We performed inelastic neutron scattering measurements on nanophase Fe powders prepared by high-energy ball milling. Neutron energy loss spectra were measured for two states of the material: (1) as milled, when the material had a characteristic nanocrystallite size of 12 nm; and (2) annealed, when the material had a characteristic crystallite size of 28 nm. The longitudinal peak in the phonon density of states (DOS) of the nanophase Fe was broadened, compared to that of the annealed material. We attribute this broadening to short phonon lifetimes in nanocrystals. The nanophase material also showed an enhanced density of states at low energies below 15 meV, which may indicate the presence of intercrystallite vibrations. These differences in phonon DOS should have only a small effect on the difference in vibrational entropy of nanocrystalline and larger-grained Fe.
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