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We measured the phonon density of states (DOS) of nanocrystalline Fe by resonant inelastic nuclear g-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, Q u , 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.[ S0031-9007(97) PACS numbers: 76.80. + y, 61.72. -y, 63.20. -e Over the past decade there has been much interest in nanocrystalline materials, generally defined as materials composed of crystallites smaller than 100 nm. Unusual mechanical properties and soft magnetic properties were topics of numerous investigations on metallic nanocrystals [1]. Very recently, neutron inelastic scattering measurements have shown some differences in the phonon density of states (DOS) of nanocrystalline and bulk materials [2-6]. One such effect was an enhancement of the phonon DOS at low energies [3][4][5][6][7]. A broadening of the peak from the longitudinal modes in the phonon DOS was also observed and attributed to the lifetime broadening of phonons in small crystals [5,6]. Unfortunately, it was not possible to measure accurately the shape of the longitudinal peak, owing to statistical and background limitations of the neutron inelastic scattering technique.In this Letter we show how a recently developed experimental technique, resonant inelastic nuclear g-ray scattering [8,9], provides new information on the shape of the phonon DOS of nanocrystalline Fe. In particular, the excellent signal-to-noise ratio of the data makes it possible to examine quantitatively the high energy tail of the phonon DOS in small samples. In our resonant inelastic nuclear g-ray scattering measurements, 14.41 keV g rays were directed onto a foil specimen, and 6.4 keV conversion x-ray radiations from the specimen were detected. This scattering is incoherent, so the data provide information on the velocity-velocity correlation function of individual 57 Fe nuclei. The experiments were performed at the undulator beamline 3-ID at the Advanced Photon Source. A high-heat-load monochromator, which consists of two symmetric silicon (1 1 1) reflections in a nondispersive setting, and a high-resolution, nested monochromator, as described previously [10], were used to provide the 14.413 keV radiation onto the specimen. The highresolution monochromator operates with asymmetric silicon (4 2 2) and symmetric silicon (10 6 4) reflections and produces a constant energy bandwidth of 5.5 meV over the tuning range. The energy of the incident radiation was tuned by rotating the (10 6 4) channel-cut crystal in steps of 2 meV. The photons were incident on the sample at 5 3 10 9

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Hydrogen transport from the surface to the deep interior and distribution in the mantle are important in the evolution and dynamics of the Earth. An aluminum oxy-hydroxide, δ-AlOOH, might influence hydrogen transport in the deep mantle because of its high stability extending to lower mantle conditions. The compressional behavior and spin states of δ-(Al,Fe3+)OOH phases were investigated with synchrotron X-ray diffraction and Mössbauer spectroscopy under high pressure and room temperature. Pressure-volume (P-V) profiles of the δ-(Al0.908(9)57Fe0.045(1))OOH1.14(3) [Fe/(Al+Fe) = 0.047(10), δ-Fe5] and the δ-(Al0.832(5)57Fe0.117(1))OOH1.15(3) [Fe/(Al+Fe) = 0.123(2), δ-Fe12] show that these hydrous phases undergo two distinct structural transitions involving changes in hydrogen bonding environments and a high- to low-spin crossover in Fe3+. A change of axial compressibility accompanied by a transition from an ordered (P21nm) to disordered hydrogen bond (Pnnm) occurs near 10 GPa for both δ-Fe5 and δ-Fe12 samples. Through this transition, the crystallographic a and b axes become stiffer, whereas the c axis does not show such a change, as observed in pure δ-AlOOH. A volume collapse due to a transition from high- to low-spin states in the Fe3+ ions is complete below 32–40 GPa in δ-Fe5 and δ-Fe12, which i ~10 GPa lower than that reported for pure ε-FeOOH. Evaluation of the Mössbauer spectra of δ-(Al0.824(10)57Fe0.126(4))OOH1.15(4) [Fe/(Al+Fe) = 0.133(3), δ-Fe13] also indicate a spin transition between 32–45 GPa. Phases in the δ-(Al,Fe)OOH solid solution with similar iron concentrations as those studied here could cause an anomalously high ρ/νΦ ratio (bulk sound velocity, defined as K/ρ at depths corresponding to the spin crossover region (~900 to ~1000 km depth), whereas outside the spin crossover region a low ρ/νΦ anomaly would be expected. These results suggest that the δ-(Al,Fe)OOH solid solution may play an important role in understanding the heterogeneous structure of the deep Earth.

An in-line monochromatization scheme suitable for 10-40 keV synchrotron radiation is presented based on the use of six crystal reflections that achieves meV and sub-meV bandwidths with high efficiency. The theoretical spectral efficiency surpasses all previous multicrystal designs and approaches that of single room-temperature back-reflecting crystals. This article presents the designs of two such devices along with their theoretical and measured performances.