Fultz, B. and Anthony, L. and Nagel, L. J. and Nicklow, R. M. and Spooner, S. (1995) Phonon densities of states and vibrational entropies of ordered and disordered Ni3Al. Physical Review B, 52 (5). pp. 3315-3321. ISSN 0163-1829. http://resolver.caltech.edu/CaltechAUTHORS:FULprb95a
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We performed inelastic neutron-scattering measurements on powdered Ni3Al. The alloy was prepared in two states of chemical order: (1) with equilibrium L12 order, and (2) with disorder (the material was a fcc solid solution prepared by high-energy ball milling). Procedures to convert the energy loss spectra into approximate phonon density of states (DOS) curves for Ni3Al in the two states of chemical order were guided by Born–von Kármán analyses with force constants obtained from previous single-crystal experiments on L12-ordered Ni3Al and fcc Ni metal. The main difference in the phonon DOS of the ordered and disordered alloys occurs near 39 meV, the energy of a peak arising from optical modes in the ordered alloy. These high-frequency optical modes involve primarily the vibrations of the aluminum-rich sublattice. The disordered alloy, which does not have such a sublattice, shows much less intensity at this energy. This difference in the phonon DOS around 39 meV is the main contributor to the difference in vibrational entropy of disordered and ordered Ni3Al, which we estimate to be Svibdis-Svibord=(+0.2±0.1)kB/atom at high temperatures.
|Additional Information:||© 1995 The American Physical Society Received 13 January 1995 We thank B. Hennion for fitting the phonon dispersion curves of Ni3Al (Ref. 26) with a Born-von Karman model, and supplying us with a set of force constants. L. Preister performed much of the powder preparation. The Oak Ridge National Laboratory is managed for the Department of Energy by Martin Marrietta Energy Systems, Oak Ridge, TN under Contract No. DE-AC05-84OR214000. This work was supported by the U.S. Department of Energy under Contract No. DE-FG03-86ER45270.|
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