Abstract
Phonon density of states (DOS) curves were measured on alloys of face-centered-cubic (fcc) Au-Fe
using nuclear resonant inelastic x-ray scattering (NRIXS) and inelastic neutron scattering (INS).
The NRIXS and INS results were combined to obtain the total phonon DOS and the partial phonon
DOS curves of Au and Fe atoms. The 57Fe partial phonon DOS of the dilute alloy Au0.97
57Fe0.03
shows a localized mode centered 4.3% above the cutoff energy of the phonons in pure Au. The
Mannheim model for impurity modes accurately reproduced this partial phonon DOS using the fcc
Au phonon DOS with a ratio of host-host to impurity-host force constants of 1.55. First-principles
calculations validated the assumption of first-nearest-neighbor forces in the Mannheim model and
gave a similar ratio of force constants. The high energy local mode broadens with increasing Fe
composition, but this has a small effect on the composition dependence of the vibrational entropy.
The main effect on the vibrational entropy of alloying comes from a stiffening of the Au partial
phonon DOS with Fe concentration. This stiffening is attributed to two main effects: 1) an increase
in electron density in the free-electron-like states, and 2) stronger sd-hybridization. These two effects
are comparable in magnitude.
