoelectronic, photovoltaic, and thermoelectric applications. Its conductivity can be tuned to either n-type or p-type through doping, providing flexibility for device design. In thermoelectric (TE) applications, however, the introduction of dopants often leads to the formation of secondary metallic phases, which can degrade TE performance. The thermoelectric performance of p-type β-FeSi₂ is generally inferior to that of n-type material. Therefore, it is necessary to investigate the crystal structures and transport properties of p-type Al-doped β-FeSi₂.

The polycrystalline FeSi_2−xAl_x samples were prepared by arc melting followed by a heat treatment process to obtain the β-phase. The crystal structures were identified using X-ray diffraction, and the phase fractions were quantified by Rietveld refinement. The electrical resistivity and Seebeck coefficient were measured using both a Hall effect measurement system (ResiTest8300) and a homemade device. Thermal conductivity was measured using a power efficiency measurement system.

As a result, undoped FeSi₂ samples were initially crystallized in the ε and α-phases and transformed into a semiconducting β-phase (~97%) after heat treatment, confirming the essential role of thermal processing in β-phase formation. The unit cell volume increases with increasing Al doping level, which can be attributed to the larger atomic radius of Al compared to Si, indicating substitutional incorporation of Al into the Si sublattice. It is found that Al incorporation significantly affected secondary phase formation, reducing the β-phase fraction to below 70% for x ≥ 0.04. This result reveals a strong destabilization effect of Al on β-FeSi₂, which has not been clearly quantified in previous studies. Meanwhile, thermal conductivity decreased with increasing Al content, likely due to enhanced phonon scattering from point defects caused by Al substitution, consistent with previous reports. This study provides useful insights for material design and thermal-to-energy conversion applications.