First-principles study of electronic structures and elasticity of Al₂Fe₃Si₃

Al₂Fe₃Si₃ intermetallic compound shows promising application in low-cost and non-toxic thermoelectric device because of its relatively high power factor of ∼700 μW m⁻¹ K⁻² at 400 K. Herein we performed the first-principles calculations with the projector augmented-wave (PAW) method to study the formation energies, elastic constants, electronic structures, and electronic transport properties of Al₂Fe₃Si₃. We discussed the thermodynamical stability of Al₂Fe₃Si₃ against other ternary crystalline compounds in Al–Fe–Si phase. The band gap of Al₂Fe₃Si₃ was particularly examined using the semilocal and hybrid functionals and the on-site Hubbard correction, which were also applied to β-FeSi₂ to calibrate the prediction reliability of our employed computational methods. Our calculations show that Al₂Fe₃Si₃ is a narrow-gap semiconductor. The semilocal functional within generalized gradient approximation (GGA) shows an exceptional agreement between the predicted band gap of Al₂Fe₃Si₃ and the available experiment data, which is in contrast to the typical trend and rationally understood through a comprehensive comparison. We found that both HSE06 and PBE0 hybrid functionals with a standard setup overestimated the band gaps of Al₂Fe₃Si₃ and β-FeSi₂ too much. The underlying reasons may be ascribed to a large electronic screening, which arises from the unique characteristics of Fe 3d states appearing in both sides of band gaps of Al₂Fe₃Si₃ and β-FeSi₂, and to a reduced delocalization error thanks to the covalent Fe–Si and Si–Si bonding nature. The chemical bonding and elasticity of Al₂Fe₃Si₃ were compared with those of β-FeSi₂ and FeAl₂. In Al₂Fe₃Si₃ the Fe–Al bonding is more ionic and the Fe–Si bonding is more covalent. The elastic moduli of Al₂Fe₃Si₃ are comparable to those of β-FeSi₂ and larger than those of FeAl₂. Our calculation results indicate that the mechanical strength of Al₂Fe₃Si₃ could be strong enough for the practical application in thermoelectric device. (paper)