FeO2 and FeOOH under deep lower-mantle conditions and Earth’s oxygen–hydrogen cycles

Iron alloys of volatile elements in the deep Earth's interior

Investigations into the compositional model of the Earth, particularly the atypical concentrations of volatile elements within the silicate portion of the early Earth, have attracted significant interest due to their pivotal role in elucidating the planet's evolution and dynamics. To understand the behavior of such volatile elements, an established 'volatility trend' has been used to explain the observed depletion of certain volatile elements. However, elements such as Se and Br remain notably over-depleted in the silicate Earth. Here we show the results from first-principles simulations that explore the potential for these elements to integrate into hcp-Fe through the formation of substitutional alloys, long presumed to be predominant constituents of the Earth's core. Based on our findings, the thermodynamic stability of these alloys suggests that these volatile elements might indeed be partially sequestered within the Earth's core. We suggest potential reservoirs for volatile elements within the deep Earth, augmenting our understanding of the deep Earth's composition.

Observation of Iron with Eight Coordination in Iron Trifluoride under High Pressure

The chemistry of hypercoordination has been a subject of fundamental interest, especially for understanding structures that challenge conventional wisdom. The small ionic radii of Fe ions typically result in coordination numbers of 4 or 6 in stable Fe-bearing ionic compounds. While 8-coordinated Fe has been observed in highly compressed oxides, the pursuit of hypercoordinated Fe still faces significant challenges due to the complexity of synthesizing the anticipated compound with another suitable anion. Through first-principles simulation and advanced crystal structure prediction methods, we predict that an orthorhombic phase of FeF3 with exclusively 8-coordinated Fe is energetically stable above 18 GPa-a pressure more feasibly achieved compared to oxides. Inspired by this theoretical result, we conducted extensive experiments using a laser-heated diamond anvil cell technique to investigate the crystal structures of FeF3 at high-pressure conditions. We successfully synthesized the predicted orthorhombic phase of FeF3 at 46 GPa, as confirmed by in situ experimental X-ray diffraction data. This work establishes a new ionic compound featuring rare 8-coordinated Fe in a simple binary Fe-bearing system and paves the way for discovering Fe hypercoordination in similar systems.

Oldhamite: a new link in upper mantle for C-O-S-Ca cycles and an indicator for planetary habitability

In the solar system, oldhamite (CaS) is generally considered to be formed by the condensation of solar nebula gas. Enstatite chondrites, one of the most important repositories of oldhamite, are believed to be representative of the material that formed Earth. Thus, the formation mechanism and the evolution process of oldhamite are of great significance to the deep understanding of the solar nebula, meteorites, the origin of Earth, and the C-O-S-Ca cycles of Earth. Until now, oldhamite has not been reported to occur in mantle rock. However, here we show the formation of oldhamite through the reaction between sulfide-bearing orthopyroxenite and molten CaCO3 at 1.5 GPa/1510 K, 0.5 GPa/1320 K, and 0.3 GPa/1273 K. Importantly, this reaction occurs at oxygen fugacities within the range of upper-mantle conditions, six orders of magnitude higher than that of the solar nebula mechanism. Oldhamite is easily oxidized to CaSO4 or hydrolysed to produce calcium hydroxide. Low oxygen fugacity of magma, extremely low oxygen content of the atmosphere, and the lack of a large amount of liquid water on the celestial body's surface are necessary for the widespread existence of oldhamite on the surface of a celestial body otherwise, anhydrite or gypsum will exist in large quantities. Oldhamites may exist in the upper mantle beneath mid-ocean ridges. Additionally, oldhamites may have been a contributing factor to the early Earth's atmospheric hypoxia environment, and the transient existence of oldhamites during the interaction between reducing sulfur-bearing magma and carbonate could have had an impact on the changes in atmospheric composition during the Permian-Triassic Boundary.

Silica-water superstructure and one-dimensional superionic conduit in Earth's mantle

Water in Earth's deep interior is predicted to be hydroxyl (OH-) stored in nominally anhydrous minerals, profoundly modulating both structure and dynamics of Earth's mantle. Here, we use a high-dimensional neuro-network potential and machine learning algorithm to investigate the weight percent water incorporation in stishovite, a main constituent of the subducted oceanic crust. We found that stishovite and water prefer forming medium- to long-range ordered superstructures, featuring one-dimensional (1D) water channels. Synthesizing single crystals of hydrous stishovite, we verified the ordering of OH- groups in the water channels through optical and nuclear magnetic resonance spectroscopy and found an average H-H distance of 2.05(3) Å, confirming simulation results. Upon heating, H atoms were predicted to behave fluid-like inside the channels, leading to an exotic 1D superionic state. Water-bearing stishovite could feature high ionic mobility and strong electrical anisotropy, manifesting as electrical heterogeneity in Earth's mantle.

The stability of FeHx and hydrogen transport at Earth's core mantle boundary

Iron hydride in Earth's interior can be formed by the reaction between hydrous minerals (water) and iron. Studying iron hydride improves our understanding of hydrogen transportation in Earth's interior. Our high-pressure experiments found that face-centered cubic (fcc) FeH_x (x ≤ 1) is stable up to 165 GPa, and our ab initio molecular dynamics simulations predicted that fcc FeH_x transforms to a superionic state under lower mantle conditions. In the superionic state, H-ions in fcc FeH become highly diffusive-like fluids with a high diffusion coefficient of ∼3.7 × 10^-4 cm² s^-1, which is comparable to that in the liquid Fe-H phase. The densities and melting temperatures of fcc FeH_x were systematically calculated. Similar to superionic ice, the extra entropy of diffusive H-ions increases the melting temperature of fcc FeH. The wide stability field of fcc FeH enables hydrogen transport into the outer core to create a potential hydrogen reservoir in Earth's interior, leaving oxygen-rich patches (ORP) above the core mantle boundary (CMB).

Oxygen-Driven Enhancement of the Electron Correlation in Hexagonal Iron at Earth's Inner Core Conditions

Earth's inner core (IC) consists of mainly iron with some light elements. Understanding its structure and related physical properties has been elusive as a result of its required extremely high pressure and temperature conditions. The phase of iron, elastic anisotropy, and density-velocity deficit at the IC have long been questions of great interest. Here, we find that the electron correlation effect is enhanced by oxygen and modifies several important features, including the stability of iron oxides. Oxygen atoms energetically stabilize hexagonal-structured iron at IC conditions and induce elastic anisotropy. Electrical resistivity is much enhanced in comparison to pure hexagonal close-packed (hcp) iron as a result of the enhanced electron correlation effect, supporting the conventional thermal convection model. Moreover, our calculated seismic velocity shows a quantitative match with geologically observed preliminary reference Earth model (PREM) data. We suggest that oxygen is the essential light element to understand and model Earth's IC.

Behavior of Au Nanoparticles under Pressure Observed by In Situ Small-Angle X-ray Scattering

The mechanical properties and stability of metal nanoparticle colloids under high-pressure conditions are investigated by means of optical extinction spectroscopy and small-angle X-ray scattering (SAXS), for colloidal dispersions of gold nanorods and gold nanospheres. SAXS allows us to follow in situ the structural evolution of the nanoparticles induced by pressure, regarding both nanoparticle size and shape (form factor) and their aggregation through the interparticle correlation function S(q) (structure factor). The observed behavior changes under hydrostatic and nonhydrostatic conditions are discussed in terms of liquid solidification processes yielding nanoparticle aggregation. We show that pressure-induced diffusion and aggregation of gold nanorods take place after solidification of the solvent. The effect of nanoparticle shape on the aggregation process is additionally discussed.

Theoretical methods for structural phase transitions in elemental solids at extreme conditions: statics and dynamics

In recent years, theoretical studies have moved from a traditionally supporting role to a more proactive role in the research of phase transitions at high pressures. In many cases, theoretical prediction leads the experimental exploration. This is largely owing to the rapid progress of computer power and theoretical methods, particularly the structure prediction methods tailored for high-pressure applications. This review introduces commonly used structure searching techniques based on static and dynamic approaches, their applicability in studying phase transitions at high pressure, and new developments made toward predicting complex crystalline phases. Successful landmark studies for each method are discussed, with an emphasis on elemental solids and their behaviors under high pressure. The review concludes with a perspective on outstanding challenges and opportunities in the field.

Quantification of Small-Scale Heterogeneity at the Core–Mantle Boundary Using Sample Entropy of SKS and SPdKS Synthetic Waveforms

Qualitative and quantitative analysis of seismic waveforms sensitive to the core–mantle boundary (CMB) region reveal the presence of ultralow-velocity zones (ULVZs) that have a strong decrease in compressional (P) and shear (S) wave velocity, and an increase in density within thin structures. However, understanding their physical origin and relation to the other large-scale structures in the lowermost mantle are limited due to an incomplete mapping of ULVZs at the CMB. The SKS and SPdKS seismic waveforms is routinely used to infer ULVZ presence, but has thus far only been used in a limited epicentral distance range. As the SKS/SPdKS wavefield interacts with a ULVZ it generates additional seismic arrivals, thus increasing the complexity of the recorded wavefield. Here, we explore utilization of the multi-scale sample entropy method to search for ULVZ structures. We investigate the feasibility of this approach through analysis of synthetic seismograms computed for PREM, 1-, 2.5-, and 3-D ULVZs as well as heterogeneous structures with a strong increase in velocity in the lowermost mantle in 1- and 2.5-D. We find that the sample entropy technique may be useful across a wide range of epicentral distances from 100° to 130°. Such an analysis, when applied to real waveforms, could provide coverage of roughly 85% by surface area of the CMB.

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.

Nanocrystalline Cubic Silicon Carbide: A Route to Superhardness

Superhard materials other than diamond and cubic boron nitride have been actively pursued in the past two decades. Cubic silicon carbide, i.e., β-SiC, is a well-known hard material with typical hardness <30 GPa. Although nanostructuring has been proven to be effective in enhancing materials' hardness by virtue of the Hall-Petch effect, it remains a significant challenge to improve hardness of β-SiC beyond the superhard threshold of 40 GPa. Here, the fabrication of nanocrystalline β-SiC bulks is reported by sintering nanoparticles under high pressure and high temperature. These β-SiC bulks are densely sintered with average grain sizes down to 10 nm depending on the sintering conditions, and the Vickers hardness increases with decreasing grain size following the Hall-Petch relation. Particularly, the bulk sintered under 25 GPa and 1400 °C shows an average grain size of 10 nm and an asymptotic Vickers hardness of 41.5 GPa. Boosting the hardness of β-SiC over the superhard threshold signifies an important progress in superhard materials research. A broader family of superhard materials is in sight through successful implementation of nanostructuring in other hard materials such as BP.

Importance of the many-body effects on the structural properties of the novel iron oxide Fe2O

The importance of many-body effects on the electronic and magnetic properties and stability of different structural phases was studied in novel iron oxide Fe₂O. It was found that while Hubbard repulsion hardly affects the electronic spectrum of this material (m*/m ≈ 1.2), it strongly changes its phase diagram, shifting critical pressures of structural transitions to much lower values. Moreover, the P3̄m1 structure previously obtained in the density functional theory (DFT) becomes energetically unstable if many-body effects are taken into consideration. It is shown that these changes are due to magnetic moment fluctuations in the DFT+DMFT (method which combines density functional theory and dynamical mean-field theory) approach, which strongly modify the phase diagram of Fe₂O.

Stabilization of S3O4 at high pressure: implications for the sulfur-excess paradox

The amount of sulfur in SO₂ discharged in volcanic eruptions exceeds that available for degassing from the erupted magma. This geological conundrum, known as the "sulfur excess", has been the subject of considerable interests but remains an open question. Here, in a systematic computational investigation of sulfur-oxygen compounds under pressure, a hitherto unknown S₃O₄ compound containing a mixture of sulfur oxidation states +II and +IV is predicted to be stable at pressures above 79 GPa. We speculate that S₃O₄ may be produced via redox reactions involving subducted S-bearing minerals (e.g., sulfates and sulfides) with iron and goethite under high-pressure conditions of the deep lower mantle, decomposing to SO₂ and S at shallow depths. S₃O₄ may thus be a key intermediate in promoting decomposition of sulfates to release SO₂, offering an alternative source of excess sulfur released during explosive eruptions. These findings provide a possible resolution of the "excess sulfur degassing" paradox and a viable mechanism for the exchange of S between Earth's surface and the lower mantle in the deep sulfur cycle.

Slab control on the mega-sized North Pacific ultra-low velocity zone

Ultra-low velocity zones (ULVZs) are localized small-scale patches with extreme physical properties at the core-mantle boundary that often gather at the margins of Large Low Velocity Provinces (LLVPs). Recent studies have discovered several mega-sized ULVZs with a lateral dimension of ~900 km. However, the detailed structures and physical properties of these ULVZs and their relationship to LLVP edges are not well constrained and their formation mechanisms are poorly understood. Here, we break the degeneracy between the size and velocity perturbation of a ULVZ using two orthogonal seismic ray paths, and thereby discover a mega-sized ULVZ at the northern edge of the Pacific LLVP. The ULVZ is almost double the size of a previously imaged ULVZ in this region, but with half of the shear velocity reduction. This mega-sized ULVZ has accumulated due to stable mantle flow converging at the LLVP edge driven by slab-debris in the lower mantle. Such flow also develops the subvertical north-tilting edge of the Pacific LLVP.

Ultrahigh-Pressure Magnesium Hydrosilicates as Reservoirs of Water in Early Earth

The origin of water on the Earth is a long-standing mystery, requiring a comprehensive search for hydrous compounds, stable at conditions of the deep Earth and made of Earth-abundant elements. Previous studies usually focused on the current range of pressure-temperature conditions in the Earth's mantle and ignored a possible difference in the past, such as the stage of the core-mantle separation. Here, using ab initio evolutionary structure prediction, we find that only two magnesium hydrosilicate phases are stable at megabar pressures, α-Mg_{2}SiO_{5}H_{2} and β-Mg_{2}SiO_{5}H_{2}, stable at 262-338 GPa and >338  GPa, respectively (all these pressures now lie within the Earth's iron core). Both are superionic conductors with quasi-one-dimensional proton diffusion at relevant conditions. In the first 30 million years of Earth's history, before the Earth's core was formed, these must have existed in the Earth, hosting much of Earth's water. As dense iron alloys segregated to form the Earth's core, Mg_{2}SiO_{5}H_{2} phases decomposed and released water. Thus, now-extinct Mg_{2}SiO_{5}H_{2} phases have likely contributed in a major way to the evolution of our planet.

Au with sp3 Hybridization in Li5AuP2

The achievement of new bonding patterns of atoms in compounds is of great importance, which usually induces interesting physical and chemical properties. Rich oxidation states, diverse bonding types, and unique aurophilic attraction endow gold (Au) as a distinctive element. Here we report that a pressure-induced Li₅AuP₂, identified by a swarm intelligence-based structural prediction, becomes the first example of Au with sp³ hybridization. The most remarkable feature of Li₅AuP₂ is that it contains various frameworks made by AuP₄, AuLi₄, LiP₄, and blende-like Li-P units, exhibiting noncentrosymmetry. The charge transfer from Li to Au makes Au 6p orbitals activate and hybridize with the 6s one. On the other hand, Li donating electrons to P and polar Au-P covalence make the constituent atoms satisfy the octet rule, rendering Li₅AuP₂ with a semiconducting character and a large second-order nonlinear optical response in the near-infrared region. Our work represents a significant step toward extending the understanding of gold chemistry.

Structural Diversity of Magnetite and Products of Its Decomposition at Extreme Conditions

Magnetite, Fe₃O₄, is the oldest known magnetic mineral and archetypal mixed-valence oxide. Despite its recognized role in deep Earth processes, the behavior of magnetite at extreme high-pressure high-temperature (HPHT) conditions remains insufficiently studied. Here, we report on single-crystal synchrotron X-ray diffraction experiments up to ∼80 GPa and 5000 K in diamond anvil cells, which reveal two previously unknown Fe₃O₄ polymorphs, γ-Fe₃O₄ with the orthorhombic Yb₃S₄-type structure and δ-Fe₃O₄ with the modified Th₃P₄-type structure. The latter has never been predicted for iron compounds. The decomposition of Fe₃O₄ at HPHT conditions was found to result in the formation of exotic phases, Fe₅O₇ and Fe₂₅O₃₂, with complex structures. Crystal-chemical analysis of iron oxides suggests the high-spin to low-spin crossover in octahedrally coordinated Fe³⁺ in the pressure interval between 43 and 51 GPa. Our experiments demonstrate that HPHT conditions promote the formation of ferric-rich Fe-O compounds, thus arguing for the possible involvement of magnetite in the deep oxygen cycle.

Unconventional Stoichiometries of Na-O Compounds at High Pressures

It has been realized that the stoichiometries of compounds may change under high pressure, which is crucial in the discovery of novel materials. This work uses systematic structure exploration and first-principles calculations to consider the stability of different stoichiometries of Na-O compounds with respect to pressure and, thus, construct a high-pressure stability field and convex hull diagram. Four previously unknown stoichiometries (NaO5, NaO4, Na4O, and Na3O) are predicted to be thermodynamically stable. Four new phases (P2/m and Cmc21 NaO2 and Immm and C2/m NaO3) of known stoichiometries are also found. The O-rich stoichiometries show the remarkable features of all the O atoms existing as quasimolecular O2 units and being metallic. Calculations of the O-O bond lengths and Bader charges are used to explore the electronic properties and chemical bonding of the O-rich compounds. The Na-rich compounds stabilized at extreme pressures (P > 200 GPa) are electrides with strong interstitial electron localization. The C2/c phase of Na3O is found to be a zero-dimensional electride with an insulating character. The Cmca phase of Na4O is a one-dimensional metallic electride. These findings of new compounds with unusual chemistry might stimulate future experimental and theoretical investigations.

Optical and Electrical Transport Evaluations of n-Type Iron Pyrite Single Crystals

Iron pyrite [cubic FeS₂ (cFeS₂)] is considered as an earth-abundant and low-cost thin-film photovoltaic material. However, the conversion efficiency of cFeS₂-based solar cells remains below 3%. To elucidate this limitation, we evaluate the optical and electrical characteristics of cFeS₂ single crystals that are grown using the flux method, thus providing us an understanding of the electron transport behavior of cFeS₂ single crystals. The oxide layer on the surface of cFeS₂, which can possibly have an influence on the electrical characteristics of cFeS₂, is removed prior to characterization via optical spectroscopy and electrical transport measurement. The optical property of cFeS₂ was found to have both indirect and direct transitions. We also observed the presence of a band tail below the conduction band. The obtained electrical transport behavior indicates that cFeS₂ bulk exhibits a high defect density and a disordered phase, thus leading to the hopping conduction mechanism. Our results will pave the way for the development of photovoltaic applications with iron pyrite.

Structural Stability and Properties of Marokite-Type γ-Mn3O4

We synthesized single crystals of marokite (CaMn₂O₄)-type orthorhombic manganese (II,III) oxide, γ-Mn₃O₄, in a multianvil apparatus at pressures of 10-24 GPa. The magnetic, electronic, and optical properties of the crystals were investigated at ambient pressure. It was found that γ-Mn₃O₄ is a semiconductor with an indirect band gap E_g of 0.96 eV and two antiferromagnetic transitions (T_N) at ∼200 and ∼55 K. The phase stability of the γ-Mn₃O₄ crystals was examined in the pressure range of 0-60 GPa using single-crystal X-ray diffraction and Raman spectroscopy. A bulk modulus of γ-Mn₃O₄ was determined to be B₀ = 235.3(2) GPa with B' = 2.6(6). The γ-Mn₃O₄ phase persisted over the whole pressure range studied and did not transform or decompose upon laser heating of the sample to ∼3500 K at 60 GPa. This result seems surprising, given the high-pressure structural diversity of iron oxides with similar stoichiometries. With an increase in pressure, the degree of distortion of MnO₆ polyhedra decreased. Furthermore, there are signs indicating a limited charge transfer between the Mn³⁺ ions in the octahedra and the Mn²⁺ ions in the trigonal prisms. Our results demonstrate that the high-pressure behavior of the structural, electronic, and chemical properties of manganese oxides strongly differs from that of iron oxides with similar stoichiometries.

Direct H-He chemical association in superionic FeO2H2He at Deep-Earth conditions

Hydrogen and helium are known to play crucial roles in geological and astrophysical environments; however, they are inert toward each other across wide pressure-temperature (P-T) conditions. Given their prominent presence and influence on the formation and evolution of celestial bodies, it is of fundamental interest to explore the nature of interactions between hydrogen and helium. Using an advanced crystal structure search method, we have identified a quaternary compound FeO₂H₂He stabilized in a wide range of P-T conditions. Ab initio molecular dynamics simulations further reveal a novel superionic state of FeO₂H₂He hosting liquid-like diffusive hydrogen in the FeO₂He sublattice, creating a conducive environment for H-He chemical association, at P-T conditions corresponding to the Earth's lowest mantle regions. To our surprise, this chemically facilitated coalescence of otherwise immiscible molecular species highlights a promising avenue for exploring this long-sought but hitherto unattainable state of matter. This finding raises strong prospects for exotic H-He mixtures inside Earth and possibly also in other astronomical bodies.

Structural, electronic and magnetic properties of TlFeSe2under high pressure

The high-pressure (HP) properties of TlFeSe₂are investigated based on the first-principles calculations combined with structure-searching method. The low-pressureC2/mphase will transform into the orthorhombicPnmaphase at 2 GPa, with 8% volume collapse, the insulator-metal transition and the bicollinear antiferromagnetic-to-nonmagnetic spin-crossover. At pressure higher than 8 GPa, the HPC2/mphase will become the ground state. BothPnmaphase and HPC2/mphase are constituted by one-dimensional chains of edge-sharing FeSe₅tetragonal pyramids. Pressuring decrease the Se-Se bond length giving rise to the transition from [Se₂]^3-to [Se₂]^2-. Negative charge transfer causes the Fe²⁺with ∼2 μ_Bmagnetic moment at ambient pressure and the nonmagnetic Fe^1.5+at higher pressure. The Fermi surfaces of HP phases are also discussed.

57Fe Mössbauer isomer shift of pure iron and iron oxides at high pressure-An experimental and theoretical study

The ⁵⁷Fe isomer shift (IS) of pure iron has been measured up to 100 GPa using synchrotron Mössbauer spectroscopy in the time domain. Apart from the expected discontinuity due to the α → ε structural and spin transitions, the IS decreases monotonically with increasing pressure. The absolute shifts were reproduced without semi-empirical calibrations by periodic density functional calculations employing extensive localized basis sets with several common density functionals. However, the best numerical agreement is obtained with the B1WC hybrid functional. Extension of the calculations to 350 GPa, a pressure corresponding to the Earth's inner core, predicted the IS range of 0.00 to -0.85 mm/s, covering the span from Fe(0) to Fe(VI) compounds measured at ambient pressure. The calculations also reproduced the pressure trend from polymorphs of prototypical iron oxide minerals, FeO and Fe₂O₃. Analysis of the electronic structure shows a strong donation of electrons from oxygen to iron at high pressure. The assignment of formal oxidation to the Fe atom becomes ambiguous under this condition.

Morphology-Tunable Synthesis of Intrinsic Room-Temperature Ferromagnetic γ-Fe2O3 Nanoflakes

Intrinsic two-dimensional (2D) magnetic materials with room-temperature ferromagnetism and air stability are highly desirable for spintronic applications. However, the experimental observations of such 2D or ultrathin ferromagnetic materials are rarely reported owing to the scarcity of these materials in nature and for the intricacy in their synthesis. Here, we report a successful controllable growth of ultrathin γ-Fe₂O₃ nanoflakes with a variety of morphologies tunable by the growth temperature alone using a facile chemical vapor deposition method and demonstrate that all ultrathin nanoflakes still show intrinsic room-temperature ferromagnetism and a semiconducting nature. The γ-Fe₂O₃ nanoflakes epitaxially grown on α-Al₂O₃ substrates take a triangular shape at low temperature and develop gradually in lateral size, forming eventually a large-scale γ-Fe₂O₃ thin film as the growth time increases due to a thermodynamic control process. The morphology of the nanoflakes could be tuned from triangular to stellated, petaloid, and dendritic crystalloids in sequence with the rise of precursor temperature, revealing a growth process from thermodynamically to kinetically dominated control. Moreover, the petaloid and dendritic nanoflakes exhibit enhanced coercivity compared with the triangular and stellated nanoflakes, and all the nanoflakes with diverse shapes possess differing electrical conductivity. The findings of such ultrathin, air-stable, and room-temperature ferromagnetic γ-Fe₂O₃ nanoflakes with tunable shape and multifunctionality may offer guidance in synthesizing other non-layered magnetic materials for next-generation electronic and spintronic devices.

Ultrasensitive Chemodynamic Therapy: Bimetallic Peroxide Triggers High pH-Activated, Synergistic Effect/H2 O2 Self-Supply-Mediated Cascade Fenton Chemistry

Recently, nanoparticle-triggered in situ catalytic Fenton/Fenton-like reaction is widely explored for tumor-specific chemodynamic therapy (CDT). However, despite the great potential of CDT in tumor treatment, insensitive response to the relatively high pH of the tumor sites and the insufficient intratumoral H₂ O₂ level leads to limited efficiency of most Fenton/Fenton-like reactions, which greatly imped its clinical conversion. This paper reports the fabrication of Fenton-type bimetallic peroxides for ultrasensitive chemodynamic therapy with high pH-activated, synergistic effect/H₂ O₂ self-supply-mediated cascade Fenton chemistry for the first time. The observations reveal that these bimetallic peroxides exhibit an ultrasensitive acid-activated decomposition-mediated Fenton-like reaction at the relatively high pH of 6.5-7.0, accompanied with highly increased •OH generation efficiency (especially, 40-60-fold increase at pH 7.0) by the metal-mediated synergistic effect-enhanced Fenton chemistry as well as in situ self-generated H₂ O₂ supplement. Moreover, the bimetallic peroxides exhibit high tumor accumulation which along with a high-efficiency tumor catalytic-therapeutic with negligible side effects in vivo. Developing these novel bimetallic peroxides, together with the already demonstrated capacity of the key metals (Fe, Mn, Cu, etc.) for magnetic resonance imaging or photodynamic/immune-enhanced therapy, will propel interest in development of smart high-efficiency nanoplatform for cancer theranostics.

Mineralogy of the deep lower mantle in the presence of H2O

Understanding the mineralogy of the Earth's interior is a prerequisite for unravelling the evolution and dynamics of our planet. Here, we conducted high pressure-temperature experiments mimicking the conditions of the deep lower mantle (DLM, 1800-2890 km in depth) and observed surprising mineralogical transformations in the presence of water. Ferropericlase, (Mg, Fe)O, which is the most abundant oxide mineral in Earth, reacts with H2O to form a previously unknown (Mg, Fe)O2H x (x ≤ 1) phase. The (Mg, Fe)O2H x has a pyrite structure and it coexists with the dominant silicate phases, bridgmanite and post-perovskite. Depending on Mg content and geotherm temperatures, the transformation may occur at 1800 km for (Mg0.6Fe0.4)O or beyond 2300 km for (Mg0.7Fe0.3)O. The (Mg, Fe)O2H x is an oxygen excess phase that stores an excessive amount of oxygen beyond the charge balance of maximum cation valences (Mg2+, Fe3+ and H+). This important phase has a number of far-reaching implications including extreme redox inhomogeneity, deep-oxygen reservoirs in the DLM and an internal source for modulating oxygen in the atmosphere.

Evidence for oxygenation of Fe-Mg oxides at mid-mantle conditions and the rise of deep oxygen

As the reaction product of subducted water and the iron core, FeO2 with more oxygen than hematite (Fe2O3) has been recently recognized as an important component in the D" layer just above the Earth's core-mantle boundary. Here, we report a new oxygen-excess phase (Mg, Fe)2O3+ δ (0 < δ < 1, denoted as 'OE-phase'). It forms at pressures greater than 40 gigapascal when (Mg, Fe)-bearing hydrous materials are heated over 1500 kelvin. The OE-phase is fully recoverable to ambient conditions for ex situ investigation using transmission electron microscopy, which indicates that the OE-phase contains ferric iron (Fe3+) as in Fe2O3 but holds excess oxygen through interactions between oxygen atoms. The new OE-phase provides strong evidence that H2O has extraordinary oxidation power at high pressure. Unlike the formation of pyrite-type FeO2Hx which usually requires saturated water, the OE-phase can be formed with under-saturated water at mid-mantle conditions, and is expected to be more ubiquitous at depths greater than 1000 km in the Earth's mantle. The emergence of oxygen-excess reservoirs out of primordial or subducted (Mg, Fe)-bearing hydrous materials may revise our view on the deep-mantle redox chemistry.

Revealing the Complex Nature of Bonding in the Binary High-Pressure Compound FeO_{2}

Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides, resulting in numerous compounds forming homologous series nFeOmFe_{2}O_{3} and the appearance of FeO_{2}. Here, based on the results of in situ single-crystal x-ray diffraction, Mössbauer spectroscopy, x-ray absorption spectroscopy, and density-functional theory+dynamical mean-field theory calculations, we demonstrate that iron in high-pressure cubic FeO_{2} and isostructural FeO_{2}H_{0.5} is ferric (Fe^{3+}), and oxygen has a formal valence less than 2. Reduction of oxygen valence from 2, common for oxides, down to 1.5 can be explained by a formation of a localized hole at oxygen sites.

Synthetic ferripyrophyllite: preparation, characterization and catalytic application

Sheet silicates, also known as phyllosilicates, contain parallel sheets of tetrahedral silicate built up by [Si₂O₅]^2- entities connected through intermediate metal-oxygen octahedral layers. The well-known minerals talc and pyrophyllite are belonging to this group based on magnesium and aluminium, respectively. Surprisingly, the ferric analogue rarely occurs in nature and is found in mixtures and conglomerates with other materials only. While partial incorporation of iron into pyrophyllites has been achieved, no synthetic protocol for purely iron-based pyrophyllite has been published yet. Here we report about the first artificial synthesis of ferripyrophyllite under exceptional mild conditions. A similar ultrathin two-dimensional (2D) nanosheet morphology is obtained as in talc or pyrophyllite but with iron(iii) as a central metal. The high surface material exhibits a remarkably high thermostability. It shows some catalytic activity in ammonia synthesis and can serve as catalyst support material for noble metal nanoparticles.

Xenon iron oxides predicted as potential Xe hosts in Earth's lower mantle

An enduring geological mystery concerns the missing xenon problem, referring to the abnormally low concentration of xenon compared to other noble gases in Earth's atmosphere. Identifying mantle minerals that can capture and stabilize xenon has been a great challenge in materials physics and xenon chemistry. Here, using an advanced crystal structure search algorithm in conjunction with first-principles calculations we find reactions of xenon with recently discovered iron peroxide FeO2, forming robust xenon-iron oxides Xe2FeO2 and XeFe3O6 with significant Xe-O bonding in a wide range of pressure-temperature conditions corresponding to vast regions in Earth's lower mantle. Calculated mass density and sound velocities validate Xe-Fe oxides as viable lower-mantle constituents. Meanwhile, Fe oxides do not react with Kr, Ar and Ne. It means that if Xe exists in the lower mantle at the same pressures as FeO2, xenon-iron oxides are predicted as potential Xe hosts in Earth's lower mantle and could provide the repository for the atmosphere's missing Xe. These findings establish robust materials basis, formation mechanism, and geological viability of these Xe-Fe oxides, which advance fundamental knowledge for understanding xenon chemistry and physics mechanisms for the possible deep-Earth Xe reservoir.

Solid state chemistry for developing better metal-ion batteries

Metal-ion batteries are key enablers in today’s transition from fossil fuels to renewable energy for a better planet with ingeniously designed materials being the technology driver. A central question remains how to wisely manipulate atoms to build attractive structural frameworks of better electrodes and electrolytes for the next generation of batteries. This review explains the underlying chemical principles and discusses progresses made in the rational design of electrodes/solid electrolytes by thoroughly exploiting the interplay between composition, crystal structure and electrochemical properties. We highlight the crucial role of advanced diffraction, imaging and spectroscopic characterization techniques coupled with solid state chemistry approaches for improving functionality of battery materials opening emergent directions for further studies.

The development of high performing metal-ion batteries require guidelines to build improved electrodes and electrolytes. Here, the authors review the current state-of-the-art in the rational design of battery materials by exploiting the interplay between composition, crystal structure and electrochemical properties.

Pressure-stabilized divalent ozonide CaO3 and its impact on Earth's oxygen cycles

High pressure can drastically alter chemical bonding and produce exotic compounds that defy conventional wisdom. Especially significant are compounds pertaining to oxygen cycles inside Earth, which hold key to understanding major geological events that impact the environment essential to life on Earth. Here we report the discovery of pressure-stabilized divalent ozonide CaO3 crystal that exhibits intriguing bonding and oxidation states with profound geological implications. Our computational study identifies a crystalline phase of CaO3 by reaction of CaO and O2 at high pressure and high temperature conditions; ensuing experiments synthesize this rare compound under compression in a diamond anvil cell with laser heating. High-pressure x-ray diffraction data show that CaO3 crystal forms at 35 GPa and persists down to 20 GPa on decompression. Analysis of charge states reveals a formal oxidation state of -2 for ozone anions in CaO3. These findings unravel the ozonide chemistry at high pressure and offer insights for elucidating prominent seismic anomalies and oxygen cycles in Earth's interior. We further predict multiple reactions producing CaO3 by geologically abundant mineral precursors at various depths in Earth's mantle.

Thinking about the Development of High-Pressure Experimental Chemistry

High-pressure chemistry is an interdisciplinary science which uses high-pressure experiments and theories to study the interactions, reactions, and transformations among atoms or molecules. It has been extensively studied thus far and achieved rapid development over the past decades. However, what is next for high-pressure chemistry? In this Perspective, we mainly focus on the development of high-pressure experimental chemistry from our own viewpoint. An overview of the series of topics is as follows: (I) high pressure used as an effective tool to help resolve scientific disputes regarding phenomena observed under ambient conditions; (II) high-pressure reactions of interest to synthetic chemists; (III) utilizing chemical methods to quench the high-pressure phase; (IV) using high pressure to achieve what chemists want to do but could not do; (V) potential applications of in situ properties under high pressure. This Perspective is expected to offer future research opportunities for researchers to develop high-pressure chemistry and to inspire new endeavors in this area to promote the field of compression chemistry science.

Piezoelectric Materials for Controlling Electro-Chemical Processes

Piezoelectric materials have been analyzed for over 100 years, due to their ability to convert mechanical vibrations into electric charge or electric fields into a mechanical strain for sensor, energy harvesting, and actuator applications. A more recent development is the coupling of piezoelectricity and electro-chemistry, termed piezo-electro-chemistry, whereby the piezoelectrically induced electric charge or voltage under a mechanical stress can influence electro-chemical reactions. There is growing interest in such coupled systems, with a corresponding growth in the number of associated publications and patents. This review focuses on recent development of the piezo-electro-chemical coupling multiple systems based on various piezoelectric materials. It provides an overview of the basic characteristics of piezoelectric materials and comparison of operating conditions and their overall electro-chemical performance. The reported piezo-electro-chemical mechanisms are examined in detail. Comparisons are made between the ranges of material morphologies employed, and typical operating conditions are discussed. In addition, potential future directions and applications for the development of piezo-electro-chemical hybrid systems are described. This review provides a comprehensive overview of recent studies on how piezoelectric materials and devices have been applied to control electro-chemical processes, with an aim to inspire and direct future efforts in this emerging research field.

Unconventional stable stoichiometry of vanadium peroxide

Peroxides have attracted considerable attention due to their intriguing electronic properties and diverse applications. However, only a few transition metal peroxides have been known thus far, limiting the variety of peroxide examples. Here, we demonstrate the stabilization of peroxides in the O-rich V-O system through first-principles calculations coupled with a swarm-intelligence structure search. As well as reproducing the known stoichiometries of VO, V₂O₃, VO₂, and V₂O₅, two hitherto unknown V₂O and VO₄ stoichiometries are predicted to be thermodynamically stable at megabar pressures. VO₄ has the highest oxygen content among the known peroxides to date. More interestingly, its electronic band gap increases with pressure, originating from the pressure-induced decrease of O-O bonding length in the peroxide group. V-rich V₂O exhibits superconductivity, becoming the first example in the V-O system. Our work not only unravels the unusual vanadium peroxide, but also provides further insight into the diverse electronic properties of vanadium oxides under high pressure.

Influence of Molecular Orbitals on Magnetic Properties of [x]

Recent discoveries of various novel iron oxides and hydrides, which become stable at very high pressure and temperature, are extremely important for geoscience. In this paper, we report the results of an investigation on the electronic structure and magnetic properties of the hydride FeO 2 H x , using density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations. An increase in the hydrogen concentration resulted in the destruction of dimeric oxygen pairs and, hence, a specific band structure of FeO 2 with strongly hybridized Fe- t 2 g -O- p z anti-bonding molecular orbitals, which led to a metallic state with the Fe ions at nearly 3+. Increasing the H concentration resulted in effective mass enhancement growth which indicated an increase in the magnetic moment localization. The calculated static momentum-resolved spin susceptibility demonstrated that an incommensurate antiferromagnetic (AFM) order was expected for FeO 2 , whereas strong ferromagnetic (FM) fluctuations were observed for FeO 2 H.

Large H2O solubility in dense silica and its implications for the interiors of water-rich planets

Sub-Neptunes are common among the discovered exoplanets. However, lack of knowledge on the state of matter in [Formula: see text]O-rich setting at high pressures and temperatures ([Formula: see text]) places important limitations on our understanding of this planet type. We have conducted experiments for reactions between [Formula: see text] and [Formula: see text]O as archetypal materials for rock and ice, respectively, at high [Formula: see text] We found anomalously expanded volumes of dense silica (up to 4%) recovered from hydrothermal synthesis above ∼24 GPa where the [Formula: see text]-type (Ct) structure appears at lower pressures than in the anhydrous system. Infrared spectroscopy identified strong OH modes from the dense silica samples. Both previous experiments and our density functional theory calculations support up to 0.48 hydrogen atoms per formula unit of ([Formula: see text])[Formula: see text] At pressures above 60 GPa, [Formula: see text]O further changes the structural behavior of silica, stabilizing a niccolite-type structure, which is unquenchable. From unit-cell volume and phase equilibrium considerations, we infer that the niccolite-type phase may contain H with an amount at least comparable with or higher than that of the Ct phase. Our results suggest that the phases containing both hydrogen and lithophile elements could be the dominant materials in the interiors of water-rich planets. Even for fully layered cases, the large mutual solubility could make the boundary between rock and ice layers fuzzy. Therefore, the physical properties of the new phases that we report here would be important for understanding dynamics, geochemical cycle, and dynamo generation in water-rich planets.

An Fe-V@NiO heterostructure electrocatalyst towards the oxygen evolution reaction

The development of a nonprecious and Earth-abundant electrocatalyst with high electrocatalytic activity for the oxygen evolution reaction (OER) is an emerging hot issue and remains a grand challenge. In the present work, we proposed a facile strategy to construct ultrathin NiO nanosheets decorated with Fe-V nanoparticles on nickel foam (Fe-V@NiO/NF) for use as an OER electrocatalyst. Due to the 3D rational configuration, the Fe-V@NiO/NF with a heterostructure shows excellent electrocatalytic activity towards the OER. Interestingly, it is found that in situ oxidation by galvanostatic electrolysis in alkaline solution is beneficial to enhance the OER performance. After 10 h of electrolysis, a current density of 50 mA cm^-2 is achieved at a low overpotential of 271.1 mV. This is because during the in situ oxidation process, iron and vanadium ions insert into the NiO lattice and lead to the generation of highly active α-FeOOH and an amorphous (oxy)-hydroxide layer. Additionally, the charge transfer resistance dramatically reduces with the prolonging of oxidation time.

Study on the High-Pressure Behavior of Goethite up to 32 GPa Using X-Ray Diffraction, Raman, and Electrical Impedance Spectroscopy

Goethite is a major iron-bearing sedimentary mineral on Earth. In this study, we conducted in situ high-pressure x-ray diffraction, Raman, and electrical impedance spectroscopy measurements of goethite using a diamond anvil cell (DAC) at room temperature and high pressures up to 32 GPa. We observed feature changes in both the Raman spectra and electrical resistance at about 5 and 11 GPa. However, the x-ray diffraction patterns show no structural phase transition in the entire pressure range of the study. The derived pressure-volume (P-V) data show a smooth compression curve with no clear evidence of any second-order phase transition. Fitting the volumetric data to the second-order Birch–Murnaghan equation of state yields V0 = 138.9 ± 0.5 Å3 and K0 = 126 ± 5 GPa.

Interaction Between FeOOH and NaCl at Extreme Conditions: Synthesis of Novel Na2FeCl4OHx Compound

Iron(III) oxide-hydroxide, FeOOH, is abundant in the banded iron formations (BIFs). Recent studies indicate that BIFs may carry water down to the lower mantle with subducting slabs. The previous experiments investigating the properties of FeOOH at extreme pressures (P) and temperatures (T) were performed in diamond anvil cells (DACs), where it was compressed inside alkali metal halide pressure-transmitting media (2). Alkali metal halides such as NaCl or KCl are expected to be chemically inert; therefore, they are widely used in DAC experiments. Here, we report the chemical interaction between FeOOH and NaCl pressure medium at 107(2) GPa and 2400(200) K. By means of single-crystal X-ray diffraction (SC-XRD) analysis applied to a multigrain sample, we demonstrate the formation of a Na2FeCl4OHx phase and provide its structural solution and refinement. Our results demonstrate that at high P-T conditions, the alkali metal halides could interact with hydrous phases and thus cannot be used as a pressure transmitting and thermal insulating medium in DAC experiments dedicated to studies of hydroxyl or water-bearing materials at high P-T.

The role of water in Earth's mantle

Geophysical observations suggest that the transition zone is wet locally. Continental and oceanic sediment components together with the basaltic and peridotitic components might be transported and accumulated in the transition zone. Low-velocity anomalies at the upper mantle–transition zone boundary might be caused by the existence of dense hydrous magmas. Water can be carried farther into the lower mantle by the slabs. The anomalous Q and shear wave regions locating at the uppermost part of the lower mantle could be caused by the existence of fluid or wet magmas in this region because of the water-solubility contrast between the minerals in the transition zone and those in the lower mantle. δ-H solid solution AlO₂H–MgSiO₄H₂ carries water into the lower mantle. Hydrogen-bond symmetrization exists in high-pressure hydrous phases and thus they are stable at the high pressures of the lower mantle. Thus, the δ-H solid solution in subducting slabs carries water farther into the bottom of the lower mantle. Pyrite FeO₂H_x is formed due to a reaction between the core and hydrated slabs. This phase could be a candidate for the anomalous regions at the core–mantle boundary.

Metallic iron limits silicate hydration in Earth's transition zone

The Earth's mantle transition zone (MTZ) is often considered an internal reservoir for water because its major minerals wadsleyite and ringwoodite can store several oceans of structural water. Whether it is a hydrous layer or an empty reservoir is still under debate. Previous studies suggested the MTZ may be saturated with iron metal. Here we show that metallic iron reacts with hydrous wadsleyite under the pressure and temperature conditions of the MTZ to form iron hydride or molecular hydrogen and silicate with less than tens of parts per million (ppm) water, implying that water enrichment is incompatible with iron saturation in the MTZ. With the current estimate of water flux to the MTZ, the iron metal preserved from early Earth could transform a significant fraction of subducted water into reduced hydrogen species, thus limiting the hydration of silicates in the bulk MTZ. Meanwhile, the MTZ would become gradually oxidized and metal depleted. As a result, water-rich region can still exist near modern active slabs where iron metal was consumed by reaction with subducted water. Heterogeneous water distribution resolves the apparent contradiction between the extreme water enrichment indicated by the occurrence of hydrous ringwoodite and ice VII in superdeep diamonds and the relatively low water content in bulk MTZ silicates inferred from electrical conductivity studies.

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.