Rare Helium-Bearing Compound FeO_{2}He Stabilized at Deep-Earth Conditions

Theoretical study of the structural and thermodynamic properties of U-He compounds under high pressure

Uranium is considered as a very important nuclear energy material because of the huge amount of energy it releases. As the main product of the spontaneous decay of uranium, it is difficult for helium to react with uranium because of its chemical inertness. Therefore, bubbles will be formed inside uranium, which could greatly reduce the performance of uranium or cause safety problems. Additionally, nuclear materials are usually operated in an environment of high-temperature and high-pressure, so it is necessary to figure out the exact state of helium inside uranium under extreme conditions. Here, we explored the structural stability of the U-He system under high pressure and high temperature by using density functional theory calculations. Two metastable phases are found between 50 and 400 GPa: U4He with space group Fmmm and U6He with space group P1̄. Both are metallic and adopt layered structures. Electron localization function calculation combined with charge density difference analysis indicates that there are covalent bonds between U and U atoms in both Fmmm-U4He and P1̄-U6He. Regarding the elastic modulus of α-U, the addition of helium has certain influence on the mechanical properties of uranium. Besides, first-principles molecular dynamics simulations were carried out to study the dynamical behavior of Fmmm-U4He and P1̄-U6He at high-temperature. It was found that Fmmm-U4He and P1̄-U6He undergo one-dimensional superionic phase transitions at 150 GPa. Our study revealed the exotic structure of U-He compounds beyond the formation of bubbles under high-pressure and high-temperature, which might be relevant to the performance and safety issues of nuclear materials under extreme conditions.

Unveiling unconventional CH4-Xe compounds and their thermodynamic properties at extreme conditions

Inert gases (e.g., He and Xe) can exhibit chemical activity at high pressure, reacting with other substances to form compounds of unexpected chemical stoichiometry. This work combines first-principles calculations and crystal structure predictions to propose four unexpected stable compounds of CH4Xe3, (CH4)2Xe, (CH4)3Xe, and (CH4)3Xe2 at pressure ranges from 2 to 100 GPa. All structures are composed of isolated Xe atoms and CH4 molecules except for (CH4)3Xe2, which comprises a polymerization product, C3H8, and hydrogen molecules. Ab initio molecular dynamics simulations indicate that pressure plays a very important role in the different temperature driving state transitions of CH4-Xe compounds. At lower pressures, the compounds follow the state transition of solid-plastic-fluid phases with increasing temperature, while at higher pressures, the stronger Xe-C interaction induces the emergence of a superionic state for CH4Xe3 and (CH4)3Xe2 as temperature increases. These results not only expand the family of CH4-Xe compounds, they also contribute to models of the structures and evolution of planetary interiors.

Synthesis and Properties of the Helium Clathrate and Defect Perovskite [He2-x □ x ][CaNb]F6

The defect double perovskite [He2-x □ x ][CaNb]F6, with helium on its A-site, can be prepared by the insertion of helium into ReO3-type CaNbF6 at high pressure. Upon cooling from 300 to 100 K under 0.4 GPa helium, ∼60% of the A-sites become occupied. Helium uptake was quantified by both neutron powder diffraction and gas insertion and release measurements. After the conversion of gauge pressure to fugacity, the uptake of helium by CaNbF6 can be described by a Langmuir isotherm. The enthalpy of absorption for helium in [He2-x □ x ][CaNb]F6 is estimated to be ∼+3(1) kJ mol-1, implying that its formation is entropically favored. Helium is able to diffuse through the material on a time scale of minutes at temperatures down to ∼150 K but is trapped at 100 K and below. The insertion of helium into CaNbF6 reduces the magnitude of its negative thermal expansion, increases the bulk modulus, and modifies its phase behavior. On compressing pristine CaNbF6, at 50 and 100 K, a cubic (Fm3̅m) to rhombohedral (R3̅) phase transition was observed at <0.20 GPa. However, a helium-containing sample remained cubic at 0.4 GPa and 50 K. CaNbF6, compressed in helium at room temperature, remained cubic to >3.7 GPa, the limit of our X-ray diffraction measurements, in contrast to prior reports that upon compression in a nonpenetrating medium, a phase transition is detected at ∼0.4 GPa.

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.

A newly predicted stable calcium argon compound byab initiocalculations under high pressure

High pressure can change the valence electron arrangement of the elements, and it can be as a new method for the emergence of unexpected new compounds. In this paper, the Ca-Ar compounds at 0-200 GPa are systematically investigated by using CALYPSO structure prediction methods combined with first principles calculations. The study of the Ca-Ar system can provide theoretical guidance for the exploration of new structures of inert elemental Ar compounds under high pressure. A stable structure:P63/mmc-CaAr and six metastable structures:Rm-CaAr2,P4/mmm-CaAr2,Pm1-CaAr3,P4/mmm-CaAr3,P21/m-CaAr4andPm1-CaAr5were obtained. Our calculations show that the only stable phaseP63/mmc-CaAr can be synthesized at high pressure of 90 GPa. All the structures are ionic compounds of metallic nature, and surprisingly all Ar atoms attract electrons and act as an oxidant under high pressure conditions. The calculation results ofab initiomolecular dynamics show thatP63/mmc-CaAr compound maintains significant thermodynamic stability at high temperatures up to 1000 K. The high-pressure structures and electronic behaviors of the Ca-Ar system are expected to expand the understanding of the high-pressure chemical reactivity of compounds containing inert elements, and provide important theoretical support for the search of novel anomalous alkaline-earth metal inert element compounds.

Predicted crystal structures of xenon and alkali metals under high pressures

The pressure-induced reaction between xenon (Xe) and other non-inert gas elements and the resultant crystal structures have attracted great interest. In this work, we carried out extensive simulations on the crystal structures of Xe-alkali metal (Xe-AM) systems under high pressures. Among all predicted compounds, KXe and RbXe are found to become stable at a pressure of ∼16 GPa by adopting a cubic symmetry of space group Pm3̄m. The stabilization of KXe and RbXe requires slightly lower pressure compared with that of previously reported CsXe (25 GPa), interestingly, which is in contrast to the electronegativity order of the AMs and unexpected. Our simulations also indicate that all predicted Xe compounds contain negatively charged Xe. Moreover, our in-depth analysis indicates that the occupation of AM d-orbitals plays a critical role in stabilizing these Xe-bearing compounds. These results shed light on the understanding of the reaction between Xe and AMs and the formation mechanism of the resultant crystal structures.

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.

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.

High pressure nanoarchitectonics and metallization of barium chloride and barium bromide

As one of the most prototypicalAX₂-type compounds, barium halide shared the cubic structure withFm-3msymmetry for BaCl₂or orthorhombic structure withPnmasymmetry for BaBr₂at ambient pressure. In this work, we explored the crystal structures of BaCl₂and BaBr₂under high pressure. We predicted a thermodynamically more favored structure with orthorhombicCmcmsymmetry for both BaCl₂and BaBr₂, at 74 and 47 GPa, respectively. Our simulations reveal that the metallic feature ofCmcmBaCl₂andCmcmBaBr₂under high pressure. The present results improve the understanding of high-pressure structures ofAX₂compounds at extremely high-pressure conditions.

High-Pressure Structures and Superconductivity of Barium Iodide

Generally, pressure is a useful tool to modify the behavior of physical properties of materials due to the change in distance between atoms or molecules in the lattice. Barium iodide (BaI2), as one of the simplest and most prototypical iodine compounds, has substantial high pressure investigation value. In this work, we explored the crystal structures of BaI2 at a wide pressure range of 0-200 GPa using a global structure search methodology. A thermodynamical structure with tetragonal I4/mmm symmetry of BaI2 was predicted to be stable at 17.1 GPa. Further electronic calculations indicated that I4/mmm BaI2 exhibits the metallic feature via an indirect band gap closure under moderate pressure. We also found that the superconductivity of BaI2 at 30 GPa is much lower than that of CsI at 180 GPa based on our electron-phonon coupling simulations. Our current simulations provide a step toward the further understanding of the high-pressure behavior of iodine compounds at extreme conditions.

Materials by design at high pressures

Pressure, a fundamental thermodynamic variable, can generate two essential effects on materials. First, pressure can create new high-pressure phases via modification of the potential energy surface. Second, pressure can produce new compounds with unconventional stoichiometries via modification of the compositional landscape. These new phases or compounds often exhibit exotic physical and chemical properties that are inaccessible at ambient pressure. Recent studies have established a broad scope for developing materials with specific desired properties under high pressure. Crystal structure prediction methods and first-principles calculations can be used to design materials and thus guide subsequent synthesis plans prior to any experimental work. A key example is the recent theory-initiated discovery of the record-breaking high-temperature superhydride superconductors H3S and LaH10 with critical temperatures of 200 K and 260 K, respectively. This work summarizes and discusses recent progress in the theory-oriented discovery of new materials under high pressure, including hydrogen-rich superconductors, high-energy-density materials, inorganic electrides, and noble gas compounds. The discovery of the considered compounds involved substantial theoretical contributions. We address future challenges facing the design of materials at high pressure and provide perspectives on research directions with significant potential for future discoveries.

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.

Structural evolution and phase transition mechanism of [Formula: see text] under high pressure

[Formula: see text] is a layered transition-metal dichalcogenide (TMD) with outstanding electronic and optical properties, which is widely used in field-effect transistor (FET). Here the structural evolution and phase transition of [Formula: see text] under high pressure are systematically studied by CALYPSO structural search method and first-principles calculations. The structural evolutions of [Formula: see text] show that the ground state structure under ambient pressure is the experimentally observed P6[Formula: see text]/mmc phase, which transfers to R3m phase at 1.9 GPa. The trigonal R3m phase of [Formula: see text] is stable up to 72.1 GPa, then, it transforms into a new P6[Formula: see text]/mmc phase with different atomic coordinates of Se atoms. This phase is extremely robust under ultrahigh pressure and finally changes to another trigonal R-3m phase under 491.1 GPa. The elastic constants and phonon dispersion curves indicate that the ambient pressure phase and three new high-pressure phases are all stable. The electronic band structure and projected density of states analyses reveal a pressure induced semiconducting to metallic transition under 72.1 GPa. These results offer a detailed structural evolution and phase diagram of [Formula: see text] under high pressure, which may also provide insights for exploration other TMDs under ultrahigh pressure.

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.

Pressure-induced high-temperature superconductivity retained without pressure in FeSe single crystals

To raise the superconducting-transition temperature (T_c) has been the driving force for the long-sustained effort in superconductivity research. Recent progress in hydrides with T_cs up to 287 K under pressure of 267 GPa has heralded a new era of room temperature superconductivity (RTS) with immense technological promise. Indeed, RTS will lift the temperature barrier for the ubiquitous application of superconductivity. Unfortunately, formidable pressure is required to attain such high T_cs. The most effective relief to this impasse is to remove the pressure needed while retaining the pressure-induced T_c without pressure. Here, we show such a possibility in the pure and doped high-temperature superconductor (HTS) FeSe by retaining, at ambient pressure via pressure quenching (PQ), its T_c up to 37 K (quadrupling that of a pristine FeSe at ambient) and other pressure-induced phases. We have also observed that some phases remain stable without pressure at up to 300 K and for at least 7 d. The observations are in qualitative agreement with our ab initio simulations using the solid-state nudged elastic band (SSNEB) method. We strongly believe that the PQ technique developed here can be adapted to the RTS hydrides and other materials of value with minimal effort.

Pressure-Induced Evolution of Crystal and Electronic Structure of Ammonia Borane

Ammonia borane (NH₃BH₃) has long attracted considerable interest for its high hydrogen content and easy dehydrogenation conditions which make it a promising hydrogen storage material. Here, we report on a computational study of the structural stability and phase transition sequence of NH₃BH₃ and associated lattice dynamics and electronic properties in a wide pressure range up to 300 GPa. The results confirm previously reported structures, including the experimentally observed orthorhombic Pmn2₁ structure at low temperature and ambient pressure, and predict the phase transition sequence Pmn2₁ → Pc → P2₁ → P1̅ for NH₃BH₃. Our calculations also reveal systematic trends of monotonically decreasing band gap with rising pressure in the three high-pressure NH₃BH₃ phases, which nevertheless all remain nonconducting up to the highest pressure of 300 GPa examined in this work. The present findings elucidate structural and electronic properties of NH₃BH₃ over an extensive pressure range, providing knowledge essential to further study of NH₃BH₃ in an expanded pressure-temperature phase space.

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.

Coexistence of plastic and partially diffusive phases in a helium-methane compound

Helium and methane are major components of giant icy planets and are abundant in the universe. However, helium is the most inert element in the periodic table and methane is one of the most hydrophobic molecules, thus whether they can react with each other is of fundamental importance. Here, our crystal structure searches and first-principles calculations predict that a He3CH4 compound is stable over a wide range of pressures from 55 to 155 GPa and a HeCH4 compound becomes stable around 105 GPa. As nice examples of pure van der Waals crystals, the insertion of helium atoms changes the original packing of pure methane molecules and also largely hinders the polymerization of methane at higher pressures. After analyzing the diffusive properties during the melting of He3CH4 at high pressure and high temperature, in addition to a plastic methane phase, we have discovered an unusual phase which exhibits coexistence of diffusive helium and plastic methane. In addition, the range of the diffusive behavior within the helium-methane phase diagram is found to be much narrower compared to that of previously predicted helium-water compounds. This may be due to the weaker van der Waals interactions between methane molecules compared to those in helium-water compounds, and that the helium-methane compound melts more easily.

Helium's placement in the Periodic Table from a crystal structure viewpoint

The present paper indicates that helium has more in common with the alkaline earths than is often considered. Not only does it share an analogous electron configuration but it also has an analogous crystal structure to that of beryllium and magnesium which is, in contrast, characteristic of the bulk material rather than of isolated atoms.

Formation of ammonia-helium compounds at high pressure

Uranus and Neptune are generally assumed to have helium only in their gaseous atmospheres. Here, we report the possibility of helium being fixed in the upper mantles of these planets in the form of NH₃-He compounds. Structure predictions reveal two energetically stable NH₃-He compounds with stoichiometries (NH₃)₂He and NH₃He at high pressures. At low temperatures, (NH₃)₂He is ionic with NH₃ molecules partially dissociating into (NH₂)^- and (NH₄)⁺ ions. Simulations show that (NH₃)₂He transforms into intermediate phase at 100 GPa and 1000 K with H atoms slightly vibrate around N atoms, and then to a superionic phase at  ~2000 K with H and He exhibiting liquid behavior within the fixed N sublattice. Finally, (NH₃)₂He becomes a fluid phase at temperatures of 3000 K. The stability of (NH₃)₂He at high pressure and temperature could contribute to update models of the interiors of Uranus and Neptune.

Chemical Bond Mechanism for Helium Revealed by Electronic Excitation

Helium chemistry is notoriously very impervious. It is therefore certainly no surprise that, for example, beryllium and helium atoms, in their ground state, do not bind. Full configuration-interaction calculations show that the same turns out to be true, save for a long-range shallow attraction, for the Be⁺ + He system. However, quite astonishingly, when one electron is re-added to Be⁺ in an excited 2p_π or 3s orbital (Be ¹P or ¹S), a bound adduct with He is formed, at an interatomic separation as short as 1.5 Å. Understanding why this happens reveals an unsuspected chemical mechanism that stabilizes helium compounds at the molecular level.

Decomposition and Recombination of Binary Interalkali Na2K at High Pressures

Under compression, "'simple'" alkali metals and their alloys exhibit complex structural and electronic properties, leading to fundamental interest in their high-pressure behaviors. Here, the swarm-intelligence structure-searching method was employed to identify the high-pressure phases of binary interalkali Na₂K, which has long been known to possess a MgZn₂-Laves phase at ambient pressure, but the high-pressure behavior remains elusive. We uncovered four new structures over a pressure range of 10-500 GPa, although the compound was found to become unstable upon decomposition into Na and K from 37 to 273 GPa. In phases before decomposition, the electrons were gradually delocalized with an increase in pressure and there was charge transfer from K to Na, whereas in phases after recombination, the electrons were gradually localized into the interstitials of the crystals, showing the unexpected opposite trend of charge transfer from Na to K, remarkably, where K was found to exhibit an oxidation state beyond the -1 valence state. The results can improve our understanding of the interaction and evolution of s electrons under compression.