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Elastic properties of B2‐Fe0.67Ni0.06Si0.27 (15 wt.% Si) alloy have been investigated by combined high‐resolution inelastic X‐ray scattering and powder X‐ray diffraction in diamond anvil cells up to 100 GPa at room temperature. Densities (ρ), compressional (VP) and shear (VS) wave velocities were extrapolated to inner core conditions to enable comparison with the preliminary reference Earth model. The modeled aggregate compressional and shear wave velocities and densities of the two‐phase mixture of B2‐Fe0.67Ni0.06Si0.27 and hcp‐Fe‐Ni are consistent with inner core PREM values of VP, VS, and ρ based on a linear mixing model with 30(5) vol % B2‐Fe0.67Ni0.06Si0.27 and 70(5) vol % hcp Fe‐Ni, which corresponds to ∼3–5 wt.% Si and ∼5–12 wt.% Ni., Plain Language Summary: The composition of the inner core holds key information about how Earth evolved and how current processes such as the geomagnetic field work. Because the core cannot be directly sampled, our best estimates of its composition are based on the comparison of geophysical data with laboratory measurements of candidate materials. Decades of study have shown the inner core to be composed mainly of iron (with a minor amount of nickel) alloyed with one or more light elements, such as silicon. However, the effect of Si on the geophysical properties of Fe‐Ni alloys is not well established. In this study, we performed laboratory experiments to determine the density and sound velocity of Fe‐Ni‐Si alloy under extreme pressure conditions. We compared our results with seismological determinations and found that Earth's inner core can be accounted for by a mechanical mixture of cubic Fe‐Ni‐Si and hexagonal Fe‐Ni alloys. This mixture has a bulk composition of ∼3–5 weight % Si and ∼5–12 weight % Ni and is consistent with geophysical constraints., Key Points: Sound velocities and densities of B2‐Fe‐Ni‐Si alloy are determined at high pressure, up to 100 GPa, using inelastic X‐ray scattering and X‐ray diffraction. Seismologically observed compressional and shear wave velocities and density of Earth's inner core can be accounted for by a two‐phase mixture of 30 vol % B2‐Fe‐Ni‐Si and 70 vol % hcp Fe‐Ni alloys. Extrapolated results at inner core boundary conditions are consistent with an inner core composition containing ∼3–5 wt.% Si and ∼5–12 wt.% Ni., Japan Society for the Promotion of Science http://dx.doi.org/10.13039/501100001691, Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659, JSPS Japanese‐German graduate externship, https://doi.org/10.6084/m9.figshare.13370795.v1, https://doi.org/10.6084/m9.figshare.13370792.v1

The phase relations of iron-rich olivine and its high-pressure polymorphs are important for planetary science and meteoritics because these minerals are the main constituents of terrestrial mantles and meteorites. The olivine–ahrensite binary loop was previously determined by thermochemical calculations in combination with high-pressure experiments; however, the transition pressures contained significant uncertainties. Here we determined the binary loop of the olivine–ahrensite transition in the (Mg,Fe)2SiO4 system at 1740 K in the pressure range of 7.5–11.2 GPa using a multi-anvil apparatus with the pressure determined using in situ X-ray diffraction, compositional analysis of quenched run products, and thermochemical calculation. Based on the determined binary loop, a user-friendly software was developed to calculate pressure from the coexisting olivine and ahrensite compositions. The software is used to estimate the shock conditions of several L6-type chondrites. The obtained olivine–ahrensite phase relations can also be applied for precise in-house multi-anvil pressure calibration at high temperatures., BMBF, H2020 European Research Council http://dx.doi.org/10.13039/100010663, Deutsches Elektronen-Synchrotron (DESY) (4201)

In iron-based superconductors, a spin-density-wave (SDW) magnetic order is suppressed with doping and unconventional superconductivity appears in close proximity to the SDW instability. The optical response of the SDW order shows clear gap features: substantial suppression in the low-frequency optical conductivity, alongside a spectral weight transfer from low to high frequencies. Here, we study the detailed temperature dependence of the optical response in three different series of the Ba122 system [Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ and BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$]. Intriguingly, we found that the suppression of the low-frequency optical conductivity and spectral weight transfer appear at a temperature $T^{\ast}$ much higher than the SDW transition temperature $T_{SDW}$. Since this behavior has the same optical feature and energy scale as the SDW order, we attribute it to SDW fluctuations. Furthermore, $T^{\ast}$ is suppressed with doping, closely following the doping dependence of the nematic fluctuations detected by other techniques. These results suggest that the magnetic and nematic orders have an intimate relationship, in favor of the magnetic-fluctuation-driven nematicity scenario in iron-based superconductors., Comment: 6 pages 5 figures

The THz spectrum of density fluctuations, $S(Q, \omega)$, of vitreous GeO$_2$ at ambient temperature was measured by inelastic x-ray scattering from ambient pressure up to pressures well beyond that of the known $\alpha$-quartz to rutile polyamorphic (PA) transition. We observe significant differences in the spectral shape measured below and above the PA transition, in particular, in the 30-80 meV range. Guided by first-principle lattice dynamics calculations, we interpret the changes in the phonon dispersion as the evolution from a quartz-like to a rutile-like coordination. Notably, such a crossover is accompanied by a cusp-like behavior in the pressure dependence of the elastic response of the system. Overall, the presented results highlight the complex fingerprint of PA phenomena on the high-frequency phonon dispersion., Comment: 18 pages, 5 figures

Nature Communications 13(1), 387 (2022). doi:10.1038/s41467-022-27991-9, Mercury���s metallic core is expected to have formed under highly reducing conditions, resulting in the presence of significant quantities of silicon alloyed to iron. Here we present the phase diagram of the Fe-FeSi system, reconstructed from in situ X-ray diffraction measurements at pressure and temperature conditions spanning over those expected for Mercury���s core, and ex situ chemical analysis of recovered samples. Under high pressure, we do not observe a miscibility gap between the cubic fcc and B2 structures, but rather the formation of a re-entrant bcc phase at temperatures close to melting. Upon melting, the investigated alloys are observed to evolve towards two distinct Fe-rich and Fe-poor liquid compositions at pressures below 35-38 GPa. The evolution of the phase diagram with pressure and temperature prescribes a range of possible core crystallization regimes, with strong dependence on the Si abundance of the core., Published by Nature Publishing Group UK, [London]

International audience; Using a highly stoichiometric magnetite, the pressure-induced phase transitions of Fe3O4 have been revisited here by performing Fe K-edge X-ray absorption and magnetic circular dichroism measurements up to P = 65 GPa at room temperature and 71 GPa at 20 K. We have observed a structural transition at around 27 GPa from magnetite to a high-pressure phase h-Fe3O4 with the loss of the net ordered magnetic moments for both temperatures. The orthorhombic CaTi2O4-type (Bbmm) structure of the high-pressure phase h-Fe3O4 has been determined by combining experimental studies and theoretical simulations of the Fe K-edge X-ray absorption near-edge structure spectra. The long-time discussed pressure induced spin crossover, and inverse to normal spinel structure transitions in Fe3O4 can be deterministically excluded from the present study up to ∼65 GPa.