Lattice dynamics and related diffusion properties of intermetallics: I. Fe₃Si

The phonon dispersion of iron-rich Fe-Si alloys of D0₃ structure (space group Fm-3m) has been studied by inelastic neutron scattering. The measurements were carried out on two crystals of different composition: Fe ₇₅Si₂₅ at 20 and 930 degrees C, and Fe₈₀Si ₂₀ at 20, 930 and 1100 degrees C. The respective degree of order was determined by powder diffraction measurements. A general decrease of phonon frequencies with increasing temperature is found, the temperature dependence being strongest for transverse phonons with ( xi xi 0) propagation. With decreasing order, phonon gaps close and the highest optical band degenerates into a broad distribution of inelastic intensity due to disorder scattering. The dependence of the dispersion on the alloy composition is not very pronounced. The migration enthalpies as well as several other thermodynamic quantities have been calculated from the densities of states. Although the low frequencies of the transverse phonons indicate particularly low migration barriers, they cannot explain the pronounced change of the diffusivity with composition. This composition dependence is tentatively explained by high vacancy concentrations.