First-principles insights into Si substitution effects in Sm2(Fe,Si)17Cx magnet

The partial substitution of Fe by Si enhances the phase stability of Sm2Fe17Cx magnets with x > 1.0. We elucidate the Si-substitution scheme and its impact on phase stability and magnetic properties in Sm2(Fe,Si)17C3 from first-principles calculations and chemical bonding analysis. The calculated substitution energies for Si at various Fe sites are negative, indicating improved phase stability. Si preferentially substitutes Fe atoms at the 9d site in Sm2(Fe,Si)17C3 while it tends to enter the Fe 18h site in Sm2(Fe,Si)17. This difference in site preference is attributed to the distinct chemical environments surrounding the Fe (Si) sites in the two compounds. Si substitution favors the formation of Sm-Si bonds while minimizing the Si-C and Si-Si interactions. Crystal orbital Hamilton populations and crystal orbital bond index calculations indicate that the partial replacement of Fe with Si strengthens the chemical bonding of Sm-Fe3 (18f) and Sm-Fe4 (18h) and improves overall phase stability in Sm2(Fe,Si)17C3. Beyond the dilution effect, Si substitution also reduces the magnetic moments of neighboring Fe atoms, a phenomenon linked to the strong Fe-Si bonding. These findings highlight the dual role of Si in modifying both the structural and magnetic characteristics of Sm2Fe17-based magnetic compounds.