Electride Formation of HCP-Iron at High Pressure: Unraveling the Origin of the Superionic State of Iron-Rich Compounds in Rocky Planets

Electride possesses electrons localized at interstitial sites without attracting nuclei. It brings outstanding material properties not only originating from its own loosely bounded characteristics but also serving as a quasiatom, which even chemically interacts with other elemental ions. In elemental metals, electride transitions have been reported in alkali metals where valence electrons can easily gain enough kinetic energy to escape nuclei. However, there are few studies on transition metals. Especially iron, the key element of human technology and geophysics, has not been studied in respect of electride formation. In this study, it is demonstrated that electride formation drives the superionic state in iron hydride under high-pressure conditions of the earth's inner core. The electride stabilizes the iron lattice and provides a pathway for hydrogen diffusion by severing the direct interaction between the metal and the volatile element. The coupling between lattice stability and superionicity is triggered near 100 GPa and enhanced at higher pressures. It is shown that the electride-driven superionicity can also be generalized for metal electrides and other rocky planetary cores by providing a fundamental interaction between the electride of the parent metal and doped light elements.

Structure Responsible for the Superconducting State in La3Ni2O7 at High-Pressure and Low-Temperature Conditions

Very recently, a new superconductor with Tc = 80 K has been reported in nickelate (La3Ni2O7) at around 15-40 GPa conditions (Nature, 621, 493, 2023), which is the second type of unconventional superconductor, besides cuprates, with Tc above liquid nitrogen temperature. However, the phase diagram plotted in this report was mostly based on the transport measurement under low-temperature and high-pressure conditions, and the assumed corresponding X-ray diffraction (XRD) results were carried out at room temperature. This encouraged us to carry out in situ high-pressure and low-temperature synchrotron XRD experiments to determine which phase is responsible for the high Tc state. In addition to the phase transition from the orthorhombic Amam structure to the orthorhombic Fmmm structure, a tetragonal phase with the space group of I4/mmm was discovered when the sample was compressed to around 19 GPa at 40 K where the superconductivity takes place in La3Ni2O7. The calculations based on this tetragonal structure reveal that the electronic states that approached the Fermi energy were mainly dominated by the eg orbitals (3dz2 and 3dx2-y2) of Ni atoms, which are located in the oxygen octahedral crystal field. The correlation between Tc and this structural evolution, especially Ni-O octahedra regularity and the in-plane Ni-O-Ni bonding angles, is analyzed. This work sheds new light to identify what is the most likely phase responsible for superconductivity in double-layered nickelate.

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).

Quantifying the partial ionization effect of gold in the transition region between condensed matter and warm dense matter

It is now well known that solids under ultra-high-pressure shock compression will enter the warm dense matter (WDM) regime which connects condensed matter and hot plasma. How condensed matter turns into the WDM, however, remains largely unexplored due to the lack of data in the transition pressure range. In this letter, by employing the unique high-Z three-stage gas gun launcher technique developed recently, we compress gold into TPa shock pressure to fill the gap inaccessible by the two-stage gas gun and laser shock experiments. With the aid of high-precision Hugoniot data obtained experimentally, we observe a clear softening behavior beyond ~560 GPa. The state-of-the-art ab-initio molecular dynamics calculations reveal that the softening is caused by the ionization of 5d electrons in gold. This work quantifies the partial ionization effect of electrons under extreme conditions, which is critical to model the transition region between condensed matter and WDM.

Piezovoltaics from PdHx

Metal hydrides have wide applications in energy science. A large pressure gradient propels the hydrogen atoms out. A piezovoltaic device, a pressure gradient-driven battery, can therefore be realized when the migrations of protons and electrons are separated by different conductors. Here we investigate the piezovoltaic performance of PdH_x with various proton conductors as electrolytes and experimentally detect an output current of ≲40 nA and a voltage of ∼0.8 V for a 3 μg sample. We also demonstrate the escape of hydrogen atoms from a palladium lattice under an increasing pressure gradient using X-ray diffraction. The relationship between piezovoltaics (chemical process) and piezoelectricity (physical process) is like that between a chemical battery and a capacitor. Our work demonstrates the piezovoltaic application of metal hydrides and provides a new way to convert mechanical energy into electrical energy.

Superionic effect and anisotropic texture in Earth's inner core driven by geomagnetic field

Seismological observations suggest that Earth's inner core (IC) is heterogeneous and anisotropic. Increasing seismological observations make the understanding of the mineralogy and mechanism for the complex IC texture extremely challenging, and the driving force for the anisotropic texture remains unclear. Under IC conditions, hydrogen becomes highly diffusive like liquid in the hexagonal-close-packed (hcp) solid Fe lattice, which is known as the superionic state. Here, we reveal that H-ion diffusion in superionic Fe-H alloy is anisotropic with the lowest barrier energy along the c-axis. In the presence of an external electric field, the alignment of the Fe-H lattice with the c-axis pointing to the field direction is energetically favorable. Due to this effect, Fe-H alloys are aligned with the c-axis parallel to the equatorial plane by the diffusion of the north-south dipole geomagnetic field into the inner core. The aligned texture driven by the geomagnetic field presents significant seismic anisotropy, which explains the anisotropic seismic velocities in the IC, suggesting a strong coupling between the IC structure and geomagnetic field.

Ordered Van der Waals Hetero-nanoribbon from Pressure-Induced Topochemical Polymerization of Azobenzene

Pressure-induced topochemical polymerization of molecular crystals with various stackings is a promising way to synthesize materials with different co-existing sub-structures. Here, by compressing the azobenzene crystal containing two kinds of intermolecular stacking, we synthesized an ordered van der Waals carbon nanoribbon (CNR) heterostructure in one step. Azobenzene polymerizes via a [4 + 2] hetero-Diels-Alder (HDA) reaction of phenylazo-phenyl in layer A and a para-polymerization reaction of phenyl in layer B at 18 GPa, as evidenced by in situ Raman and IR spectroscopies, X-ray diffraction, as well as gas chromatography-mass spectrometry and the solid-state nuclear magnetic resonance of the recovered products. The theoretical calculation shows that the obtained CNR heterostructure has a type II (staggered) band gap alignment. Our work highlights a high-pressure strategy to synthesize bulk CNR heterostructures.

From Biomass to Functional Crystalline Diamond Nanothread: Pressure-Induced Polymerization of 2,5-Furandicarboxylic Acid

2,5-Furandicarboxylic acid (FDCA) is one of the top-12 value-added chemicals from sugar. Besides the wide application in chemical industry, here we found that solid FDCA polymerized to form an atomic-scale ordered sp³-carbon nanothread (CNTh) upon compression. With the help of perfectly aligned π-π stacked molecules and strong intermolecular hydrogen bonds, crystalline poly-FDCA CNTh with uniform syn-configuration was obtained above 11 GPa, with the crystal structure determined by Rietveld refinement of the X-ray diffraction (XRD). The in situ XRD and theoretical simulation results show that the FDCA experienced continuous [4 + 2] Diels-Alder reactions along the stacking direction at the threshold C···C distance of ∼2.8 Å. Benefiting from the abundant carbonyl groups, the poly-FDCA shows a high specific capacity of 375 mAh g^-1 as an anode material of a lithium battery with excellent Coulombic efficiency and rate performance. This is the first time a three-dimensional crystalline CNTh is obtained, and we demonstrated it is the hydrogen bonds that lead to the formation of the crystalline material with a unique configuration. It also provides a new method to move biomass compounds toward advanced functional carbon materials.

Preservation of high-pressure volatiles in nanostructured diamond capsules

High pressure induces dramatic changes and novel phenomena in condensed volatiles^1,2 that are usually not preserved after recovery from pressure vessels. Here we report a process that pressurizes volatiles into nanopores of type 1 glassy carbon precursors, converts glassy carbon into nanocrystalline diamond by heating and synthesizes free-standing nanostructured diamond capsules (NDCs) capable of permanently preserving volatiles at high pressures, even after release back to ambient conditions for various vacuum-based diagnostic probes including electron microscopy. As a demonstration, we perform a comprehensive study of a high-pressure argon sample preserved in NDCs. Synchrotron X-ray diffraction and high-resolution transmission electron microscopy show nanometre-sized argon crystals at around 22.0 gigapascals embedded in nanocrystalline diamond, energy-dispersive X‑ray spectroscopy provides quantitative compositional analysis and electron energy-loss spectroscopy details the chemical bonding nature of high-pressure argon. The preserved pressure of the argon sample inside NDCs can be tuned by controlling NDC synthesis pressure. To test the general applicability of the NDC process, we show that high-pressure neon can also be trapped in NDCs and that type 2 glassy carbon can be used as the precursor container material. Further experiments on other volatiles and carbon allotropes open the possibility of bringing high-pressure explorations on a par with mainstream condensed-matter investigations and applications.

Novel Valence Transition in Elemental Metal Europium around 80 GPa

Valence transition could induce structural, insulator-metal, nonmagnetic-magnetic and superconducting transitions in rare-earth metals and compounds, while the underlying physics remains unclear due to the complex interaction of localized 4f electrons as well as their coupling with itinerant electrons. The valence transition in the elemental metal europium (Eu) still has remained as a matter of debate. Using resonant x-ray emission scattering and x-ray diffraction, we pressurize the states of 4f electrons in Eu and study its valence and structure transitions up to 160 GPa. We provide compelling evidence for a valence transition around 80 GPa, which coincides with a structural transition from a monoclinic (C2/c) to an orthorhombic phase (Pnma). We show that the valence transition occurs when the pressure-dependent energy gap between 4f and 5d electrons approaches the Coulomb interaction. Our discovery is critical for understanding the electrodynamics of Eu, including magnetism and high-pressure superconductivity.

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.

Pressure-Induced Hydrogen Transfer in 2-Butyne via a Double CH···π Aromatic Transition State

Hydrogen transfer (H-transfer) is an important elementary reaction in chemistry and bioscience. It is often facilitated by the hydrogen bonds between the H-donor and acceptor. Here, at room temperature and high pressure, we found that solid 2-butyne experienced a concerted two-in-two-out intermolecular CH···π H-transfer, which initiated the subsequent polymerization. Such double H-transfer goes through an aromatic Hückel six-membered ring intermediate state via intermolecular CH···π interactions enhanced by external pressure. Our work shows that H-transfer can occur via the CH···π route in appropriate conformations under high pressure, which gives important insights into the H-transfer in solid-state hydrocarbons.

Topological Ordering of Memory Glass on Extended Length Scales

Identifying ordering in non-crystalline solids has been a focus of natural science since the publication of Zachariasen's random network theory in 1932, but it still remains as a great challenge of the century. Literature shows that the hierarchical structures, from the short-range order of first-shell polyhedra to the long-range order of translational periodicity, may survive after amorphization. Here, in a piece of AlPO₄, or berlinite, we combine X-ray diffraction and stochastic free-energy surface simulations to study its phase transition and structural ordering under pressure. From reversible single crystals to amorphous transitions, we now present an unambiguous view of the topological ordering in the amorphous phase, consisting of a swarm of Carpenter low-symmetry phases with the same topological linkage, trapped in a metastable intermediate stage. We propose that the remaining topological ordering is the origin of the switchable "memory glass" effect. Such topological ordering may hide in many amorphous materials through disordered short atomic displacements.

Superionic iron alloys and their seismic velocities in Earth's inner core

Earth's inner core (IC) is less dense than pure iron, indicating the existence of light elements within it¹. Silicon, sulfur, carbon, oxygen and hydrogen have been suggested to be the candidates^2,3, and the properties of iron-light-element alloys have been studied to constrain the IC composition^4-19. Light elements have a substantial influence on the seismic velocities^4-13, the melting temperatures^14-17 and the thermal conductivities^18,19 of iron alloys. However, the state of the light elements in the IC is rarely considered. Here, using ab initio molecular dynamics simulations, we find that hydrogen, oxygen and carbon in hexagonal close-packed iron transform to a superionic state under the IC conditions, showing high diffusion coefficients like a liquid. This suggests that the IC can be in a superionic state rather than a normal solid state. The liquid-like light elements lead to a substantial reduction in the seismic velocities, which approach the seismological observations of the IC^20,21. The substantial decrease in shear-wave velocity provides an explanation for the soft IC²¹. In addition, the light-element convection has a potential influence on the IC seismological structure and magnetic field.

Crystalline Fully Carboxylated Polyacetylene Obtained under High Pressure as a Li-Ion Battery Anode Material

Substituted polyacetylene is expected to improve the chemical stability, physical properties, and combine new functions to the polyacetylene backbones, but its diversity is very limited. Here, by applying external pressure on solid acetylenedicarboxylic acid, we report the first crystalline poly-dicarboxylacetylene with every carbon on the trans-polyacetylene backbone bonded to a carboxyl group, which is very hard to synthesize by traditional methods. The polymerization is evidenced to be a topochemical reaction with the help of hydrogen bonds. This unique structure combines the extremely high content of carbonyl groups and high conductivity of a polyacetylene backbone, which exhibits a high specific capacity and excellent cycling/rate performance as a Li-ion battery (LIB) anode. We present a completely functionalized crystalline polyacetylene and provide a high-pressure solution for the synthesis of polymeric LIB materials and other polymeric materials with a high content of active groups.

Future Study of Dense Superconducting Hydrides at High Pressure

The discovery of a record high superconducting transition temperature (Tc) of 288 K in a pressurized hydride inspires new hope to realize ambient-condition superconductivity. Here, we give a perspective on the theoretical and experimental studies of hydride superconductivity. Predictions based on the BCS-Eliashberg-Midgal theory with the aid of density functional theory have been playing a leading role in the research and guiding the experimental realizations. To date, about twenty hydrides experiments have been reported to exhibit high-Tc superconductivity and their Tc agree well with the predicted values. However, there are still some controversies existing between the predictions and experiments, such as no significant transition temperature broadening observed in the magnetic field, the experimental electron-phonon coupling beyond the Eliashberg-Midgal limit, and the energy dependence of density of states around the Fermi level. To investigate these controversies and the origin of the highest Tc in hydrides, key experiments are required to determine the structure, bonding, and vibrational properties associated with H atoms in these hydrides.

Pressure Gradient Squeezing Hydrogen out of MnOOH: Thermodynamics and Electrochemistry

Pressure of gigapascal (GPa) is a robust force for driving phase transitions and chemical reactions with negative volume change and is intensely used for promoting combination/addition reactions. Here, we find that the pressure gradient between the high-pressure region and the ambient-pressure environment in a diamond anvil cell is an even stronger force to drive decomposition/elimination reactions. A pressure difference of tens of GPa can "push" hydrogen out from its compounds in the high-pressure region to the environment. More importantly, in transition metal hydroxides such as MnOOH, the protons and electrons of hydrogen can even be separated via different conductors, pushed out by the high pressure, and recombine outside under ambient conditions, producing continuous current. A pressure-gradient-driven battery is hence proposed. Our investigation demonstrated that a pressure gradient is a special and powerful force to drive decomposition and electrochemical reactions.

Yi-Gang Xu: the Earth's deep interior holds the key to habitability

The deep Earth is the engine of whole Earth systems and plays a key role in surface evolution and geological hazards. Scientists have been deciphering the internal processes that shape our habitable planet, especially since the formulation of plate tectonics theory. To date, how the deep Earth works remains mysterious. At the end of 2020, the Chinese Academy of Sciences (CAS) started to set up the Center for Excellence in Deep Earth Science, headquartered in the Guangzhou Institute of Geochemistry (GIG), with long-term support for these emerging and interdisciplinary research areas. NSR recently talked to Professor Yi-Gang Xu, GIG’s Director, about why the study of the Earth's interior is essential, the current progress of deep Earth science in China, and what makes our planet habitable.

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.

Phase transition and chemical reactivity of 1H-tetrazole under high pressure up to 100 GPa

The pressure-induced phase transition and polymerization of nitrogen-rich molecules are widely focused on due to their extreme importance for the development of green high-energy-density materials. Here, we present a study of the phase-transition behaviour and chemical reaction of 1H-tetrazole up to 100 GPa using in situ Raman, IR, X-ray diffraction, neutron diffraction techniques and theoretical calculations. A phase transition above 2.6 GPa was identified and the high-pressure structure was determined with one molecule in a unit cell instead of two molecules as reported before. The 1H-tetrazole polymerized reversibly below 100 GPa, probably through carbon-nitrogen bonding instead of nitrogen-nitrogen bonding. Our studies update the structure model of the high-pressure phase of 1H-tetrazole, and present the possible intermolecular bonding route for the first time, which gives new insights to understand the phase transition and chemical reaction of nitrogen-rich compounds, and is of benefit for designing new high-energy-density materials.

Scalable High-Pressure Synthesis of sp2-sp3 Carbon Nanoribbon via [4 + 2] Polymerization of 1,3,5-Triethynylbenzene

Pressure-induced polymerization of aromatics is an effective method to construct extended carbon materials, including the diamond-like nanothread and graphitic structures, but the reaction pressure of phenyl is typically around 20 GPa and too high to be applied for large-scale preparation. Here by introducing ethynyl to phenyl, we obtained a sp²-sp³ carbon nanoribbon structure by compressing 1,3,5-triethynylbenzene (TEB), and the reaction pressure of phenyl was successfully decreased to 4 GPa, which is the lowest reaction pressure of aromatics at room temperature. Using experimental and theoretical methods, we figured out that the ethynylphenyl of TEB undergoes [4 + 2] dehydro-Diels-Alder (DDA) reaction with phenyl upon compression at an intermolecular C···C distance above 3.3 Å, which is much longer than those of benzene and acetylene. Our research suggested that the DDA reaction between ethynylphenyl and phenyl is a promising route to decrease the reaction pressure of aromatics, which allows the scalable high-pressure synthesis of nanoribbon materials.

Structural, Vibrational, and Electronic Properties of 1D-TlInTe2 under High Pressure: A Combined Experimental and Theoretical Study

Analogous to 2D layered transition-metal dichalcogenides, the TlSe family of quasi-one dimensional chain materials with the Zintl-type structure exhibits novel phenomena under high pressure. In the present work, we have systematically investigated the high-pressure behavior of TlInTe₂ using Raman spectroscopy, synchrotron X-ray diffraction (XRD), and transport measurements, in combination with first principles crystal structure prediction (CSP) based on evolutionary approach. We found that TlInTe₂ undergoes a pressure-induced semiconductor-to-semimetal transition at 4 GPa, followed by a superconducting transition at 5.7 GPa (with T_c = 3.8 K). An unusual giant phonon mode (A_g) softening appears at ∼10-12 GPa as a result of the interaction of optical phonons with the conduction electrons. The high-pressure XRD and Raman spectroscopy studies reveal that there is no structural phase transitions observed up to the maximum pressure achieved (33.5 GPa), which is in agreement with our CSP calculations. In addition, our calculations predict two high-pressure phases above 35 GPa following the phase transition sequence as I4/mcm (B37) → Pbcm → Pm3̅m (B2). Electronic structure calculations suggest Lifshitz (L1 & L2-type) transitions near the superconducting transition pressure. Our findings on TlInTe₂ open up a new avenue to study unexplored high-pressure novel phenomena in TlSe family induced by Lifshitz transition (electronic driven), giant phonon softening, and electron-phonon coupling.

Metallization of Quantum Material GaTa4Se8 at High Pressure

Pressure is a unique thermodynamic variable to explore the phase competitions and novel phases inaccessible at ambient conditions. The resistive switching material GaTa₄Se₈ displays several quantum phases under pressure, such as a J_eff = 3/2 Mott insulator, a correlated quantum magnetic metal, and d-wave topological superconductivity, which has recently drawn considerable interest. Using high-pressure Raman spectroscopy, X-ray diffraction, extended X-ray absorption, transport measurements, and theoretical calculations, we reveal a complex phase diagram for GaTa₄Se₈ at pressures exceeding 50 GPa. In this previously unattained pressure regime, GaTa₄Se₈ ranges from a Mott insulator to a metallic phase and exhibits superconducting phases. In contrast to previous studies, we unveil a hidden correlation between the structural distortion and band gap prior to the insulator-to-metal transition, and the metallic phase shows superconductivity with structural and magnetic properties that are distinctive from the lower-pressure phase. These discoveries highlight that GaTa₄Se₈ is a unique material to probe novel quantum phases from a structural, metallicity, magnetism, and superconductivity perspective.

Coupled deep-mantle carbon-water cycle: Evidence from lower-mantle diamonds

Diamonds form in a variety of environments between subducted crust, lithospheric and deep mantle. Recently, deep source diamonds with inclusions of the high-pressure H₂O-phase ice-VII were discovered. By correlating the pressures of ice-VII inclusions with those of other high-pressure inclusions, we assess quantitatively the pressures and temperatures of their entrapment. We show that the ice-VII-bearing diamonds formed at depths down to 800 ± 60 km but at temperatures 200–500 K below average mantle temperature that match the pressure-temperature conditions of decomposing dense hydrous mantle silicates. Our work presents strong evidence for coupled recycling of water and carbon in the deep mantle based on natural samples.

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A novel approach was developed to assess the pressure-temperature conditions of entrapment of inclusions in diamonds

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The viscoelastic relaxation of diamond has a significant effect on the evolution of pressure and temperature

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Ice-VII-bearing diamonds have formed in wet, cool environments at depths down to 800 km

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The coupled recycling of water and carbon is present in the deep mantle

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.

Quenchable amorphous glass-like material from VF3

The quite simple but relatively stable VF3-type compounds are known to be of major interest due to their building blocks - octahedra that are extremely important in perovskites as well. Here, we show that the VF6 octahedron in VF3 varies over a fairly wide pressure range (0-50 GPa), maintaining undisturbed rhombohedral crystal symmetry. Half of this pressure, VF6 rotates easily while the other undergoes strong uniaxial deformation in a "super-dense" condition. The congested sphere packing ultimately does not endure and drives the material to amorphize. We observed that the amorphous state could be quenched and acquire a transparent glass-like appearance when unloaded to ambient conditions. Dramatic, pressure-induced changes are clarified by phonon dispersion curves with the imaginary phonon mode, the so-called phonon soft mode, which indicates the structural instability. The distortion of the VF6 octahedra is attributed to the distinctive amorphization that could be further searched for throughout the whole almost identical VF3-type series providing metal trifluorides of various amorphous species.

Probing the Electronic Band Gap of Solid Hydrogen by Inelastic X-Ray Scattering up to 90 GPa

Metallization of hydrogen as a key problem in modern physics is the pressure-induced evolution of the hydrogen electronic band from a wide-gap insulator to a closed gap metal. However, due to its remarkably high energy, the electronic band gap of insulating hydrogen has never before been directly observed under pressure. Using high-brilliance, high-energy synchrotron radiation, we developed an inelastic x-ray probe to yield the hydrogen electronic band information in situ under high pressures in a diamond-anvil cell. The dynamic structure factor of hydrogen was measured over a large energy range of 45 eV. The electronic band gap was found to decrease linearly from 10.9 to 6.57 eV, with an 8.6 times densification (ρ/ρ_{0}∼8.6) from zero pressure up to 90 GPa.

Regulating off-centering distortion maximizes photoluminescence in halide perovskites

Metal halide perovskites possess unique atomic and electronic configurations that endow them with high defect tolerance and enable high-performance photovoltaics and optoelectronics. Perovskite light-emitting diodes have achieved an external quantum efficiency of over 20%. Despite tremendous progress, fundamental questions remain, such as how structural distortion affects the optical properties. Addressing their relationships is considerably challenging due to the scarcity of effective diagnostic tools during structural and property tuning as well as the limited tunability achievable by conventional methods. Here, using pressure and chemical methods to regulate the metal off-centering distortion, we demonstrate the giant tunability of photoluminescence (PL) in both the intensity (>20 times) and wavelength (>180 nm/GPa) in the highly distorted halide perovskites [CH₃NH₃GeI₃, HC(NH₂)₂GeI₃, and CsGeI₃]. Using advanced in situ high-pressure probes and first-principles calculations, we quantitatively reveal a universal relationship whereby regulating the level of off-centering distortion towards 0.2 leads to the best PL performance in the halide perovskites. By applying this principle, intense PL can still be induced by substituting CH₃NH₃⁺ with Cs⁺ to control the distortion in (CH₃NH₃)_1-xCs_xGeI₃, where the chemical substitution plays a similar role as external pressure. The compression of a fully substituted sample of CsGeI₃ further tunes the distortion to the optimal value at 0.7 GPa, which maximizes the emission with a 10-fold enhancement. This work not only demonstrates a quantitative relationship between structural distortion and PL property of the halide perovskites but also illustrates the use of knowledge gained from high-pressure research to achieve the desired properties by ambient methods.

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.

Origin of Plasticity in Nanostructured Silicon

The mechanism of plasticity in nanostructured Si has been intensively studied over the past decade but still remains elusive. Here, we used in situ high-pressure radial x-ray diffraction to simultaneously monitor the deformation and structural evolution of a large number of randomly oriented Si nanoparticles (SiNPs). In contrast to the high-pressure β-Sn phase dominated plasticity observed in large SiNPs (∼100  nm), small SiNPs (∼9  nm) display a high-pressure simple hexagonal phase dominated plasticity. Meanwhile, dislocation activity exists in all of the phases, but significantly weakens as the particle size decreases and only leads to subtle plasticity in the initial diamond cubic phase. Furthermore, texture simulations identify major active slip systems in all of the phases. These findings elucidate the origin of plasticity in nanostructured Si under stress and provide key guidance for the application of nanostructured Si.

Novel Superstructure-Phase Two-Dimensional Material 1T-VSe2 at High Pressure

A superstructure can elicit versatile new properties in materials by breaking their original geometrical symmetries. It is an important topic in the layered graphene-like two-dimensional transition metal dichalcogenides, but its origin remains unclear. Using diamond-anvil cell techniques, synchrotron X-ray diffraction, X-ray absorption, and first-principles calculations, we show that the evolution from weak van der Waals bonding to Heisenberg covalent bonding between layers induces an isostructural transition in quasi-two-dimensional 1T-type VSe₂ at high pressure. Furthermore, our results show that high pressure induces a novel superstructure at 15.5 GPa rather than suppresses it as it would normally, which is unexpected. It is driven by Fermi-surface nesting, enhanced by pressure-induced distortion. The results suggest that the superstructure not only appears in the two-dimensional structure but also can emerge in the pressure-tuned three-dimensional structure with new symmetry and develop superconductivity.

Evidence for the stability of ultrahydrous stishovite in Earth's lower mantle

The distribution and transportation of water in Earth's interior depends on the stability of water-bearing phases. The transition zone in Earth's mantle is generally accepted as an important potential water reservoir because its main constituents, wadsleyite and ringwoodite, can incorporate weight percent levels of H2O in their structures at mantle temperatures. The extent to which water can be transported beyond the transition zone deeper into the mantle depends on the water carrying capacity of minerals stable in subducted lithosphere. Stishovite is one of the major mineral components in subducting oceanic crust, yet the capacity of stishovite to incorporate water beyond at lower mantle conditions remains speculative. In this study, we combine in situ laser heating with synchrotron X-ray diffraction to show that the unit cell volume of stishovite synthesized under hydrous conditions is ∼2.3 to 5.0% greater than that of anhydrous stishovite at pressures of ∼27 to 58 GPa and temperatures of 1,240 to 1,835 K. Our results indicate that stishovite, even at temperatures along a mantle geotherm, can potentially incorporate weight percent levels of H2O in its crystal structure and has the potential to be a key phase for transporting and storing water in the lower mantle.

Probing Cerium 4f States across the Volume Collapse Transition by X-ray Raman Scattering

Understanding the volume collapse phenomena in rare-earth materials remains an important challenge due to a lack of information on 4f electronic structures at different pressures. Here, we report the first high-pressure inelastic X-ray scattering measurement on elemental cerium (Ce) metal. By overcoming the ultralow signal issue in the X-ray measurement at the Ce N_4,5-edge, we observe the changes of unoccupied 4f states across the volume collapse transition around 0.8 GPa. To help resolve the longstanding debate on the Anderson-Kondo and Mott-Hubbard models, we further compare the experiments with extended multiplet calculations that treat both screening channels on equal footing. The results indicate that a modest change in the 4f-5d Kondo coupling can well describe the spectral redistribution across the volume collapse, whereas the hybridization between neighboring atoms in the Hubbard model appears to play a minor role. Our study helps to constrain the theoretical models and opens a promising new route for systematic investigation of volume collapse phenomena in rare-earth materials.

High-pressure integrated synchrotron infrared spectroscopy system at the Shanghai Synchrotron Radiation Facility

We describe a new integrated optical spectroscopy facility for high-pressure research in materials research and mineral science located at the beamline BL01B of the Shanghai Synchrotron Radiation Facility. The system combines infrared synchrotron Fourier-Transform spectroscopy with broadband laser visible/near infrared and conventional laser Raman spectroscopy in one instrument. The system utilizes a custom-built microscope optics designed for a variety of diamond anvil cell experiments, which include low-temperature and ultrahigh pressure studies. We demonstrate the capabilities of the facility for studies of a variety of high-pressure phenomena such as phase and electronic transitions and chemical transformations.

Structure-Controlled Oxygen Concentration in Fe2O3 and FeO2

Solid-solid reaction, particularly in the Fe-O binary system, has been extensively studied in the past decades because of its various applications in chemistry and materials and earth sciences. The recently synthesized pyrite-FeO₂ at high pressure suggested a novel oxygen-rich stoichiometry that extends the achievable O-Fe ratio in iron oxides by 33%. Although FeO₂ was synthesized from Fe₂O₃ and O₂, the underlying solid reaction mechanism remains unclear. Herein, combining in situ X-ray diffraction experiments and first-principles calculations, we identified that two competing phase transitions starting from Fe₂O₃: (1) without O₂, perovskite-Fe₂O₃ transits to the post-perovskite structure above 50 GPa; (2) if free oxygen is present, O diffuses into the perovskite-type lattice of Fe₂O₃ leading to the pyrite-type FeO₂ phase. We found the O-O bonds in FeO₂ are formed by the insertion of oxygen into the Pv lattice via the external stress and such O-O bonding is only kinetically stable under high pressure. This may provide a general mechanism of adding extra oxygen to previous known O saturated oxides to produce unconventional stoichiometries. Our results also shed light on how O is enriched in mantle minerals under pressure.

Altered chemistry of oxygen and iron under deep Earth conditions

A drastically altered chemistry was recently discovered in the Fe-O-H system under deep Earth conditions, involving the formation of iron superoxide (FeO₂Hx with x = 0 to 1), but the puzzling crystal chemistry of this system at high pressures is largely unknown. Here we present evidence that despite the high O/Fe ratio in FeO₂Hx, iron remains in the ferrous, spin-paired and non-magnetic state at 60-133 GPa, while the presence of hydrogen has minimal effects on the valence of iron. The reduced iron is accompanied by oxidized oxygen due to oxygen-oxygen interactions. The valence of oxygen is not -2 as in all other major mantle minerals, instead it varies around -1. This result indicates that like iron, oxygen may have multiple valence states in our planet's interior. Our study suggests a possible change in the chemical paradigm of how oxygen, iron, and hydrogen behave under deep Earth conditions.

Pressure-Driven Reversible Switching between n- and p-Type Conduction in Chalcopyrite CuFeS2

Temperature-dependent switching between p- and n-type conduction is a newly observed phenomenon in very few Ag-based semiconductors, which may promote fascinating applications in modern electronics. Pressure, as an efficient external stimulus that has driven collective phenomena such as spin-crossover and Mott transition, is also expected to initialize a conduction-type switching in transition metal-based semiconductors. Herein, we report the observation of a pressure-driven dramatic switching between p- and n-type conduction in chalcopyrite CuFeS₂ associated with a structural phase transition. Under compression around 8 GPa, CuFeS₂ undergoes a phase transition with symmetry breakdown from space group I-42 d to space group I-4 accompanying with a remarkable volume shrinkage of the FeS₄ tetrahedra. A high-to-low spin-crossover of Fe²⁺ ( S = 2 to S = 0) is manifested along with this phase transition. Instead of pressure-driven metallization, a surprising semiconductor-to-semiconductor transition is observed associated with the structural and electronic transformations. Significantly, both photocurrent and Hall coefficient measurements confirm that CuFeS₂ undergoes a reversible pressure-driven p- n conduction type switching accompanying with the structural phase transition. The absence of cationic charge transfer between copper and iron during the phase transition is confirmed by both X-ray absorption near-edge spectra (Cu/Fe, K-edge) and total-fluorescence-yield X-ray absorption spectra (Fe, K-edge) results, and the valence distribution maintains Cu²⁺Fe²⁺S₂ in the high-pressure phase. The observation of an abrupt pressure-driven p- n conduction type switching in a transition metal-based semiconductor paves the way to novel pressure-responsive switching devices.

Reactivity of Hydrogen-Helium and Hydrogen-Nitrogen Mixtures at High Pressures

Through a series of Raman spectroscopy studies, we investigate the behavior of hydrogen-helium and hydrogen-nitrogen mixtures at high pressure across a wide range of concentrations. We find that there is no evidence of chemical association or increased miscibility of hydrogen and helium in the solid state up to pressures of 250 GPa at 300 K. In contrast, we observe the formation of concentration-dependent N_{2}-H_{2} van der Waals solids, which react to form N-H bonded compounds above 50 GPa. Through this combined study, we can demonstrate that the recently reported chemical association of H_{2}-He can be attributed to significant N_{2} contamination and subsequent formation of N_{2}-H_{2} compounds.

Evolution of a Novel Ribbon Phase in Optimally Doped Bi2Sr2CaCu2O8+δ at High Pressure and Its Implication to High- TC Superconductivity

One challenge in studying high-temperature superconductivity (HTSC) stems from a lack of direct experimental evidence linking lattice inhomogeneity and superconductivity. Here, we apply synchrotron hard X-ray nanoimaging and small-angle scattering to reveal a novel micron-scaled ribbon phase in optimally doped Bi₂Sr₂CaCu₂O_8+δ (Bi-2212, with δ = 0.1). The morphology of the ribbon-like phase evolves simultaneously with the dome-shaped T_C behavior under pressure. X-ray absorption studies show that the increasing of T_C is associated with oxygen-hole redistribution in the CuO₂ plan, while T_C starts to decrease with pressure when oxygen holes become immobile. Additional X-ray irradiation experiments reveal that nanoscaled short-range ordering of oxygen vacancies could further lower T_C, which indicates that the optimal T_C is affected not only by an optimal morphology of the ribbon phase, but also an optimal distribution of oxygen vacancies. Our studies thereby provide for the first time compelling experimental evidence correlating the T_C with micron to nanoscale inhomogeneity.

New high-pressure tetragonal polymorphs of SrTiO3-molecular orbital and Raman band change under pressure

The vibrational and structural properties of SrTiO₃ were investigated using Raman spectroscopy, synchrotron x-ray powder diffraction up to 55 GPa at 300 K, and ab initio quantum chemical molecular orbital (MO) calculations. These measurements and calculations revealed the structure transformation from cubic to tetragonal phase at about 9 GPa. Above 9 GPa, sharper new peaks were associated with a tetragonal structure. At about 30 GPa some bands disappeared and several new bands emerged. Structure transformation from I4/mcm to a new structure of P4/mbm was found at above 30 GPa by Rietveld profile fitting analysis. The diffraction pattern gave no indication of a Cmcm orthorhombic phase. Ab initio MO calculation proved the change of the molecular orbital coupling with a structure transformation. The Mulliken charge of Ti is increased with increasing pressure in the cubic phase, but the Sr charge continuously decreased. The d-p-π hybridization of the Ti-O bond and localizing the electron density are decreased with increasing pressure. The Ti-O bond becomes shorted in the P4/mbm phase and the change in the Ti charge accelerated more. All present investigations by x-ray diffraction, Raman spectra study and MO calculation show consistent results.

Emergent superconductivity in an iron-based honeycomb lattice initiated by pressure-driven spin-crossover

The discovery of iron-based superconductors (FeSCs), with the highest transition temperature (T_c) up to 55 K, has attracted worldwide research efforts over the past ten years. So far, all these FeSCs structurally adopt FeSe-type layers with a square iron lattice and superconductivity can be generated by either chemical doping or external pressure. Herein, we report the observation of superconductivity in an iron-based honeycomb lattice via pressure-driven spin-crossover. Under compression, the layered FePX₃ (X = S, Se) simultaneously undergo large in-plane lattice collapses, abrupt spin-crossovers, and insulator-metal transitions. Superconductivity emerges in FePSe₃ along with the structural transition and vanishing of magnetic moment with a starting T_c ~ 2.5 K at 9.0 GPa and the maximum T_c ~ 5.5 K around 30 GPa. The discovery of superconductivity in iron-based honeycomb lattice provides a demonstration for the pursuit of transition-metal-based superconductors via pressure-driven spin-crossover.

Correlated High-Pressure Phase Sequence of VO2 under Strong Compression

Understanding how the structures of a crystal behave under compression is a fundamental issue both for condensed matter physics and for geoscience. Traditional description of a crystal as the stacking of a unit cell with special symmetry has gained much success on the analysis of physical properties. Unfortunately, it is hard to reveal the relationship between the compressed phases. Taking the family of metal dioxides (MO₂) as an example, the structural evolution, subject to fixed chemical formula and highly confined space, often appears as a set of random and uncorrelated events. Here we provide an alternative way to treat the crystal as the stacking of the coordination polyhedron and then discover a unified structure transition pattern, in our case VO₂. X-ray diffraction (XRD) experiments and first-principles calculations show that the coordination increase happens only at one apex of the V-centered octahedron in an orderly fashion, leaving the base plane and the other apex topologically intact. The polyhedron evolves toward increasing their sharing, indicating a general rule for the chemical bonds of MO₂ to give away the ionicity in exchange for covalency under pressure.

When water meets iron at Earth's core–mantle boundary

Hydrous minerals in subducted crust can transport large amounts of water into Earth's deep mantle. Our laboratory experiments revealed the surprising pressure-induced chemistry that, when water meets iron at the core–mantle boundary, they react to form an interlayer with an extremely oxygen-rich form of iron, iron dioxide, together with iron hydride. Hydrogen in the layer will escape upon further heating and rise to the crust, sustaining the water cycle. With water supplied by the subducting slabs meeting the nearly inexhaustible iron source in the core, an oxygen-rich layer would cumulate and thicken, leading to major global consequences in our planet. The seismic signature of the D″ layer may echo the chemical complexity of this layer. Over the course of geological time, the enormous oxygen reservoir accumulating between the mantle and core may have eventually reached a critical eruption point. Very large-scale oxygen eruptions could possibly cause major activities in the mantle convection and leave evidence such as the rifting of supercontinents and the Great Oxidation Event.

Pressure-Induced Polymerization and Disproportionation of Li2C2 Accompanied with Irreversible Conductivity Enhancement

Li₂C₂ has the highest theoretical capacity (1400 mA·h·g^-1) as the electrode material for Li-ion battery, but suffers from low conductivity. Here we found that under external pressure its conductivity was irreversibly enhanced by 10⁹-fold. To explain that, we performed X-ray diffraction, Raman, IR, gas chromatography-mass spectrometry, and theoretical investigations under external pressure. We found that the C₂^2- anions approached to each other and polymerized upon compression, which is responsible for the irreversible enhancement of conductivity. The polymer has a ribbon structure and disproportionates into Li₃C₄ (Li_2-0.5C₂) ribbon structure, Li₆C₃ (Li propenide) and Li₄C₃ (Li allenide) upon decompression, implying that the carbon skeletal is highly electrochemically active. Our work reported polymerized Li₂C₂ for the first time, demonstrated that applying pressure is an effective method to prepare novel Li-C frameworks, and hence shed light on the search for novel carbon-based electrode materials.

Decompression-Driven Superconductivity Enhancement in In2 Se3

An unexpected superconductivity enhancement is reported in decompressed In₂ Se₃ . The onset of superconductivity in In₂ Se₃ occurs at 41.3 GPa with a critical temperature (T_c ) of 3.7 K, peaking at 47.1 GPa. The striking observation shows that this layered chalcogenide remains superconducting in decompression down to 10.7 GPa. More surprisingly, the highest T_c that occurs at lower decompression pressures is 8.2 K, a twofold increase in the same crystal structure as in compression. It is found that the evolution of T_c is driven by the pressure-induced R-3m to I-43d structural transition and significant softening of phonons and gentle variation of carrier concentration combined in the pressure quench. The novel decompression-induced superconductivity enhancement implies that it is possible to maintain pressure-induced superconductivity at lower or even ambient pressures with better superconducting performance.

Local structure of liquid gallium under pressure

In situ high energy X-ray pair distribution function (PDF) measurements, microtomography and reverse Monte Carlo simulations were used to characterize the local structure of liquid gallium up to 1.9 GPa. This pressure range includes the well-known solid-solid phase transition from Ga-I to Ga-II at low temperature. In term of previous research, the local structure of liquid gallium within this domain was suggested a mixture of two local structures, Ga I and Ga II, based on fitting experimental PDF to known crystal structure, with a controversy. However, our result shows a distinctly different result that the local structure of liquid gallium resembles the atomic arrangement of both gallium phase II and III (the high pressure crystalline phase). A melting mechanism is proposed for Ga, in which the atomic structure of phase Ι breaks up at the onset of melting, providing sufficient free volume for atoms to rearrange, to form the melt.

Stability of Ar(H2)2 to 358 GPa

"Chemical precompression" through introducing impurity atoms into hydrogen has been proposed as a method to facilitate metallization of hydrogen under external pressure. Here we selected Ar(H2)2, a hydrogen-rich compound with molecular hydrogen, to explore the effect of "doping" on the intermolecular interaction of H2 molecules and metallization at ultrahigh pressure. Ar(H2)2 was studied experimentally by synchrotron X-ray diffraction to 265 GPa, by Raman and optical absorption spectroscopy to 358 GPa, and theoretically using the density-functional theory. Our measurements of the optical bandgap and the vibron frequency show that Ar(H2)2 retains 2-eV bandgap and H2 molecular units up to 358 GPa. This is attributed to reduced intermolecular interactions between H2 molecules in Ar(H2)2 compared with that in solid H2 A splitting of the molecular vibron mode above 216 GPa suggests an orientational ordering transition, which is not accompanied by a change in lattice symmetry. The experimental and theoretical equations of state of Ar(H2)2 provide direct insight into the structure and bonding of this hydrogen-rich system, suggesting a negative chemical pressure on H2 molecules brought about by doping of Ar.