Face centered cubic (fcc) FeH_X was synthesized at pressures of 18–68 GPa and temperatures exceeding 1,500 K. Thermally quenched samples were evaluated using synchrotron X‐ray diffraction (XRD) and nuclear resonant inelastic X‐ray scattering (NRIXS) to determine sample composition and sound velocities to 82 GPa. To aid in the interpretation of nonideal (X ≠ 1) stoichiometries, two equations of state for fcc FeH_X were developed, combining an empirical equation of state for iron with two distinct synthetic compression curves for interstitial hydrogen. Matching the density deficit of the Earth's core using these equations of state requires 0.8–1.1 wt % hydrogen at the core‐mantle boundary and 0.2–0.3 wt % hydrogen at the interface of the inner and outer cores. Furthermore, a comparison of Preliminary Reference Earth Model (PREM) to a Birch's law extrapolation of our experimental results suggests that an iron alloy containing ∼0.8–1.3 wt % hydrogen could reproduce both the density and compressional velocity (V_P) of the Earth's outer core.

Constraining the accommodation, distribution, and circulation of hydrogen in the Earth's interior is vital to our broader understanding of the deep Earth due to the significant influence of hydrogen on the material and rheological properties of minerals. Recently, a great deal of attention has been paid to the high-pressure polymorphs of FeOOH (space groups P21nm and Pnnm). These structures potentially form a hydrogen-bearing solid solution with AlOOH and phase H (MgSiO4H2) that may transport water (OH–) deep into the Earth's lower mantle. Additionally, the pyrite-type polymorph (space group Pa3 of FeOOH), and its potential dehydration have been linked to phenomena as diverse as the introduction of hydrogen into the outer core (Nishi et al. 2017), the formation of ultralow-velocity zones (ULVZs) (Liu et al. 2017), and the Great Oxidation Event (Hu et al. 2016). In this study, the high-pressure evolution of FeOOH was re-evaluated up to ~75 GPa using a combination of synchrotron-based X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and optical absorption spectroscopy. Based on these measurements, we report three principal findings: (1) pressure-induced changes in hydrogen bonding (proton disordering or hydrogen bond symmetrization) occur at substantially lower pressures in ε-FeOOH than previously reported and are unlikely to be linked to the high-spin to low-spin transition; (2) ε-FeOOH undergoes a 10% volume collapse coincident with an isostructural Pnnm → Pnnm transition at approximately 45 GPa; and (3) a pressure-induced band gap reduction is observed in FeOOH at pressures consistent with the previously reported spin transition (40 to 50 GPa).

Oceanic plates contain numerous hydrous phases including amphiboles, which are important carriers of water into subduction zones. The hydroxyl bound within the crystalline structure of hydrous minerals, as well as changes in hydrogen bond symmetry, can impact the bulk properties of these minerals. In this study, 12 natural amphibole samples spanning a range of 10 compositions were probed with synchrotron infrared spectroscopy at room temperature and pressures up to 60 GPa. Infrared spectra were collected at atmospheric pressure and at regular intervals during compression, allowing for the collection of spectra centered on the typical O-H stretching region at 3600–3700 cm⁻¹ as they evolved with pressure for each composition. The number of O-H bands within each sample was found to vary with composition, but the pressure dependence of O-H frequency shifting more closely correlated with mode frequency at ambient pressure than with composition. Combined with earlier results, these data reveal a linear relationship between mode frequency at ambient pressure and the pressure dependence of O-H stretching modes in amphiboles and sheet silicates. Two sample preparation methods utilized in this study allowed for direct comparison between quasi-hydrostatic neon-loaded sample conditions and the conditions achieved with a KBr pressure medium. Samples loaded in neon preserved sharper peaks, allowing greater spectral resolution, especially at higher pressures when peaks are most likely to broaden or disappear due to crystalline disorder and pressure gradients across the sample. This new quasi-hydrostatic loading method proved valuable to tracing O-H stretching behavior in amphiboles to higher pressures than previously obtained and will lend itself to future study of O-H stretching pressure dependence in a wide range of hydrous minerals.

The alkali halides are often used as optically-transparent pressure-transmitting media and thermal insulators in laser-heated diamond anvil cell experiments. High P–T equations of state for these materials would allow them to be used simultaneously as sensitive in situ pressure markers, making sample preparation and data analysis simpler. KCl is especially useful for this application because its high melting point, crystallographic simplicity, and low electron density are ideal for use in high-temperature x-ray diffraction experiments. However, the high-temperature static equation of state data for this material is limited in pressure to 8 GPa. As experiments routinely exceed 100 GPa in pressure and thousands of Kelvin, it is necessary for the equation of state of KCl to be determined experimentally at these conditions if it is to be used as an equation of state calibrant in the future. This paper combines new high-pressure, high-temperature data of the B2 phase of KCl (up to 167 GPa and 2400 K) with the previously published room-temperature data of Dewaele et al. [ Phys. Rev. B 85, 214105 (2012)] to produce a Mie-Grüneisen-Debye thermal equation of state for this material. The room-temperature equation of state parameters are similar to those reported previously: $V_0=32.0\left(3\right) {\mathrm{cm}}^3/\mathrm{mole}, K_0=24\left(1\right)$ GPa, and $K_0^{\prime }=4.56\left(5\right)$. The thermal parameters, ${\gamma }_0$ and $q$ are 2.9(4) and 1.0(1), respectively. While a $q$ of 1 is expected, the ${\gamma }_0$ is higher than expected from previous calculations, lower pressure experimental data, and the available shockwave data. Thus, previously-reported equations of state underestimate the pressure of KCl at high temperatures.