Inelastic x-ray scattering from polycrystalline materials at low momentum transfer

Bosak, A.; Krisch, M.; Fischer, Irmengard; Huotari, Simo; Monaco, G.

doi:10.1103/physrevb.75.064106

Cited by 30 publications

(32 citation statements)

References 32 publications

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“…When dealing with a polycrystal, the intrinsic orientational average automatically adds to the measured spectrum the contribution from these quasi-transverse modes also in first Brillouin zone. In the case of polycrystalline sodium, a simple elasticity-based model (21) indeed confirms on a quantitative ground this interpretation. As a matter of fact, the larger the elastic anisotropy of the system, the more visible is the quasi-transverse component in the Sðq;ωÞ spectrum.…”

Section: Resultssupporting

confidence: 62%

Fingerprints of order and disorder on the high-frequency dynamics of liquids

Giordano

Monaco

2010

Proc. Natl. Acad. Sci. U.S.A.

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It is largely accepted that liquids are characterized by a short-range order usually corresponding to that of the solid phase at the same density. It is less clear to what extent dynamic properties of liquids and crystals can be compared. In particular, high-frequency collective excitations reminiscent of phonons in solids exist as well in liquids. They are however traditionally discussed in terms of relaxation processes characteristic of the liquid phase. We report here on a quantitative comparison of the collective excitations in liquid and polycrystalline sodium. We show that liquid sodium exhibits acoustic excitations of both longitudinal and transverse polarization at frequencies strictly related to those of the corresponding crystal. The only relevant difference between the liquid and the polycrystal appears in the broadening of the excitations: An additional disorder-induced contribution comes into play in the case of the liquid, which we show to be related to the distribution of local structures around the average one. These results establish a direct connection between structural and dynamic properties of liquids, with shortrange order and overall structural disorder leaving very specific fingerprints.vibrational dynamics | X-ray scattering T he short-range order in liquids, characterized by parameters like, e.g., the first neighbors distance and number, is usually similar to that of the corresponding solids (1, 2). Ab initio calculations of liquid structures spanning a large density range do support this idea (3). For what concerns dynamic properties, the situation is more complex. Local vibrations appear in the liquid as in the corresponding solid, although they are usually much broader (1). This broadening is generally related to the distribution of local structures that is present in the liquid phase (4) and that is, e.g., characterized by the distribution of first neighbors distances. Collective excitations exist as well in liquids, and their broadening is usually attributed to relaxation processes (5-7). In fact, the response of a crystal to an external perturbation can be formally expressed in terms of phonons, the modes of lattice vibration that can be defined in presence of a long-range, periodic structure. In liquids this description is instead formally not appropriate, and the concept of relaxation is usually introduced with reference to a dynamic process that controls the reequilibration of the system after it has been perturbed (8). Several distinct relaxation processes can be introduced to describe the dynamics of liquids, each one characterized by a characteristic time, τ, that sets the timescale over which it is active and observable. Indeed, if the perturbation frequency, ω, is such that ωτ ≪ 1, the relaxation is active, whereas when ωτ ≫ 1 the relaxation process does not have the time to take place and is frozen. In the infinite frequency limit, where all relaxations are frozen, the liquid behaves like a solid: This limit corresponds to the elastic regime, where only the instantaneous ...

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Section: Resultssupporting

confidence: 62%

Fingerprints of order and disorder on the high-frequency dynamics of liquids

Giordano

Monaco

2010

Proc. Natl. Acad. Sci. U.S.A.

Self Cite

119

133

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“…Another significant difference is in the number, relative position and momentum resolution of the points used to constrain the phonon dispersion within the first Brillouin zone: Antonangeli et al (2012) used 8 to 9 points in the 3.5 to 12.5 nm −1 range, with a momentum resolution of 0.25 nm . A detailed discussion of the influence of q-settings and qresolution on the derived velocity can be found in Bosak et al (2007). Finally, especially when comparing the results of Antonangeli et al (2012) with those of Ohtani et al (2013), we cannot rule out a difference due to absolute energy calibration.…”

Section: Density Dependence Of the Compressional Sound Velocity In Hcmentioning

confidence: 99%

“…V P and V S can then be derived by straightforward averaging. However, in the case of measurements on polycrystalline samples (as typically the case for experiments at very high pressures), the loss of directional information limits the measurements to the averaged phonon dispersions of the aggregate (Bosak et al, 2007). V P can be then obtained from a sine fit to the aggregate phonon dispersion and V S derived by combining the measured V P with results from an equation of state (density and adiabatic bulk modulus) (Antonangeli et al, 2004b), but data analysis and interpretation now depend on how well the data sample the linear part of the phonon dispersion and on the knowledge of sample texture (Bosak et al, 2007).…”

Section: Experimental Techniques For Sound Velocity Determination Undmentioning

confidence: 99%

Sound velocity of hcp-Fe at high pressure: experimental constraints, extrapolations and comparison with seismic models

Antonangeli

Ohtani

2015

Prog. in Earth and Planet. Sci.

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Determining the sound velocity of iron under extreme thermodynamic conditions is essential for a proper interpretation of seismic observations of the Earth's core but is experimentally challenging. Here, we review techniques and methodologies used to measure sound velocities in metals at megabar pressures, with specific focus on the compressional sound velocity of hexagonal close-packed iron. A critical comparison of literature results, coherently analyzed using consistent metrology (pressure scale, equation of state), allows us to propose reference relations for the pressure and density dependence of the compressional velocity of hexagonal close-packed iron at ambient temperature. This provides a key base line upon which to add complexity, including high-temperature effects, pre-melting effects, effects of nickel and/or light element incorporation, necessary for an accurate comparison with seismic models, and ultimately to constrain Earth's inner core composition.

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“…Up to very recently, the information which could be extracted from polycrystals was somewhat limited. At small momentum transfer, Q, approximate longitudinal aggregate velocities can be extracted [Bosak et al, 2007], whereas the large Q-region allows the reconstruction of the (generalized) VDOS from the INS/IXS spectra under conditions of (i) correct directional averaging and (ii) an appropriate reciprocal space sampling [Needham et al, 1993;De Wette and Rahman, 1968;Squires, 1978;Bosak and Krisch, 2005].…”

Section: Introductionmentioning

confidence: 99%

Lattice dynamics of stishovite from powder inelastic X‐ray scattering

Bosak

Fischer

Krisch

et al. 2009

Geophysical Research Letters

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[1] We have retrieved the full lattice dynamics of stishovite, SiO 2 , by a combination of powder inelastic x-ray scattering (IXS) and ab initio calculations. The extracted dispersion curves are confirmed by experiments on a single crystal. Thereby we have shown that IXS powder measurements over a large range of momentum transfers allow an unambiguous validation of lattice dynamics models, which then in turn can be used to derive elastic and thermodynamic properties. The proposed method can be extended to high pressure measurements on in situ synthesized samples and promises to be a valuable tool in studies of materials for which no single crystals can be obtained.

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Inelastic x-ray scattering from polycrystalline materials at low momentum transfer

Cited by 30 publications

References 32 publications

Fingerprints of order and disorder on the high-frequency dynamics of liquids

Fingerprints of order and disorder on the high-frequency dynamics of liquids

Sound velocity of hcp-Fe at high pressure: experimental constraints, extrapolations and comparison with seismic models

Lattice dynamics of stishovite from powder inelastic X‐ray scattering

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