Density functional calculations for small iron clusters with substitutional phosphorus

Embrittlement is known to be caused by P segregation at grain boundaries in Fe alloys. Effects of P substitutions on binding energies and electronic structures of octahedral Fe cluster are investigated using density functional calculations in order to understand the nature of bonding between P and Fe atoms at grain boundaries. The binding energies increase in Fe3P3 and Fe-rich clusters while they decrease in P-rich clusters. The changes in binding energies are closely connected to the charge transfer from Fe to P atoms. The charge transfer leads to both stronger and weaker bonds in mixed Fe–P clusters. The weaker bonds due to less charge cause embrittlement. The calculations indicate that the binding energies and chemical bonding are affected by atomic configurations of P atoms in Fe–P clusters.