First-principles study of lattice dynamics and diffusion in DO3-type Fe3Si

The collective and single-particle dynamics in the intermetallic compound Fe3Si with the DO3 structure have been investigated using first-principles density-functional calculations in combination with statistical mechanics in the grand-canonical ensemble. The dispersion relations and the density of states of phonons have been calculated using a direct force-constant approach based on analytic Hellmann-Feynman forces. The defect formation parameters Delta epsilon(D) and Delta V-D describing the change in energy and volume on removing an atom from the system (D=vacancy) or on substituting an atom by another atomic species (D=antistructure defect) have been determined by total-energy calculations on large supercells. With this information, the grand-canonical potential is calculated as a function of the defect concentrations. Minimization of this potential with respect to the defect concentrations, together with the Gibbs-Duhem relation and the condition of particle-number conservation at fixed composition, determines the effective defect formation enthalpies and volumes. Defect migration enthalpies have been derived from transition states determined using the nudged-elastic-band method. All calculations are based on large 128-atom supercells, as required by the long-range nature of interatomic forces in intermetallic compounds. Phonon dispersion relations and defect formation enthalpies are in good agreement with the available experimental data. The analysis of the defect formation and migration enthalpies explains the pronounced asymmetry between Fe and Si diffusion characteristic of Fe3Si and leads to a convincing atomistic scenario for the diffusion events.