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J A Muñoz,
M S Lucas,
O Delaire, M L Winterrose,
L Mauger,
Chen W Li,
A O Sheets,
M B Stone,
D L Abernathy,
Yuming Xiao,
Paul Chow,
B Fultz
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ABSTRACT: Inelastic neutron scattering and nuclear resonant inelastic x-ray scattering were used to measure phonon spectra of FeV as a B2 ordered compound and as a bcc solid solution. The two data sets were combined to give an accurate phonon density of states, and the phonon partial densities of states for V and Fe atoms. Contrary to the behavior of ordering alloys studied to date, the phonons in the B2 ordered phase are softer than in the solid solution. Ordering increases the vibrational entropy by +0.22±0.03 kB/atom, which stabilizes the ordered phase to higher temperatures. First-principles calculations show that the number of electronic states at the Fermi level increases upon ordering, enhancing the screening between ions, and reducing the interatomic force constants. The effect of screening is larger at the V atomic sites than at the Fe atomic sites.
Physical Review Letters 09/2011; 107(11):115501. · 7.37 Impact Factor
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M L Winterrose,
L Mauger,
I Halevy,
A F Yue,
M S Lucas,
J A Muñoz,
H Tan,
Y Xiao,
P Chow,
W Sturhahn,
T S Toellner,
E E Alp,
B Fultz
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ABSTRACT: The 57 Fe phonon partial density of states (PDOS) in L1 2 -ordered Pd 3 Fe was studied at high pressures by nuclear resonant inelastic x-ray scattering (NRIXS) measurements and density functional theory (DFT) calculations. The NRIXS spectra showed that the stiffening of the 57 Fe PDOS with decreasing volume was slower from 12 to 24 GPa owing to the pressure-induced Invar transition in Pd 3 Fe, with a change from a high-moment ferromagnetic (FM) state to a low-moment (LM) state observed by nuclear forward scattering. Force constants obtained from fitting to a Born–von Kármán model showed a relative softening of the first-nearest-neighbor (1NN) Fe-Pd longitudinal force constants at the magnetic transition. For the FM low-pressure state, the DFT calculations gave a PDOS and 1NN longitudinal force constants in good agreement with experiment, but discrepancies for the high-pressure LM state suggest the presence of short-range magnetic order.
Physical Review B 04/2011; 83. · 3.69 Impact Factor
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M. S. Lucas,
J. A. Muñoz,
O. Delaire,
N. D. Markovskiy,
M. B. Stone,
D. L. Abernathy,
I. Halevy,
L. Mauger,
J. B. Keith, M. L. Winterrose,
Yuming Xiao,
M. Lerche,
B. Fultz
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ABSTRACT: The phonon densities of states of body-centered-cubic Fe-V alloys across the full composition range were studied by inelastic neutron scattering, nuclear resonant inelastic x-ray scattering, and ab initio calculations. The average phonon energy followed the inverse of the electronic heat capacity and the inverse of the electronic density of states at the Fermi level, showing how the interatomic forces depend on electronic screening. These quantities, including phonon energy, changed rapidly near the composition of the paramagnetic-ferromagnetic transition. For Fe- and V-rich alloys, the thermal phonon softening deviated from quasiharmonic behavior but better agreement was found for intermediate compositions. The Fe partial phonon density of states has a distinctly different shape than V for alloys with less than 50 at. % Fe.
Phys. Rev. B. 10/2010; 82(14).
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ABSTRACT: Synchrotron x-ray diffraction (XRD) measurements, nuclear forward scattering (NFS) measurements, and density functional theory (DFT) calculations were performed on L1_{2}-ordered Pd3Fe. Measurements were performed at 300 K at pressures up to 33 GPa, and at 7 GPa at temperatures up to 650 K. The NFS revealed a collapse of the 57Fe magnetic moment between 8.9 and 12.3 GPa at 300 K, coinciding with a transition in bulk modulus found by XRD. Heating the sample under a pressure of 7 GPa showed negligible thermal expansion from 300 to 523 K, demonstrating Invar behavior. Zero-temperature DFT calculations identified a ferromagnetic ground state and showed several antiferromagnetic states had comparable energies at pressures above 20 GPa.
Physical Review Letters 07/2009; 102(23):237202. · 7.37 Impact Factor
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M. S. Lucas,
O. Delaire, M. L. Winterrose,
T. Swan-Wood,
M. Kresch,
I. Halevy,
B. Fultz,
Jingzhu Hu,
M. Lerche,
M Y Hu,
M. Somayazulu
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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.
