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Published December 2009 | Published
Journal Article Open

Effects of vacancies on phonon entropy of B2 FeAl

Abstract

The phonon density of states (DOS) and phonon entropy of B2 FeAl were determined as functions of the Fe site vacancy concentration using several scattering techniques and were computed from first principles. Measurements at elevated temperature and pressure were performed to explore volume effects, test the usefulness of the quasiharmonic (QH) approximation, and provide comparison for the first-principles calculations. The average temperature and pressure dependencies of phonons were consistent with the QH model. The decrease in specific volume associated with the introduction of vacancies causes a stiffening of the DOS that was captured well with the experimentally determined Grüneisen parameter. Features associated with vacancies in the DOS are not well explained by this model, however, especially in the gap between the acoustic and optic branches. First-principles calculations indicated that these modes are primarily associated with vibrations of Al atoms in the first-nearest-neighbor shell of the vacancy, with some vibration amplitude also involving the second-nearest-neighbor Fe atoms. At the vacancy concentrations of study, the phonon entropy of vacancy formation was found to be approximately −1.7k_B/atom, about half as large and of opposite sign as the configurational entropy of vacancy formation.

Additional Information

© 2009 The American Physical Society. Received 22 June 2009; revised 17 November 2009; published 21 December 2009. We thank C. T. Liu for providing us with the samples for inelastic neutron scattering. This work was benefited from the use of the Intense Pulsed Neutron Source at Argonne National Laboratory. This facility is funded by the U.S. Department of Energy, BES-Materials Science, under Contract No. W-31-109-Eng-38. This work was supported by the Department of Energy through the Basic Energy Sciences Grant No. DE-FG02-03ER46055 and DOE BES-MS Grant No. W-31-109-ENG-38. Portions of this work were performed at HPCAT Sector 16, Advanced Photon Source APS, Argonne National Laboratory. The use of the HPCAT facility was supported by DOE-BES, DOE-NNSA CDAC, NSF, DOD TACOM, and the W.M. Keck Foundation. The use of the APS was supported by DOE-BES under Contract No. DE-AC02-06CH11357.

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