Iron silicide is an earth-abundant and environmentally friendly material system that crystallizes in several phases, including metallic ε-FeSi and α-Fe₂Si₅, and semiconducting β-FeSi₂. Among them, β-FeSi₂ has attracted considerable interest for optoelectronic, photovoltaic, and thermoelectric applications due to its tunable electrical conductivity. Both n-type and p-type conduction can be achieved through transition-metal doping; however, performance optimization is often limited by dopant solubility and the emergence of secondary metallic phases. Previous studies mainly inferred solubility limits from lattice parameter shifts or qualitative phase identification, with a lack of quantitatively correlating bulk phase fractions and local elemental distribution.

Polycrystalline Fe_1−xM_xSi₂ (M = Mn, Co, Ni) samples were prepared by arc melting followed by a two-step heat treatment to promote the formation of the β phase. Dopant concentrations were systematically varied within controlled ranges. Phase identification and quantitative analysis were performed using X-ray diffraction combined with Rietveld refinement, while microstructure and elemental distributions were examined by SEM-EDS at multiple locations to evaluate reproducibility and local dopant incorporation.

The evolution of phase fractions reveals that increasing dopant concentration promotes the formation of metallic ε and α-phases. Although the β-phase remains dominant (>95%) over a finite composition range, local compositional analysis indicates earlier saturation of dopant incorporation within the β-matrix. The estimated solid-solution limits are approximately 6.3% for Mn and 8.8% for Co, while Ni exhibits significantly lower solubility. Beyond these limits, excess dopants segregate and contribute to metallic phase formation, which correlates with degradation in thermoelectric performance. The combined bulk and local analyses provide a practical strategy to evaluate dopant solubility and phase stability in β-FeSi₂, offering guidance for optimizing transition-metal doping in semiconductor-based energy materials.