Calcium dissolution in bridgmanite in the Earth's deep mantle.

Accurate knowledge of the mineralogy is essential for understanding the lower mantle, which represents more than half of Earth's volume. CaSiO3 perovskite is believed to be the third-most-abundant mineral throughout the lower mantle, following bridgmanite and ferropericlase1-3. Here we experimentally show that the calcium solubility in bridgmanite increases steeply at about 2,300 kelvin and above 40 gigapascals to a level sufficient for a complete dissolution of all CaSiO3 component in pyrolite into bridgmanite, resulting in the disappearance of CaSiO3 perovskite at depths greater than about 1,800 kilometres along the geotherm4,5. Hence we propose a change from a two-perovskite domain (TPD; bridgmanite plus CaSiO3 perovskite) at the shallower lower mantle to a single-perovskite domain (SPD; calcium-rich bridgmanite) at the deeper lower mantle. Iron seems to have a key role in increasing the calcium solubility in bridgmanite. The temperature-driven nature can cause large lateral variations in the depth of the TPD-to-SPD change in response to temperature variations (by more than 500 kilometres). Furthermore, the SPD should have been thicker in the past when the mantle was warmer. Our finding requires revision of the deep-mantle mineralogy models and will have an impact on our understanding of the composition, structure, dynamics and evolution of the region.

Ultrahigh-pressure isostructural electronic transitions in hydrogen.

High-pressure transitions are thought to modify hydrogen molecules to a molecular metallic solid and finally to an atomic metal1, which is predicted to have exotic physical properties and the topology of a two-component (electron and proton) superconducting superfluid condensate2,3. Therefore, understanding such transitions remains an important objective in condensed matter physics4,5. However, measurements of the crystal structure of solid hydrogen, which provides crucial information about the metallization of hydrogen under compression, are lacking for most high-pressure phases, owing to the considerable technical challenges involved in X-ray and neutron diffraction measurements under extreme conditions. Here we present a single-crystal X-ray diffraction study of solid hydrogen at pressures of up to 254 gigapascals that reveals the crystallographic nature of the transitions from phase I to phases III and IV. Under compression, hydrogen molecules remain in the hexagonal close-packed (hcp) crystal lattice structure, accompanied by a monotonic increase in anisotropy. In addition, the pressure-dependent decrease of the unit cell volume exhibits a slope change when entering phase IV, suggesting a second-order isostructural phase transition. Our results indicate that the precursor to the exotic two-component atomic hydrogen may consist of electronic transitions caused by a highly distorted hcp Brillouin zone and molecular-symmetry breaking.

Thermal conductivity of Fe-Si alloys and thermal stratification in Earth's core.

Light elements in Earth's core play a key role in driving convection and influencing geodynamics, both of which are crucial to the geodynamo. However, the thermal transport properties of iron alloys at high-pressure and -temperature conditions remain uncertain. Here we investigate the transport properties of solid hexagonal close-packed and liquid Fe-Si alloys with 4.3 and 9.0 wt % Si at high pressure and temperature using laser-heated diamond anvil cell experiments and first-principles molecular dynamics and dynamical mean field theory calculations. In contrast to the case of Fe, Si impurity scattering gradually dominates the total scattering in Fe-Si alloys with increasing Si concentration, leading to temperature independence of the resistivity and less electron-electron contribution to the conductivity in Fe-9Si. Our results show a thermal conductivity of ∼100 to 110 W⋅m-1⋅K-1 for liquid Fe-9Si near the topmost outer core. If Earth's core consists of a large amount of silicon (e.g., > 4.3 wt %) with such a high thermal conductivity, a subadiabatic heat flow across the core-mantle boundary is likely, leaving a 400- to 500-km-deep thermally stratified layer below the core-mantle boundary, and challenges proposed thermal convection in Fe-Si liquid outer core.

Verwey-Type Charge Ordering and Site-Selective Mott Transition in Fe4O5 under Pressure.

The metal-insulator transition driven by electronic correlations is one of the most fundamental concepts in condensed matter. In mixed-valence compounds, this transition is often accompanied by charge ordering (CO), resulting in the emergence of complex phases and unusual behaviors. The famous example is the archetypal mixed-valence mineral magnetite, Fe3O4, exhibiting a complex charge-ordering below the Verwey transition, whose nature has been a subject of long-time debates. In our study, using high-resolution X-ray diffraction supplemented by resistance measurements and DFT+DMFT calculations, the electronic, magnetic, and structural properties of recently synthesized mixed-valence Fe4O5 are investigated under pressure to ∼100 GPa. Our calculations, consistent with experiment, reveal that at ambient conditions Fe4O5 is a narrow-gap insulator characterized by the original Verwey-type CO. Under pressure Fe4O5 undergoes a series of electronic and magnetic-state transitions with an unusual compressional behavior above ∼50 GPa. A site-dependent collapse of local magnetic moments is followed by the site-selective insulator-to-metal transition at ∼84 GPa, occurring at the octahedral Fe sites. This phase transition is accompanied by a 2+ to 3+ valence change of the prismatic Fe ions and collapse of CO. We provide a microscopic explanation of the complex charge ordering in Fe4O5 which "unifies" it with the behavior of two archetypal examples of charge- or bond-ordered materials, magnetite and rare-earth nickelates (RNiO3). We find that at low temperatures the Verwey-type CO competes with the "trimeron"/"dimeron" charge ordered states, allowing for pressure/temperature tuning of charge ordering. Summing up the available data, we present the pressure-temperature phase diagram of Fe4O5.

Fe0.79Si0.07B0.14 metallic glass gaskets for high-pressure research beyond 1†Mbar.

A gasket is an important constituent of a diamond anvil cell (DAC) assembly, responsible for the sample chamber stability at extreme conditions for X-ray diffraction studies. In this work, we studied the performance of gaskets made of metallic glass Fe0.79Si0.07B0.14 in a number of high-pressure X-ray diffraction (XRD) experiments in DACs equipped with conventional and toroidal-shape diamond anvils. The experiments were conducted in either axial or radial geometry with X-ray beams of micrometre to sub-micrometre size. We report that Fe0.79Si0.07B0.14 metallic glass gaskets offer a stable sample environment under compression exceeding 1 Mbar in all XRD experiments described here, even in those involving small-molecule gases (e.g. Ne, H2) used as pressure-transmitting media or in those with laser heating in a DAC. Our results emphasize the material's importance for a great number of delicate experiments conducted under extreme conditions. They indicate that the application of Fe0.79Si0.07B0.14 metallic glass gaskets in XRD experiments for both axial and radial geometries substantially improves various aspects of megabar experiments and, in particular, the signal-to-noise ratio in comparison to that with conventional gaskets made of Re, W, steel or other crystalline metals.

Phase transitions beyond post-perovskite in NaMgF3 to 160 GPa.

Neighborite, NaMgF3, is used as a model system for understanding phase transitions in ABX3 systems (e.g., MgSiO3) at high pressures. Here we report diamond anvil cell experiments that identify the following phases in NaMgF3 with compression to 162 GPa: NaMgF3 (perovskite) → NaMgF3 (post-perovskite) → NaMgF3 (Sb2S3-type) → NaF (B2-type) + NaMg2F5 (P2 1 /c) → NaF (B2) + MgF2 (cotunnite-type). Our results demonstrate the existence of an Sb2S3-type post-post-perovskite ABX3 phase. We also experimentally demonstrate the formation of the P2 1 /c AB2X5 phase which has been proposed theoretically to be a common high-pressure phase in ABX3 systems. Our study provides an experimental observation of the full sequence of phase transitions from perovskite to post-perovskite to post-post-perovskite followed by 2-stage breakdown to binary compounds. Notably, a similar sequence of transitions is predicted to occur in MgSiO3 at ultrahigh pressures, where it has implications for the mineralogy and dynamics in the deep interior of large, rocky extrasolar planets.

Tungsten Hexanitride with Single-Bonded Armchairlike Hexazine Structure at High Pressure.

WN_{6} phase discovered at 126-165 GPa after heating of W in nitrogen. XRD refinements reveal a unit cell in space group R3[over ¯]m which is consistent with the WN_{6} structure with armchairlike hexazine (N_{6}) rings, while strong A_{1g} Raman mode confirms its N─N single bonds. Density functional theory (DFT) calculations reveal balanced contributions of attractive interactions between W and covalent N_{6} rings, and repulsions between N_{6} rings that make WN_{6} ultrastiff and tough. The WN_{6} phase displays long bond lengths in the nearest N-N and pressure-enhanced electronic band gap, which pave the way for finding novel nitrides.

Superconducting Phase Induced by a Local Structure Transition in Amorphous Sb_{2}Se_{3} under High Pressure.

Superconductivity and Anderson localization represent two extreme cases of electronic behavior in solids. Surprisingly, these two competing scenarios can occur in the same quantum system, e.g., in an amorphous superconductor. Although the disorder-driven quantum phase transition has attracted much attention, its structural origins remain elusive. Here, we discovered an unambiguous correlation between superconductivity and density in amorphous Sb_{2}Se_{3} at high pressure. Superconductivity first emerges in the high-density amorphous (HDA) phase at about 24 GPa, where the density of glass unexpectedly exceeds its crystalline counterpart, and then shows an enhanced critical temperature when pressure induces crystallization at 51 GPa. Ab initio simulations reveal that the bcc-like local geometry motifs form in the HDA phase, arising from distinct "metavalent bonds." Our results demonstrate that HDA phase is critical for the incipient superconductive behavior.

Reconciliation of Experiments and Theory on Transport Properties of Iron and the Geodynamo.

We measure the electrical resistivity of hcp iron up to ∼170 GPa and ∼3000 K using a four-probe van der Pauw method coupled with homogeneous flattop laser heating in a DAC, and compute its electrical and thermal conductivity by first-principles molecular dynamics including electron-phonon and electron-electron scattering. We find that the measured resistivity of hcp iron increases almost linearly with temperature, and is consistent with our computations. The results constrain the resistivity and thermal conductivity of hcp iron to ∼80±5 µΩ cm and ∼100±10 W m^{-1} K^{-1}, respectively, at conditions near the core-mantle boundary. Our results indicate an adiabatic heat flow of ∼10±1 TW out of the core, supporting a present-day geodynamo driven by thermal and compositional convection.

Novel Hydrogen Clathrate Hydrate.

We report a new hydrogen clathrate hydrate synthesized at 1.2 GPa and 298 K documented by single-crystal x-ray diffraction, Raman spectroscopy, and first-principles calculations. The oxygen sublattice of the new clathrate hydrate matches that of ice II, while hydrogen molecules are in the ring cavities, which results in the trigonal R3c or R3[over ¯]c space group (proton ordered or disordered, respectively) and the composition of (H_{2}O)_{6}H_{2}. Raman spectroscopy and theoretical calculations reveal a hydrogen disordered nature of the new phase C_{1}^{'}, distinct from the well-known ordered C_{1} clathrate, to which this new structure transforms upon compression and/or cooling. This new clathrate phase can be viewed as a realization of a disordered ice II, unobserved before, in contrast to all other ordered ice structures.

Prediction and Synthesis of Dysprosium Hydride Phases at High Pressure.

Crystal structure prediction (CSP) methods recently proposed a series of new rare-earth (RE) hydrides at high pressures with novel crystal structures, unusual stoichiometries, and intriguing features such as high-Tc superconductivity. RE trihydrides (REH3) generally undergo a phase transition from ambient P63/mmc or P3̅c1 to Fm3̅m at high pressure. This cubic REH3 (Fm3̅m) was considered to be a precursor to further synthesize RE polyhydrides such as YH4, YH6, YH9, and CeH9 with higher hydrogen contents at higher pressures. However, the structural stability and equation of state (EOS) of any of the REH3 have not been fully investigated at sufficiently high pressures. This work presents high-pressure X-ray diffraction (XRD) measurements in a laser-heated diamond anvil cell up to 100 GPa and ab initio evolutionary CSP of stable phases of DyH3 up to 220 GPa. Experiments observed the Fm3̅m phase of DyH3 to be stable at pressures from 17 to 100 GPa and temperatures up to ∼2000 K. After complete decompression, the P3̅c1 and Fm3̅m phases of DyH3 recovered under ambient conditions. Our calculations predicted a series of phases for DyH3 at high pressures with the structural phase transition sequence P3̅c1 → Imm2 → Fm3̅m → Pnma → P63/mmc at 11, 35, 135, and 194 GPa, respectively. The predicted P3̅c1 and Fm3̅m phases are consistent with experimental observations. Furthermore, electronic band structure calculations were carried out for the predicted phases of DyH3, including the 4f states, within the DFT+U approach. The inclusion of 4f states shows significant changes in electronic properties, as more Dy d states cross the Fermi level and overlap with H 1s states. The structural phase transition from P3̅c1 to Fm3̅m observed in DyH3 is systematically compared with other REH3 compounds at high pressures. The phase transition pressure in REH3 shows an inverse relation with the ionic radius of RE atoms.

A Boosted Critical Temperature of 166†K in Superconducting D3 S Synthesized from Elemental Sulfur and Hydrogen.

The discovery of superconductivity in H3 S at 203 K marked an advance towards room-temperature superconductivity and demonstrated the potential of H-dominated compounds to possess a high critical temperature (Tc ). There have been numerous reports of the H-S system over the last five years, but important questions remain unanswered. It is crucial to verify whether the Tc was determined correctly for samples prepared from compressed H2 S, since they are inevitably contaminated with H-depleted byproducts. Here, we prepare stoichiometric H3 S by direct in situ synthesis from elemental S and excess H2 . The Im 3 ‾ m phase of D3 S samples exhibits a Tc significantly higher than previously reported values (ca. 150 K), reaching a maximum Tc of 166 K at 157 GPa. Furthermore, we confirm that the sharp decrease in Tc below 150 GPa is accompanied by continuous rhombohedral structural distortions and demonstrate that the Cccm phase is non-metallic, with molecular H2 units in the crystal structure.

The O-O Bonding and Hydrogen Storage in the Pyrite-type PtO2.

We have synthesized pyrite-type PtO2 (py-PtO2) at 50-60 GPa and successfully recovered it at 1 bar. The observed O-O stretching vibration in Raman spectra provides direct evidence for inter-oxygen bonding in the structure. We also identified the O-H vibrations in py-PtO2 synthesized from the low-temperature areas, indicating hydrogenation, py-PtO2H x ( x ≤ 1). Diffraction patterns are consistent with a range of degrees of hydrogenation controlled by temperature. We found that py-PtO2 has a high bulk modulus, 314 ± 4 GPa. The chemical behaviors found in py-PtO2 have implications for the hydrogen storage in materials with anion-anion bonding, and the geochemistry of oxygen, hydrogen, and transition metals in the deep planetary interiors.

Helium-hydrogen immiscibility at high pressures.

Hydrogen and helium are the most abundant elements in the universe, and they constitute the interiors of gas giant planets. Thus, their equations of states, phase, chemical state, and chemical reactivity at extreme conditions are of great interest. Applying Raman spectroscopy, visual observation, and synchrotron X-ray diffraction in diamond anvil cells, we performed experiments on H2-He 1:1 and D2-He 1:10 compressed gas mixtures up to 100 GPa at 300 K. By comparing with the available data on pure bulk materials, we find no sign of miscibility, chemical reactivity, and new compound formation. This result establishes a new baseline for future investigations of miscibility in the He-H2 system at extreme P-T conditions.

Synthesis of Xenon and Iron-Nickel Intermetallic Compounds at Earth's Core Thermodynamic Conditions.

Using in situ synchrotron x-ray diffraction and Raman spectroscopy in concert with first principles calculations we demonstrate the synthesis of stable Xe(Fe,Fe/Ni)_{3} and XeNi_{3} compounds at thermodynamic conditions representative of Earth's core. Surprisingly, in the case of both the Xe-Fe and Xe-Ni systems Fe and Ni become highly electronegative and can act as oxidants. The results indicate the changing chemical properties of elements under extreme conditions by documenting that electropositive at ambient pressure elements could gain electrons and form anions.

Effects of pressure on the structure and lattice dynamics of ammonium perchlorate: A combined experimental and theoretical study.

Ammonium perchlorate NH4ClO4 (AP) was studied using synchrotron angle-dispersive X-ray powder diffraction (XRPD) and Raman spectroscopy. A diamond-anvil cell was used to compress AP up to 50 GPa at room temperature (RT). Density functional theory (DFT) calculations were performed to provide further insight and comparison to the experimental data. A high-pressure barite-type structure (Phase II) forms at ≈4 GPa and appears stable up to 40 GPa. Refined atomic coordinates for Phase II are provided, and details for the Phase I → II transition mechanics are outlined. Pressure-dependent enthalpies computed for DFT-optimized crystal structures confirm the Phase I → II transition sequence, and the interpolated transition pressure is in excellent agreement with the experiment. Evidence for additional (underlying) structural modifications include a marked decrease in the Phase II b'-axis compressibility starting at 15 GPa and an unambiguous stress relaxation in the normalized stress-strain response at 36 GPa. Above 47 GPa, XRD Bragg peaks begin to decrease in amplitude and broaden. The apparent loss of crystalline long-range order likely signals the onset of amorphization. Three isostructural modifications were discovered within Phase II via Raman spectroscopy. A revised RT isothermal phase diagram is discussed based on the findings of this study.

Carbonate-Metal Reactions in the Lower Mantle.

Carbonates are important carbon-bearing phases in the mantle. While their role in upper mantle petrologic processes has been well studied, their effect on phase relations, melting, and transport properties in the lower mantle is less understood. The stability of carbonates in the mantle depends on a host of factors, including pressure, temperature, oxygen fugacity, and reactions with surrounding mantle phases. To understand the stability of carbonates in the presence of metal in the lower mantle, carbonate-metal reaction experiments on the Fe-Si-Ca-Mg-C-O system were conducted up to 124 GPa and 3200 K. We find that carbonates react with iron alloys to form silicates, iron carbides, and oxides. However, the temperature at which these reactions occur increases with pressure, indicating that along a geotherm in the lowermost mantle carbonates are the stable carbon-bearing phase. Carbon is found to be less siderophilic at high-pressure compared to silicon.

Pressure-induced hydrogen bond symmetrization in iron oxyhydroxide.

Under high pressures the hydrogen bonds were predicted to transform from a highly asymmetric soft O-H···O to a symmetric rigid configuration in which the proton lies midway between the two oxygen atoms. Despite four decades of research on hydroxyl containing compounds, pressure induced hydrogen bond symmetrization remains elusive. Following single crystal x-ray diffraction, Mössbauer and Raman spectroscopy measurements supported by ab initio calculations, we report the H-bonds symmetrization in iron oxyhydroxide, FeOOH, resulting from the Fe(3+) high-to-low spin crossover at above 45 GPa.

Superconductor-insulator transitions in three-dimensional indium-oxide at high pressures.

Experiments investigating magnetic-field-tuned superconductor-insulator transition (HSIT) mostly focus on two-dimensional material systems where the transition and its proximate ground-state phases, often exhibit features that are seemingly at odds with the expected behavior. Here we present a complementary study of a three-dimensional pressure-packed amorphous indium-oxide (InOx) powder where granularity controls the HSIT. Above a low threshold pressure of ∼0.2 GPa, vestiges of superconductivity are detected, although neither a true superconducting transition nor insulating behavior are observed. Instead, a saturation at very high resistivity at low pressure is followed by saturation at very low resistivity at higher pressure. We identify both as different manifestations of anomalous metallic phases dominated by superconducting fluctuations. By analogy with previous identification of the low resistance saturation as a 'failed superconductor', our data suggests that the very high resistance saturation is a manifestation of a 'failed insulator'. Above a threshold pressure of ∼6 GPa, the sample becomes fully packed, and superconductivity is robust, withTCtunable with pressure. A quantum critical point atPC∼ 25 GPa marks the complete suppression of superconductivity. For a finite pressure belowPC, a magnetic field is shown to induce a HSIT from a true zero-resistance superconducting state to a weakly insulating behavior. Determining the critical field,HC, we show that similar to the 2D behavior, the insulating-like state maintains a superconducting character, which is quenched at higher field, above which the magnetoresistance decreases to its fermionic normal state value.

Pressure-induced high-spin/low-spin disproportionated state in the Mott insulator FeBO3.

The pressure-induced Mott insulator-to-metal transitions are often accompanied by a collapse of magnetic interactions associated with delocalization of 3d electrons and high-spin to low-spin (HS-LS) state transition. Here, we address a long-standing controversy regarding the high-pressure behavior of an archetypal Mott insulator FeBO3 and show the insufficiency of a standard theoretical approach assuming a conventional HS-LS transition for the description of the electronic properties of the Mott insulators at high pressures. Using high-resolution x-ray diffraction measurements supplemented by Mössbauer spectroscopy up to pressures ~ 150 GPa, we document an unusual electronic state characterized by a "mixed" HS/LS state with a stable abundance ratio realized in the [Formula: see text] crystal structure with a single Fe site within a wide pressure range of ~ 50-106 GPa. Our results imply an unconventional cooperative (and probably dynamical) nature of the ordering of the HS/LS Fe sites randomly distributed over the lattice, resulting in frustration of magnetic moments.