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Isotopic fractionation has been linked to the lattice vibrations of materials through their phonon spectra. The Lamb-Mössbauer factor (fLM) has the potential to provide information about the lattice vibrations in materials. We constrain the temperature evolution of the fLM of γ- and ε-Fe at in situ high-P-T conditions between 1650 K and the melting point. We find that the vibrations of γ- and ε-Fe can be described using a quasiharmonic model with a pressure- and temperature-dependent Debye temperature computed from the measured fLM. From the Debye temperature, we derive the equilibrium isotopic fractionation β-factor of iron. Our results show that the quasiharmonic behavior of metallic iron would lower the value of lnβFe57/54 by 0.1‰ at 1600–2800 K and 50 GPa when compared to the extrapolation of room temperature nuclear resonant inelastic X‑ray scattering data. Our study suggests that anharmonicity may be more prevalent in Fe metal than in lower mantle minerals at 2800 K and 50 GPa, a relevant condition for the core formation, and the silicate mantle may be isotopically heavy in iron. [ABSTRACT FROM AUTHOR]

Operando nuclear resonant inelastic X‐ray scattering (NRIXS) and X‐ray absorption fine‐structure spectroscopy (XAFS) measurements were used to gain insight into the structure and surface composition of FeCu and FeAg nanoparticles (NPs) during the electrochemical CO2 reduction (CO2RR) and to extract correlations with their catalytic activity and selectivity. The formation of a core–shell structure during CO2RR for FeAg NPs was inferred from the analysis of the operando NRIXS data (phonon density of states, PDOS) and XAFS measurements. Electrochemical analysis of the FeAg NPs revealed a faradaic selectivity of 36 % for CO in 0.1 M KHCO3 at −1.1 V vs. RHE, similar to that of pure Ag NPs. In contrast, a predominant selectivity towards H2 evolution is obtained in the case of the FeCu NPs, analogous to the results obtained for pure Fe NPs, although small Cu NPs have also been shown to favor H2 production. [ABSTRACT FROM AUTHOR]

The increase in superconducting transition temperature (TC) of Sn nanostructures in comparison to bulk, was studied. Changes in the phonon density of states (PDOS) of the weakly coupled superconductor Sn were analyzed and correlated with the increase in TC measured by magnetometry. The PDOS of all nanostructured samples shows a slightly increased number of low-energy phonon modes and a strong decrease in the number of high-energy phonon modes in comparison to the bulk Sn PDOS. The phonon densities of states, which were determined previously using nuclear resonant inelastic X-ray scattering, were used to calculate the superconducting transition temperature using the Allen-Dynes-McMillan (ADMM) formalism. Both the calculated as well as the experimentally determined values of TC show an increase compared to the bulk superconducting transition temperature. The good agreement between these values indicates that phonon softening has a major influence on the superconducting transition temperature of Sn nanostructures. The influence of electron confinement effects appears to be minor in these systems. [ABSTRACT FROM AUTHOR]

A new synchrotron radiation experimental capability of coupling nuclear resonant inelastic X-ray scattering with the cryogenically cooled high-pressure diamond anvil cell technique is presented. The new technique permits measurements of phonon density of states at low temperature and high pressure simultaneously, and can be applied to studies of phonon contribution to pressure- and temperature-induced magnetic, superconducting and metal-insulator transitions in resonant isotope-bearing materials. In this report, a pnictide sample, EuFe2As2, is used as an example to demonstrate this new capability at beamline 3-ID of the Advanced Photon Source, Argonne National Laboratory. A detailed description of the technical development is given. The Fe-specific phonon density of states and magnetism from the Fe sublattice in Eu57Fe2As2 at high pressure and low temperature were derived by using this new capability. [ABSTRACT FROM AUTHOR]

We introduce the combination of nuclear resonant inelastic X-ray scattering and synchrotron Mössbauer spectroscopy with the laser-heated diamond anvil cell technique for studying magnetic, elastic, thermodynamic, and vibrational properties of materials under high pressures and high temperatures. An Nd:YLF laser, operating in continuous donut mode (TEM 01 ), has been used to heat samples inside a diamond anvil cell from both sides. Temperatures of the laser-heated sample are measured by means of spectral radiometry and by the detailed balance principle of the energy spectra. The temperature measured by the detailed balance principle is in very good agreement with values determined from the thermal radiation spectra fitted to the Planck radiation function up to 1700 K. Nuclear resonant scattering on 57 Fe-containing materials (i.e., Fe, FeO, Fe 2 O 3 ) has been studied up to 2500 K and 100 GPa. A detailed description of the laser-heating optics, temperature determination, the X-ray monochromatization, and the X-ray focusing optics is given in this article. [ABSTRACT FROM AUTHOR]

Abstract: The global carbon cycle may involve iron carbide as a component of the Earth''s inner core. In this study, we report the first experimental measurements of phonon density of states, sound velocities, and Debye temperatures of Fe3C up to 47GPa and 1450K using the nuclear resonant inelastic X-ray scattering (NRIXS) method in a laser-heated diamond anvil cell. Within the investigated pressure and temperature range, the effect of temperature on compressional velocity v p falls within the measurement uncertainties. The shear velocity vs decreases with increasing temperature in a non-linear fashion: at ~15GPa, vs decreases by ~12% between 300K and 740K; At ~46GPa, temperature produces a ~5% reduction between 300 and 960K and a ~6% further reduction between 960K and 1370–1450K. The high-temperature vs of Fe3C deviates from Birch''s law behavior towards smaller values, potentially matching the anomalously low vs in the inner core under the relevant pressure and temperature conditions. Our results support Fe3C as a major component of the inner core. [Copyright &y& Elsevier]

Sound velocities of cementite Fe 3 C have been measured up to 1.5 Mbar and at 300 K in a diamond anvil cell using the nuclear resonant inelastic X-ray scattering (NRIXS) technique. From the partial phonon density of states (pDOS) and equation of state (EOS) of Fe 3 C, we derived its elastic parameters including shear modulus, compressional ( V P ) and shear-wave ( V S ) velocities to core pressures. A pressure-induced spin-pairing transition in the powdered Fe 3 C sample was found to occur gradually between 10 and 50 GPa by the X-ray Emission Spectroscopy (XES) measurements. Following the completion of the spin-pairing transition, the V P and V S of low-spin Fe 3 C increased with pressure at a markedly lower rate than its high-spin counterpart. Our results suggest that the incorporation of carbon in solid iron to form iron carbide phases, Fe 3 C and Fe 7 C 3 , could effectively lower the V S but respectively raise the Poisson's ratio by 0.05 and 0.07 to approach the seismically observed values for the Earth's inner core. The comparison with the preliminary reference Earth model (PREM) implies that an inner core composition containing iron and its carbon-rich alloys can satisfactorily explain the observed seismic properties of the inner core. [ABSTRACT FROM AUTHOR]

Sum rules of the moments of the nuclear resonant inelastic x-ray scattering (NRIXS) spectrum provide means of data analysis and information on atomic dynamics. We extend existing work by calculating the third and fourth moment beyond the harmonic approximation and show that NRIXS can provide a direct measurement of the anharmonic terms in lattice potentials. Projected partial phonon density of states (ppDOS) extracted from measured spectra provide vibrational mode-specific information on lattice dynamics. Furthermore, unique contributions to thermodynamic properties can be defined and calculated as the moments and other weighted integrals of ppDOS. A summary of some of these thermodynamic quantities is given. The directional dependence of NRIXS and its effects are emphasized. We derive explicit relationships between the moments of phonon excitation probability function and those of ppDOS. The comparison between the two sets of moments provides a consistency check and insights to the lattice dynamics of the system under study. [ABSTRACT FROM AUTHOR]

Abstract: Many seismological studies have confirmed that Vp travels 3–4% faster along the rotation axis of the Earth than along the equatorial plane in the inner core, indicating that the inner core is elastically anisotropic. However, seismic and mineral physics observations of the polarized Vs are still emerging. Thus far, the Vs anisotropy of the constitute iron crystals at relevant pressures of the Earth''s core has remained mostly theoretical mainly because of the technical difficulties involved in measuring reliable Vs velocities of iron crystals. Here we have measured azimuthal Vs anisotropy of highly textured hcp-Fe at high pressures using nuclear resonant inelastic X-ray scattering, a technique sensitive to Vs, in a diamond anvil cell. Our results show that the azimuthal Vs is 2–4% faster along the crystallographic c axis than along the a axis at 158GPa and 172GPa. If one describes the Vp anisotropy of the inner core as a result of the textured hcp-Fe crystals, it is conceivable that azimuthal and polarized Vs anisotropies with a magnitude of a few percent also exist in the region. Since Vp and Vs of candidate iron phases behave quite differently in theoretical predictions, our results here indicate that future seismic observations of the Vs and Vp anisotropies of the inner core thus hold the key to deciphering the causes for the seismic and dynamic signatures as well as constitute iron phase(s) of the region. [Copyright &y& Elsevier]

To unravel the effects of phonon confinement, the influence of size and morphology on the atomic vibrations is investigated in Sn nanoislands and cluster-assembled films. Nuclear resonant inelastic x-ray scattering is used to probe the phonon densities of states of the Sn nanostructures which show significant broadening of the features compared to bulk phonon behavior. Supported by ab initio calculations, the broadening is attributed to phonon scattering and can be described within the damped harmonic oscillator model. Contrary to the expectations based on previous research, the appearance of high-energy modes above the cutoff energy is not observed. From the thermodynamic properties extracted from the phonon densities of states, it was found that grain boundary Sn atoms are bound by weaker forces than bulk Sn atoms. [ABSTRACT FROM AUTHOR]