Reactions of xenon with iron and nickel are predicted in the Earth's inner core

Metallic PtC monolayer as a promising hydrogen evolution electrocatalyst

Reasonable design of hydrogen evolution reaction (HER) electrocatalysts with low Pt loading and excellent catalytic performance is a key challenge in finding efficient and cost attractive catalysts. Pt with its unique d-electrons provides new opportunities for the development of HER catalysts when it forms compounds with highly earth-abundant C. Herein, we focused on designing highly efficient catalysts composed of Pt and C elements using first-principles structure search simulations, identifying four stability PtCx monolayers. The novel PtC monolayer with a zigzag C chain not only possesses lower Pt loading but also shows inherent metallicity. Meanwhile, its H2O adsorption and dissociation abilities are efficient and facile. The HER activity of the PtC monolayer is comparable to that of commercial Pt, with desirable ΔGH* values and larger exchange current density, which are mainly attributed to lower charge donation of Pt, larger occupation of Pt PDOS at the Fermi level, and paired electrons of the zigzag C chain. Moreover, its excellent HER activity can be maintained even at high H coverage under strain and solvent effect. All these attractive properties render the PtC monolayer an appropriate HER catalyst.

Quantitatively Deciphering the Local Structure and Luminescence Spectroscopy of Pr3+-Doped Yttrium Lithium Fluoride

Praseodymium (Pr3+)-doped LiYF4 nanophosphors have garnered significant interest for their potential applications in lasers, phosphors, and quantum memories. However, there remains a lack of comprehensive research on the local coordination environment and luminescence spectroscopy of Pr3+:LiYF4 nanocrystals. This study presents the first investigation of the ground-state structure of Pr3+:LiYF4 crystals by employing the crystal structure prediction method, and a [PrF8]5- ligand complex with S4 local symmetry is determined. The complete energy levels of the Pr3+ ions in LiYF4 nanocrystals are unveiled by using our newly developed well-established parametrization matrix diagonalization method. A novel set of free-ion and crystal-field parameters is derived through a good simulation with 45 experimental energy levels. Many of the emissions of Pr3+-doped LiYF4 are successfully reproduced based on Judd-Ofelt theory, and these transitions are comparable to the experimental ones. Moreover, two new prominent emission bands with their peaks at 675 and 849 nm originating from 1I6 → 3F4 and 1I6 → 1G4 transitions, respectively, are predicted by us for the first time. This study could provide a feasible method to search for practical laser transition channels of solid-state lasers based on Pr3+: LiYF4 nanophosphors.

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.

Structural Phase Transition and Decomposition of XeF2 under High Pressure and Its Formation of Xe-Xe Covalent Bonds

Noble gases with inert chemical properties have rich bonding modes under high pressure. Interestingly, Xe and Xe form covalent bonds, originating from the theoretical simulation of the pressure-induced decomposition of XeF2, which has yet to be experimentally confirmed. Moreover, the structural phase transition and metallization of XeF2 under high pressure have always been controversial. Therefore, we conducted extensive experiments using a laser-heated diamond anvil cell technique to investigate the above issues of XeF2. We propose that XeF2 undergoes a structural phase transition and decomposition above 84.1 GPa after laser heating, and the decomposed product Xe2F contains Xe-Xe covalent bonds. Neither the pressure nor temperature alone could bring about these changes in XeF2. With our UV-vis absorption experiment, I4/mmm-XeF2 was metalized at 159 GPa. This work confirms the existence of Xe-Xe covalent bonds and provides insights into the controversy surrounding XeF2, enriching the research on noble gas chemistry under high pressure.

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.

A pressure-induced superhard SiCN4 compound uncovered by first-principles calculations

Silicon-carbon-nitride (Si-C-N) compounds are a family of potential superhard materials with many excellent chemical and physical properties; however, only SiCN, Si2CN4 and SiC2N4 were synthesized. Here, we theoretically report a new SiCN4 compound with P41212, Fdd2 and R3̄ structures by first-principles structural predictions based on the particle swarm optimization algorithm. Pressure-induced structural phase transitions from P41212 to Fdd2, and then to the R3̄ phase were determined at 2 GPa and 249 GPa. By comparing enthalpy differences with 1/3Si3N4 + C + 4/3N2, it was found that these structures tend to decompose at ambient pressure. However, with the increase of pressure, the enthalpy differences of Fdd2 and R3̄ structures turn to be negative and they can be stabilized at a pressure of more than 41 GPa. They are also dynamically stable as no imaginary frequencies were found in their stabilized pressure ranges. The calculated band gap is 4.37 eV for P41212, 3.72 eV for Fdd2 and 3.81 eV for the R3̄ phase by using the Heyd-Scuseria-Ernzerhof (HSE06) method and the estimated Vickers hardness values are higher than 40 GPa by adopting the elastic modulus based hardness formula, which confirmed their superhard characteristics. These results provide significant insights into Si-C-N systems and will inevitably promote the future experimental works.

2D Metal-Free BSi5 with an Intrinsic Metallicity and Remarkable HER Activity

One of the most urgent and attractive topics in electrocatalytic water splitting is the exploration of high-performance and low-cost catalysts. Herein, we have proposed three fresh two-dimensional nanostructures (BSi5, BSi4, and BSi3) with inherent metallicity contributed by delocalized π electrons based on first-principles calculations. Their planar atoms arrangement, akin to graphene, is in favor of the availability of active atoms and H adsorption/deadsorption. Among them, the BSi5 monolayer shows the best HER activity, even superior to a commercial Pt catalyst. Moreover, its extraordinary HER activity can be maintained under high H coverage and large biaxial strain, mainly originating from the fact that B 2pz orbital electrons are responsible for the B-H interaction. Further analysis reveals that there appears to be a linear correlation between the magnitude of B 2pz DOS at the Fermi level and Gibbs free energy in both three proposed nanostructures and five hypothetical B-Si nanostructures. Our work represents a significant step forward toward the design of metal-free HER catalysts.

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.

MAGUS: machine learning and graph theory assisted universal structure searcher

Crystal structure predictions based on first-principles calculations have gained great success in materials science and solid state physics. However, the remaining challenges still limit their applications in systems with a large number of atoms, especially the complexity of conformational space and the cost of local optimizations for big systems. Here, we introduce a crystal structure prediction method, MAGUS, based on the evolutionary algorithm, which addresses the above challenges with machine learning and graph theory. Techniques used in the program are summarized in detail and benchmark tests are provided. With intensive tests, we demonstrate that on-the-fly machine-learning potentials can be used to significantly reduce the number of expensive first-principles calculations, and the crystal decomposition based on graph theory can efficiently decrease the required configurations in order to find the target structures. We also summarized the representative applications of this method on several research topics, including unexpected compounds in the interior of planets and their exotic states at high pressure and high temperature (superionic, plastic, partially diffusive state, etc.); new functional materials (superhard, high-energy-density, superconducting, photoelectric materials), etc. These successful applications demonstrated that MAGUS code can help to accelerate the discovery of interesting materials and phenomena, as well as the significant value of crystal structure predictions in general.

Mechanochemistry and the Evolution of Ionic Bonds in Dense Silver Iodide

External mechanical stress alters the nature of chemical bonds and triggers novel reactions, providing interesting synthetic protocols to supplement traditional solvent- or thermo-based chemical approaches. The mechanisms of mechanochemistry have been well studied in organic materials made of a carbon-centered polymeric framework and covalence force field. They convert stress into anisotropic strain which will engineer the length and strength of targeted chemical bonds. Here, we show that by compressing silver iodide in a diamond anvil cell, the external mechanical stress weakens the Ag-I ionic bonds and activate the global diffusion of super-ions. In contrast to conventional mechanochemistry, mechanical stress imposes unbiased influence on the ionicity of chemical bonds in this archetypal inorganic salt. Our combined synchrotron X-ray diffraction experiment and first-principles calculation demonstrate that upon the critical point of ionicity, the strong ionic Ag-I bonds break down, leading to the recovery of elemental solids from a decomposition reaction. Instead of densification, our results reveal the mechanism of an unexpected decomposition reaction through hydrostatic compression and suggest the sophisticated chemistry of simple inorganic compounds under extreme conditions.

Phase Transitions and Electric Properties of PbBr2 under High Pressure: A First-Principles Study

PbBr₂ has recently attracted considerable attention as a precursor for lead halide perovskite-based devices because of its attractive properties. It is known that pressure can modify the chemical and physical properties of materials by altering the distance between atoms in the lattice. Here, a global structure-searching scheme was used to explore the high-pressure structures of PbBr₂, whose structures and properties at high pressure are still far from clear. Three new phases of PbBr₂ were predicted in the pressure range of 0-200 GPa, and the pressure-driven phase transition sequence of orthorhombic Pnma (0-52 GPa) → tetragonal I4/mmm (52-80 GPa) → orthorhombic Cmca (80-153.5 GPa) → orthorhombic Immm (153.5-200 GPa) is proposed. Electronic calculations indicate a semiconductor-to-metallic transition of PbBr₂ in the Cmca phase at ~120 GPa. Our present results could be helpful in improving the understanding of fundamental physical properties and provide insights to modulate the structural and related photoelectric properties of PbBr₂.

Crystal Structures and Electronic Properties of BaAu Compound under High Pressure

The investigations of Au-bearing alloy materials have been of broad research interest as their relevant features exhibit significant advantages compared with pure Au. Here, we extensively investigate the compression behaviors of BaAu compounds via first-principles calculations and find that a high-pressure cubic phase is calculated to be stable above 12 GPa. Further electronic calculations indicate that despite the low electronegativity of Ba, Fd-3m-structured BaAu exhibits metallic characteristics, which is different from those of semiconducting alkali metal aurides that possess slight characteristics of an ionic compound. These findings provide a step toward a further understanding of the electronic properties of BaAu compounds and provide key insight for exploring the other Au-bearing alloy materials under extreme conditions.

Pressure- and Temperature-Induced Structural Phase Diagram of Lead-Free (K0.5Na0.5)NbO3-0.05LiNbO3 Single Crystals: Raman Scattering and Infrared Study

Ferroelectric lead-free K_xNa_1-xNbO₃ (KNN) perovskite, whose piezoelectric properties can be comparable to those of traditional Pb-based systems, has aroused wide concern in recent years. However, the specific influences of the stress field on KNN's structure and piezoelectric properties have not been well clarified and there are few descriptions about the temperature-pressure phase diagram. Here, we analyzed the phonon mode behavior and structural evolution of K_0.5Na_0.5NbO₃-0.05LiNbO₃ (KNN-LN) and MnO₂-doped single crystals with pressure- and temperature-dependent phase structure variations by theoretical calculation, polarized Raman scattering, and infrared reflectance spectra. The different phase structures can be predicted at high pressure using the CALYPSO method with its same-name code. The rhombohedral → orthorhombic → tetragonal → cubic phase transition process can be discovered in detail by Raman spectra under different temperatures and pressures. The phase coexistence on the thermal phase boundary was confirmed by basic anastomosis. Meanwhile, it was found that the substitution of Mn in the NbO₆ octahedron aggravates the deformation of high pressure on KNN-LN and the substitution of Mn at the B-site intensifies the structural evolution more severely than at the A-site. The present study aims at exploring octahedra tilt, phonon vibrations, and the internal structure on the general critical phase boundary in KNN-LN crystals. It provides effective help for the study of lead-free perovskite phase transformation and the improvement in piezoelectric properties under a high-pressure field.

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.

Design of Novel Transition-Metal-Doped C6N2 with High-Efficiency Polysulfide Anchoring and Catalytic Performances toward Application in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are highly expected because of their high theoretical specific capacity and energy density. However, its application still faces challenges, including the shuttle effect affecting the sulfur reduction reaction, the high decomposition energy barrier of Li₂S during charging, the volume change of sulfur, and the poor conductivity during charging and discharging. Here, combined with density functional theory and particle swarm optimization algorithm for the nitrogen carbide monolayer structural search (C_mN_8-m, m = 1-8), the surprising discovery is that a single metal-atom-doped C₆N₂ monolayer could effectively accelerate the conversion of lithium polysulfide and anchor lithium polysulfide during discharging and decrease the decomposition energy barrier of Li₂S during charging. This "anchoring and catalyzing" mechanism effectively reduces the shuttle effect and greatly improves the reaction kinetics. Among a series of metal atoms, Cr is the best doping element, and it exhibits suitable adsorption energy for polysulfides and the lowest decomposition energy barrier for Li₂S. This work opens up a new way for the development of transition-metal-doped carbon-nitrogen materials with an excellent catalytic activity for lithium polysulfide as cathode materials for Li-S batteries.

Pressure-induced evolution of structures and promising superconductivity of ScB6

Unique multicenter bonding in boron-rich materials leads to the formation of complicated structures and intriguing properties. ScB₆, as a sister compound, possibly possesses high hardness and superconducting critical temperature in this family under ambient pressure. Here, phase transitions, chemical bonding states and electronic properties of ScB₆ at high pressure are uncovered using particle swarm optimization (PSO) combined with first-principles calculations. The phase sequence of P2₁/m → C2/m → Cmcm for ScB₆ has been identified under high pressure. Interestingly, the evolution of a boron framework is from a graphene-like layer to a planar B₄ ring, B₆ and B₇ cycle, and non-planar B₈ cycle, which interconnect a graphene-like network. These phases of ScB₆ are expected to be hard materials due to the excellent mechanical behaviors by the mechanical property calculations. Although the metallic features of the three phases reduce their hardness, the further electron-phonon coupling calculations indicate that the three phases of ScB₆ are superconducting phases under high pressures.

Electronegativity and chemical hardness of elements under pressure

SignificanceOver the years, many unusual chemical phenomena have been discovered at high pressures, yet our understanding of them is still very fragmentary. Our paper addresses this from the fundamental level by exploring the key chemical properties of atoms-electronegativity and chemical hardness-as a function of pressure. We have made an appropriate modification to the definition of Mulliken electronegativity to extend its applicability to high pressures. The change in atomic properties, which we observe, allows us to provide a unified framework explaining (and predicting) many chemical phenomena and the altered behavior of many elements under pressure.

A new MoCN monolayer containing stable cyano structural units as a high-efficiency catalyst for the hydrogen evolution reaction

In the hydrogen evolution reaction (HER), it is essential to find a high-efficiency and nonprecious electrocatalyst comparable to Pt, which needs to have rich inherently active sites and good conductivity. By combining a global minimum structure search and first-principles calculations, a hitherto unknown 2D MoCN monolayer was found, which can be considered as a structure in which Mo atoms interact with the stable CN units through triple bonds. The resultant MoCN monolayer possesses superior thermodynamic, dynamic, thermal, and mechanical stabilities, as well as inherent metallicity. In particular, it can exhibit outstanding HER catalytic activity due to the presence of many active sites with near-zero ΔG_H* values, whose density totals 1.80 × 10¹⁵ sites per cm², even more than Pt. In addition, we also propose a series of other 2D monolayers containing stable CN units (i.e., MoC₂N, MoCN₂ and MoC₂N₂), all of which can uniformly show high stability and good HER catalytic activity. Applying strain can further effectively improve the activities of C-rich (MoC₂N) and N-rich (MoCN₂) monolayers, inducing considerably high HER catalytic performance. For the MoCN, MoC₂N and MoCN₂ monolayers, the most active sites are located at the Mo-C-N chain involved. All these fascinating findings can not only provide new excellent candidates but also new insights into the design of highly efficient and nonprecious HER electrocatalysts as an alternative to Pt in the near future.

Janus MoPC Monolayer with Superior Electrocatalytic Performance for the Hydrogen Evolution Reaction

Designing the earth's abundant and high-performance electrocatalysts, which possess high stability, excellent electrical conductivity, inherent active sites, and catalytic activity identical with Pt, is challenging but crucial for the hydrogen evolution reaction (HER). By first-principles structure search simulations, we identify a new two-dimensional (2D) MoPC material with the Janus structure as a promising catalyst. This novel 2D monolayer has superior stability and metallic conductivity. Especially, it exhibits a remarkable HER catalytic activity, where all of the constituent atoms, including Mo, P, and C, can uniformly act as active sites in view of the near-zero ΔG_H* value. Its active site density counts up to 1.46 × 10¹⁵ site/cm², larger than that of many reported materials and even comparable to Pt. The excellent HER catalytic activity can also be maintained at a very high H coverage with or without external strain. The MoPC monolayer can produce H₂ spontaneously through the favorable Volmer-Heyrovsky pathway. The detailed catalytic mechanism behind the high HER activity has been also analyzed. Our work provides a feasible action for the experimental synthesis of excellent HER catalysts.

Growth mechanism, electronic properties and spectra of aluminum clusters

Density functional theory (DFT) and particle swarm optimization (PSO) have been applied to study the growth behavior, electronic properties and spectra of neutral, anionic and cationic aluminum clusters with 3-20 atoms. Many isomers have been obtained through a comprehensive structural search. The results indicate that the ground state structures of neutral and anionic aluminum clusters follow an identical periodic growth law. When the number of atoms is 6-11 and 13-18, Al atoms in these clusters grow around an octahedral cluster nucleus and an icosahedral cluster nucleus, respectively. For Al_n⁺ (n ≤ 14 and n ≠ 7) clusters, the most stable structure is different from that of Al_n or Al_n^-clusters. When n > 14, the ground state structure of Al_n⁺ clusters is similar to that of Al_n or Al_n^-clusters. The electronic properties of aluminum clusters have been analyzed by the averaged binding energy, second-order difference of energy, energy gap and dissociation energy. It is found that the Al₇⁺ and Al₁₃^- clusters have very high stability and a large energy gap and can be regarded as two superatoms. The aluminum cluster with 18 or 40 valence electrons are the least likely to lose an electron. The dissociation behavior of Al_n⁺ clusters caused by collision is reasonably explained by means of the dissociation energy. The optical absorption spectra of neutral aluminum clusters have been simulated by using the time-dependent density functional theory. The ground states of anionic aluminum clusters have been determined by comparing theoretical photoelectron spectra (PES) with experimental findings. Infrared and Raman spectra of cationic aluminum clusters have been forecasted and can assist in identifying the most stable structure in future experiments.

Route to a direct-gap silicon allotrope Si32

Using swarm-intelligence-based structure prediction methods, we predict a novel direct bandgap silicon allotrope with open channels at ambient conditions. This silicon phase, termed Si₃₂, can be produced by removing Sr atoms from a newCmcm-SrSi₈clathrate-like compound, which is calculated to be thermodynamically stable under epitaxial strain at high pressures. Si₃₂is predicted to have a direct bandgap of ∼1.15 eV and exceptional optical properties. The prediction of novel silicon clathrate-like structure paves the way for the exploration of novel silicon phases with extensive application possibilities.

First-Principles Study of High-Pressure Phase Stability and Electron Properties of Be-P Compounds

New, stable stoichiometries in Be-P systems are investigated up to 100 GPa by the CALYPSO structure prediction method. Along with the BeP₂-I4₁/amd structure, we identify two novel compounds of Be₃P₂-P-42₁m and Be₃P₂-C2/m. It should be noted that the Be-P compounds are predicted to be energetically unfavorable above 40 GPa. As can be seen, interesting structures may be experimentally synthesizable at modest pressure. Our results indicate that at 33.2 GPa, the most stable ambient-pressure tetragonal Be₃P₂-P-42₁m transitions to the monoclinic Be₃P₂-C2/m structure. Moreover, the predicted Be₃P₂-P-42₁m and Be₃P₂-C2/m phases are energetically favored compared with the Be₃P₂-Ia-3 structure synthesized experimentally. Electronic structure calculations reveal that BeP₂-I4₁/amd, Be₃P₂-P-42₁m, and Be₃P₂-C2/m are all semiconductors with a narrow band gap. The present findings offer insight and guidance for exploration toward further fundamental understanding and potential applications in the semiconductor field.

Pressure-stabilized hexafluorides of first-row transition metals

Fluorine chemistry was demonstrated to show the importance of stretching the limits of chemical synthesis, oxidation state, and chemical bonding at ambient conditions. Thus far, the highest fluorine stoichiometry of a neutral first-row transition-metal fluoride is five, in VF₅ and CrF₅. Pressure can stabilize new stoichiometric compounds that are inaccessible at ambient conditions. Here, we attempted to delineate the fluorination limits of first-row transition metals at a high pressure through first-principles swarm-intelligence structure searching simulations. Besides reproducing the known compounds, our extensive search has resulted in a plethora of unreported compounds: CrF₆, MnF₆, FeF₄, FeF₅, FeF₆, and CoF₄, indicating that the application of pressure achieves not only the fluorination limit (e.g., hexafluoride) but also the long-sought bulky tetrafluorides. Our current results provide a significant step forward towards a comprehensive understanding of the fluorination limit of first-row transition metals.

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.

Exploring the structure and electronic properties of germanium doped boron clusters using density functional theory based global optimization method

Density functional theory calculations to investigate the effect of single and double germanium atom doping on the geometric structure and electronic properties of boron clusters with 10 to 20 atoms.

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.

Janus Mo2P3 Monolayer as an Electrocatalyst for Hydrogen Evolution

The rational design of low-cost electrocatalysts with the desired performance is the core of the large-scale hydrogen production from water. Two-dimensional materials with high specific surface area and excellent electron properties are ideal candidates for electrocatalytic water splitting. Herein, we identify a hitherto unknown Mo₂P₃ monolayer with a Janus structure (i.e., out-of-plane asymmetry) through first-principle structure search calculations. Its inherent metallicity ensures good electrical conductivity. Notably, its catalytic activity is comparable to that of Pt and the density of active sites is up to 2.65 × 10¹⁵ site/cm² owing to the Mo → P charge transfer enhancing the catalytic activity of P atoms and asymmetric structure exposing more active sites to the surface. The Mo₂P₃ monolayer can spontaneously produce hydrogen through the Volmer-Heyrovsky pathway. These excellent performances can be well maintained under strain. The coexistence of covalent and ionic bonds results in Mo₂P₃ having high stability. All these excellent properties make the Mo₂P₃ monolayer a promising candidate for electrocatalytic water splitting.

Nodal-loop half metallicity in a two-dimensional Fe4N2 pentagon crystal with room-temperature ferromagnetism

Two-dimensional (2D) materials with fully spin-polarized nodal-loop band crossing are a class of topological magnetic materials, holding promise for high-speed low-dissipation spintronic devices. Recently, several 2D nodal-loop materials have been reported in theory and experiment, such as Cu₂Si, Be₂C, CuSe, and Cr₂S₃ monolayers, adopting triangular, tetragonal, hexagonal, or complex lattices. However, a 2D nodal-loop half metal with room-temperature magnetism is still less reported. Here, we report that the 2D Fe₄N₂ pentagon crystal is a nodal-loop half metal with room-temperature magnetism over 428 K and a global minimum structure via first-principles calculations and global structure search. The Dirac nodal lines in Fe₄N₂ form a flat nodal loop at the Fermi level and a spin-polarized type-II nodal-loop above the Fermi level, which are protected by mirror symmetry. Our results establish Fe₄N₂ as a platform to obtain nodal-loop half metallicity in the 2D pentagon lattice and provide opportunities to build high-speed low-dissipation spintronics in the nanoscale.

Two-Dimensional TeB Structures with Anisotropic Carrier Mobility and Tunable Bandgap

Two-dimensional (2D) semiconductors with desirable bandgaps and high carrier mobility have great potential in electronic and optoelectronic applications. In this work, we proposed α-TeB and β-TeB monolayers using density functional theory (DFT) combined with the particle swarm-intelligent global structure search method. The high dynamical and thermal stabilities of two TeB structures indicate high feasibility for experimental synthesis. The electronic structure calculations show that the two structures are indirect bandgap semiconductors with bandgaps of 2.3 and 2.1 eV, respectively. The hole mobility of the β-TeB sheet is up to 6.90 × 10² cm² V^-1 s^-1. By reconstructing the two structures, we identified two new horizontal and lateral heterostructures, and the lateral heterostructure presents a direct band gap, indicating more probable applications could be further explored for TeB sheets.

Crystal structures and superconductivity of lithium and fluorine implanted gold hydrides under high pressures

The investigations on gold science have been capturing research interest due to its diverse physical and chemical properties. Gold hydrides in the solid state, as a member of the Au compound family, are rare since the reaction of Au with H is hindered in terms of their similar electronegativity. It is expected that Li and F can provide electrons and holes, respectively, to help stabilize gold hydrides under high pressure. Herein, by means of a crystal structural search based on particle swarm optimization methodology accompanied by first-principles calculations, four hitherto unknown Li-Au-H compounds (i.e., LiAuH, LiAu₂H, Li₂Au₂H, and Li₆AuH) are predicted to be stable under compression. Intriguingly, Au-H bonding is found in LiAuH, LiAu₂H, and Li₂Au₂H. As the gold content increases, Au atom arrangements exhibit diverse forms, from the chain in Li₆AuH, the square layer in LiAuH, the network in Li₂Au₂H, and eventually to the coexistence of square and pyramid layers in LiAu₂H. Additionally, Li₆AuH has a unique cage-type lithium structure. Furthermore, electron-phonon coupling calculations show that these Li-Au-H phases are phonon-modulated superconductors with a superconducting critical temperature of 1.3, 0.06, and 0.02 K at 25 GPa and 2.79 K at 100 GPa. In contrast, we also identified two solid F₄AuH and F₆AuH phases with unexpected semiconductivity. They have structural configurations of H-bridged AuF₄ quasi-square components and distorted AuF₆ octahedrons, respectively, and have no gold-to-hydrogen bonds. Our current results indicate that electron doping at suitable concentrations under pressure can stabilize unique gold hydrides, and provide deep insights into the structures, electron properties, bonding behavior, and stability mechanism of ternary Li-Au-H and F-Au-H compounds.

Emerging Yttrium Phosphides with Tetrahedron Phosphorus and Superconductivity under High Pressures

Metal phosphides have triggered growing interest for their exotic structures and striking properties. Hence, within advanced structure search and first-principle calculations, several unprecedented Y-P compounds (e. g., Y₃ P, Y₂ P, Y₃ P₂ , Y₂ P₃ , YP₂ , and YP₃ ) were identified under compression. Interestingly, as phosphorus content increases, P atoms exhibit diverse behaviors corresponding to standalone anion, dumbbell, zigzag chain, planar sheet, crossing chain-like network, buckled layer, three-dimensional framework, and wrinkled layer. Particularly, Fd-3m YP₂ can be viewed as assemblage of diamond-like Y structure and rare vertex-sharing tetrahedral P₄  units. Impressively, electron-phonon coupling (EPC) calculations elucidate that Pm-3m Y₃ P possesses the highest superconducting critical temperature T_c of 10.2 K among binary transition metal phosphides. Remarkably, the EPC of Pm-3m Y₃ P mainly arises from the contribution of low-frequency soft phonon modes, whereas mid-frequency phonon modes of Fd-3m YP₂ dominate. These results strengthen knowledge of metal phosphides and pave a way for seeking superconductive transition metal phosphides.

A novel high-pressure phase of ScN5with higher stability predicted from first-principles calculations

For binary compounds of Sc-N, the stable structures and stoichiometries were studied from ambient condition to high pressure of 100 GPa, adopting CALYPSO method. The newly predictedP2₁/c-ScN₅compound was more energetically stable under high pressureP= 62 GPa comparing with the three previously reported phases ofP1-ScN₅,Cm-ScN₅andC2/m-ScN₅. Furthermore, the high-pressure phase ofP2₁/c-ScN₅was dynamically stable at ambient condition, so the ambient-pressure recovery is possible. In this paper, the study suggested that the energetic polynitrides can be obtained in transition metal nitrides under high pressure. And we identified one novel 3D extended puckered poly-nitrogen network in theP2₁/c-ScN₅structure, which is similar to theC2/m-ScN₅. The decomposition ofP2₁/c-ScN₅to ScN and N₂under ambient pressure was estimated to release 5.02 eV energy per formula unit (f.u.), corresponding to 4.19 kJ g^-1in energy density, which was expected to be highly exothermic. The present results can conduce to obtain more polynitrogen forms and theoretically encourages experimental discovery in these promising materials.

X-ray Free Electron Laser-Induced Synthesis of ε-Iron Nitride at High Pressures

The ultrafast synthesis of ε-Fe₃N_1+x in a diamond-anvil cell (DAC) from Fe and N₂ under pressure was observed using serial exposures of an X-ray free electron laser (XFEL). When the sample at 5 GPa was irradiated by a pulse train separated by 443 ns, the estimated sample temperature at the delay time was above 1400 K, confirmed by in situ transformation of α- to γ-iron. Ultimately, the Fe and N₂ reacted uniformly throughout the beam path to form Fe₃N_1.33, as deduced from its established equation of state (EOS). We thus demonstrate that the activation energy provided by intense X-ray exposures in an XFEL can be coupled with the source time structure to enable exploration of the time-dependence of reactions under high-pressure conditions.

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.

Exploring the structures and properties of nickel silicides at the pressures of the Earth's core

Given the highly possible existence of nickel and silicon in the Earth's core, the study of the reaction between Ni and Si and the resulting structures at the pressure corresponding to that of the Earth's core is highly required. Therefore, we have investigated the crystal structures of Ni-Si compounds at pressures of 0-350 GPa by adopting a crystal structure search algorithm in conjunction with first-principles calculations. We uncover two high Ni-content Ni5Si and Ni6Si compounds with 12-coordination Si bonded with Ni, with both showing strong chemical stability in the Earth's core. Bonding analysis reveals that the Ni atoms in these Ni-Si compounds present oxidant features and act as electron acceptors. This distinctive anomaly is the natural result of the energy shifts of the Ni 3d and Si 3p bands, resulting in charge transfer from Si to Ni. By examining the key properties (e.g., density and sound velocities) of the Ni5Si and Ni6Si compounds, the obtained density lies within the range of the Earth's inner core, and the estimated sound velocities are found to be consistent with seismic data. These results indicate that these two compounds could be considered as possible core constituents. Our findings provide valuable insights into the enigmatic Earth's core as well as geophysical and geochemical processes.

Noble Gases in Solid Compounds Show a Rich Display of Chemistry With Enough Pressure

In this review, we summarize the rapid progress that has been made in the study of noble gas chemistry in solid compounds under high pressure. Thanks to the recent development of first-principles crystal structure search methods, many new noble gas compounds have been predicted and some have been synthesized. Strikingly, almost all types of chemical roles and interactions are found or predicted in these high-pressure noble gas compounds, ranging from cationic and anionic noble gases to covalent bonds between noble gas atoms, and to hydrogen bond-like noble gas bonds. Besides, the recently discovered He insertion reactions reveal a unique chemical force that displays no local chemical bonding, providing evidence that research into noble gas reactions can advance the frontier of chemistry at the very basic level.

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.

Unraveling the local structure and luminescence evolution in Nd3+-doped LiYF4: a new theoretical approach

Neodymium ion (Nd3+)-doped yttrium lithium fluoride (LiYF4, YLF) laser crystals have shown significant prospects as excellent laser materials in many kinds of solid-state laser systems. However, the origins of the detailed information of their local structure and luminescence evolution are still poorly understood. Herein, we use an unbiased CALYPSO structure searching technique and density functional theory to study the local structure of Nd3+-doped YLF. Our results reveal a new stable phase with the P4[combining macron] (No. 81) space group for Nd3+-doped YLF, indicating that the host Y3+ ion site was naturally occupied by the Nd3+ ion impurity. On the basis of our newly developed WEPMD method, we adopt a specific type of orthogonal correlation crystal field to obtain a new set of crystal-field parameters as well as 182 complete Stark energy levels. Many absorption and emission lines for Nd3+-doped YLF are calculated and discussed based on Judd-Ofelt theory, and our results indicate that some of the observed absorption and emission lines are perfectly reproduced by our theoretical calculations. Additionally, we predict several promising transition lines in the visible and near-infrared spectral regions, including the electronic dipole emission lines 4F5/2 → 4I9/2 at 808 nm and 2H9/2 → 4I9/2 at 799 nm, as well as the magnetic dipole emission lines 4F3/2(27) → 4I11/2(6) at 1047 nm and 4F3/2(27) → 4I11/2(8) at 1052 nm. These transition channels indicate that Nd3+-doped YLF laser crystals have greatly promising laser actions for serving as a solid-state laser material.

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.

The geometrical structure and electronic properties of trivalent Ho3+ doped Y2O3 crystals: a first-principles study

Trivalent rare-earth holmium ion (Ho3+) doped yttrium oxide (Y2O3) has attracted great research interest owing to its unique optoelectronic properties and excellent performances in many new-type laser devices. But the crystal structures of the Ho3+-doped Y2O3 system (Y2O3 : Ho) are still unclear. Here, we have carried out a first-principle study on the structural evolution of the trivalent Ho3+ doped Y2O3 by using the CALYPSO structure search method. The results indicate that the lowest-energy structure of Ho3+-doped Y2O3 possesses a standardized monoclinic P2 phase. It is found that the doped Ho3+ ion are likely to occupy the sites of Y3+ in the host crystal lattice, forming the [HoO6]9- local structure with C 2 site symmetry. Electronic structure calculations reveal that the band gap value of Ho3+-doped Y2O3 is approximately 4.27 eV, suggesting the insulating character of Y2O3 : Ho system. These findings could provide fundamental insights to understand the atomic interactions in crystals as well as the information of electronic properties for other rare-earth-doped materials.

Highly Stable Two-Dimensional Iron Monocarbide with Planar Hypercoordinate Moiety and Superior Li-Ion Storage Performance

Stable planar hypercoordinate motifs have been recently demonstrated in two-dimensional (2D) confinement systems, while perfectly planar hypercoordinate motifs in 2D carbon-transition metal systems are rarely reported. Here, by using comprehensive ab initio computations, we discover two new iron monocarbide (FeC) binary sheets stabilized at 2D confined space, labeled as tetragonal-FeC (t-FeC) and orthorhombic-FeC (o-FeC), which are energetically more favorable compared with the previously reported square and honeycomb lattices. The proposed t-FeC is the global minimum configuration in the 2D space, and each carbon atom is four-coordinated with four ambient iron atoms, considered as the quasi-planar tetragonal lattice. Strikingly, the o-FeC monolayer is an orthorhombic phase with a perfectly planar pentacoordinate carbon moiety and a planar seven-coordinate iron moiety. These monolayers are the first example of a simultaneously pentacoordinate carbon and planar seven-coordinate Fe-containing material. State-of-the-art theoretical calculations confirm that all these monolayers have significantly dynamic, mechanical, and thermal stabilities. Among these two monolayers, the t-FeC monolayer shows a higher theoretical capacity (395 mAh g^-1) and can stably adsorb Li up to t-FeCLi₄ (1579 mAh g^-1). The low migration energy barrier is predicted as small as 0.26 eV for Li, which results in the fast diffusion of Li atoms on this monolayer, making it a promising candidate for lithium-ion battery material.

Crystal structure and properties of iodine monofluoride compounds at high pressure

The pressure-induced structural phase transitions of halogen compound IF under high pressure were studied by using the unbiased CALYPSO structure prediction method. The phase transition sequence of IF under high pressure is determined to be P1 → P21/c-I → P21/c-II → I4/mmm, and the corresponding phase transition pressures are 3 GPa, 16.5 GPa and 46 GPa, respectively. The physical properties of each predicted phase were thus fully studied, and it was found that the P1 phase was unstable which is in excellent agreement with the experiment result. Furthermore, our exploration of the high pressure phases for IF will provide fundamental insights for further exploration the structural phase transition of other halogen compounds under high pressure.

Structural, mechanical and electronic properties and hardness of ionic vanadium dihydrides under pressure from first-principles computations

Based on a combination of the CALYPSO method for crystal structure prediction and first-principles calculations, we explore the crystal structures of VH2 under the pressure range of 0-300 GPa. The cubic Fm-3m phase with regular VH8 cubes is predicted to transform into orthorhombic Pnma structure with fascinating distorted VH9 tetrakaidecahedrons at 47.36 GPa. Both the Fm-3m phase at 0 GPa and the Pnma phase at 100 GPa are mechanically and dynamically stable, as verified with the calculations of elastic constants and phonon dispersions, respectively. Moreover, the calculated electronic band structure and density of states indicate both stable phases are metallic. Remarkably, the analyses of the Poisson's ratio, electron localization function (ELF) and Bader charge substantiate that both stable phases are ionic crystals on account of effective charges transferring from V atom to H. On the basis of the microscopic hardness model, the Fm-3m and Pnma crystals of VH2 are potentially incompressible and hard materials with the hardness values of 17.83 and 17.68 GPa, respectively.

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.