In Print: Physical Review Letters 79, 937 (1997).

B. Fultz1, C. C. Ahn1, E. E. Alp2, W. Sturhahn2, T. S. Toellner2

1 Div. Engineering and Applied Science, 138Ð78 California Institute of Technology Pasadena, California 91125, USA
2 Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA

We measured the phonon density of states (DOS) of nanocrystalline Fe by resonant inelastic nuclear gamma-ray scattering. The nanophase material shows large distortions of its phonon DOS. We attribute the high energy part of it to lifetime broadening. A damped harmonic oscillator model for the phonons provides a low quality factor, Qu, averaging about 5, but the longitudinal modes may have been broadened most. The nanocrystalline Fe also shows an enhancement in its phonon DOS at energies below 15 meV. The difference in vibrational entropy of the bulk and nanocrystalline Fe was small, owing to competing changes in the nanocrystalline phonon DOS at low and high energies.

Transmission electron micrographs of ballistically-consolidated nanocrystals of 57Fe. Dark field image from (110) bcc Fe diffraction. Large white crystallites are about 20 nm diameter.

Top: Phonon DOS of bulk bcc Fe, calculated with force constants from inelastic neutron scattering.
Bottom: crosses are phonon DOS curves extracted from experimental data of Fig. 4. Solid curves are calculated as described in text.