Cohesive, structural, and electronic properties of Fe-Si compounds

Phase stability, structural, and electronic properties of iron silicides in the Fe₃Si, FeSi, and FeSi₂ compositions are investigated by first-principle density-functional calculations based on ultrasoft pseudopotentials and all-electron methods. Structural stabilization versus spin-polarization effects are discussed at the Fe₃Si composition, while for {ε}-FeSi and β-FeSi₂ we investigate their structural properties and the corresponding semiconducting band properties. All the computed results are analyzed and compared to available experimental data. The stability of the bulk phases, the lattice parameters, the cohesive energies and magnetic properties are found to be in good agreement with experiment when using the generalized gradient approximations for the exchange-correlation functional. Density-functional calculations are unable to account for the small bulk modulus of ε-FeSi despite that the computed lattice constant and internal atomic positions coincide with the experimental results. Both full-potential and ultrasoft-pseudopotential methods confirm for β-FeSi₂ the indirect nature of the fundamental gap, which is attributed to a transition between Y to 0.6×{}{Λ} being 30% smaller than the experimental gap. Ultrasoft pseudopotential calculations of Fe-Si magnetic phases and of various nonequilibrium metallic phases at the FeSi and FeSi₂ composition are presented. These calculations provide ab initio information concerning the stabilization of metallic pseudomorphic phases via high pressures or epitaxy.