Phonons in Nanocrystalline 57Fe

We measured the phonon density of states (DOS) of nanocrystalline Fe by resonant inelastic nuclear γ-ray scattering. The nanophase material shows large distortions in its phonon DOS. We attribute the high energy distortion 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

Atom clusters and vibrational excitations in chemically-disordered Pt357Fe

Inelastic nuclear resonant scattering spectra of Fe-57 atoms were measured on crystalline alloys of Pt3Fe-57 that were chemically disordered, partially ordered, and L1(2) ordered. Phonon partial density of states curves for Fe-57 were obtained from these spectra. Upon disordering, about 10% of the spectral intensity underwent a distinct shift from 25 to 19 meV. This change in optical modes accounted for most of the change of the vibrational entropy of disordering contributed by Fe atoms, which was (+0.10 +/- 0.03) k(B) (Fe atom)(-1). Prospects for parametrizing the vibrational entropy with low-order cluster variables were assessed. To calculate the difference in vibrational entropy of the disordered and ordered alloys, the clusters must be large enough to account for the abundances of several of the atom configurations of the first-nearest-neighbor shell about the Fe-57 atoms

Vibrational modes in nanocrystalline iron under high pressure

The phonon density of states (DOS) of nanocrystalline 57Fe was measured using nuclear resonant inelastic x-ray scattering (NRIXS) at pressures up to 28 GPa in a diamond anvil cell. The nanocrystalline material exhibited an enhancement in its DOS at low energies by a factor of 2.2. This enhancement persisted throughout the entire pressure range, although it was reduced to about 1.7 after decompression. The low-energy regions of the spectra were fitted to the function AEn, giving values of n close to 2 for both the bulk control sample and the nanocrystalline material, indicative of nearly three-dimensional vibrational dynamics. At higher energies, the van Hove singularities observed in both samples were coincident in energy and remained so at all pressures, indicating that the forces conjugate to the normal coordinates of the nanocrystalline materials are similar to the interatomic potentials of bulk crystals

Local Chemical Environments and the Phonon Partial Densities of States of 57Fe in 57Fe3Al

Inelastic nuclear resonant scattering spectra were measured on alloys of Fe3Al that were chemically disordered, partially ordered, and D03 ordered. The features in the phonon partial density of states of 57Fe were found to change systematically with chemical short-range order in the alloy. Changes in the phonon partial density of states were modeled successfully by assigning vibrational spectra to 57Fe atoms in different first-nearest-neighbor chemical environments

Dynamics of iron atoms across the pressure-induced Invar transition in Pd_3Fe

The ^(57)Fe phonon partial density of states (PDOS) in L1_2-ordered Pd_3Fe 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_3Fe, 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

Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films

We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic X-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phase which provide a microscopic proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase. This results in a notable change in lattice entropy, which is important for the comparison between thin film and bulk results. Our calculations confirm the recently reported lattice instability in the AF phase. The imaginary frequencies at the $X$-point depend critically on the Fe magnetic moment and atomic volume. Analyzing these non vibrational modes leads to the discovery of a stable monoclinic ground state structure which is robustly predicted from DFT but not verified in our thin film experiments. Specific heat, entropy and free energy calculated within the quasiharmonic approximation suggest that the new phase is possibly suppressed because of its relatively smaller lattice entropy. In the bulk phase, lattice degrees of freedom contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as the electronic and magnetic subsystems and therefore needs to be included in thermodynamic modeling.Comment: 15 pages, 12 figure

Strongly Anisotropic Magnesiowüstite in Earth's Lower Mantle

The juxtaposition of a liquid iron‐dominant alloy against a mixture of silicate and oxide minerals at Earth's core‐mantle boundary is associated with a wide range of complex seismological features. One category of observed structures is ultralow‐velocity zones, which are thought to correspond to either aggregates of partially molten material or solid, iron‐enriched assemblages. We measured the phonon dispersion relations of (Mg,Fe) O magnesiowüstite containing 76 mol % FeO, a candidate ultralow‐velocity zone phase, at high pressures using high‐energy resolution inelastic X‐ray scattering. From these measurements, we find that magnesiowüstite becomes strongly elastically anisotropic with increasing pressure, potentially contributing to a significant proportion of seismic anisotropy detected near the base of the mantle