Anionic Chemistry of Noble Gases: Formation of Mg–NG (NG = Xe, Kr, Ar) Compounds under Pressure

Structural Phase Transition and Disproportionation of MnF3 Under High Pressure and High Temperature

Materials that once suffered under high-pressure and high-temperature conditions often display unusual phenomena that challenge traditional understanding. MnF3, an intermediate valence state Mn-F compound, exhibits a distorted octahedral crystal structure influenced by the Jahn-Teller effect. Here we report the structural phase transition and self-disproportionation of MnF3 under high pressure and high temperature. The initial octahedra phase I2/a-MnF3 transforms into the hendecahedra Pnma phase under high pressure. Subsequently, we found that molten Pnma-MnF3 self-disproportionate into MnF2 and MnF4 with the aid of laser heating at a pressure above 57.1 GPa. Raman spectra and UV-vis absorption experiments confirmed these changes that were ultimately confirmed by synchrotron radiation XRD. The equation of states for the volume with the pressure of these Mn-F compounds was also given. This work expands the study of Mn-F systems and provides guidance for the behavior of transition metal fluorides under high pressures and high temperatures.

Noble Gas Anions: An Overview of Strategies and Bonding Motifs

This review article aims to provide an overview of the strategies employed to prepare noble gas anions under different environments and experimental conditions, and of the bonding motifs typically occurring in these species. Observed systems include anions fixed into synthesized salts, detected in the gas phase or in high-pressure devices. The major role of the theoretical calculations is also highlighted, not only in support of the experiments, but also as effective in predicting still unreported species. The chemistry of noble gas anions overall appears as a varied and rich paint, offering fascinating opportunities for both experimentalists and theoreticians.

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.

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.

Pressure induced weakness of electrostatic interaction and solid decomposition in Cs-I compounds

This work utilized first-principles calculations and the CALYPSO structure search technique to systematically investigate the crystal structure stability of CsxIy compounds under high pressures ranging from 0 to 500 GPa. Several new phases with both conventional and unconventional stoichiometries were predicted. Interestingly, we discovered a counter-intuitive phenomenon where Cs-I compounds decompose into Cs and I elemental solids under pressure. To understand the physical mechanism behind this pressure-induced decomposition, we examine the phenomenon from two distinct perspectives: enthalpy of formation and interatomic interactions. Our results suggest that the main cause is the weakening of electrostatic interactions leading to the decomposition, while the weak covalent interaction plays a minor role. From an energy perspective, the decrease in the formation of enthalpy (ΔH) is primarily due to a reduction in the difference of internal energy (ΔU). These findings provide valuable insights into the decomposition mechanism and high-pressure properties of alkali metal halides. The counterintuitive phenomenon of high-pressure charge transfer and decomposition may inspire new ideas and perspectives in the fields of geology and the study of alkali metal halides under extreme conditions.

Emerging d-d orbital coupling between non-d-block main-group elements Mg and I at high pressure

d-d orbital coupling, which increases anisotropic and directional bonding, commonly occurs between d-block transition metals. Here, we report an unexpected d-d orbital coupling in the non-d-block main-group element compound Mg2I based on first-principles calculations. The unfilled d orbitals of Mg and I atoms under ambient conditions become part of the valence orbitals and couple with each other under high pressures, resulting in the formation of highly symmetric I-Mg-I covalent bonding in Mg2I, which forces the valence electrons of Mg atoms into the lattice voids to form interstitial quasi-atoms (ISQs). In turn, the ISQs highly interact with the crystal lattice, contributing to lattice stability. This study greatly enriches the fundamental understanding of chemical bonding between non-d-block main-group elements at high pressures.

Potential rules for stable transition metal hexafluorides with high oxidation states under high pressures

High pressure is a powerful tool in material sciences which can lead to the discovery of novel inorganic species in high oxidation states. Based on the prediction of the stability of PdF₆ with a high Pd oxidation state of +6, we propose three potential guiding rules for finding stable transition metal (TM) fluorides with high +6 oxidation states: (1) the existence of a large (>7 eV) valence orbitals energy differences of atoms between the TM d orbital and the F 2p orbital; (2) an appropriate number of valence electrons within the range of 6-11; and (3) suitable electronegativity values less than 2.3 on the Pauli scale. More importantly, by synergistically invoking all of these rules, we predict, by combining a particle swarm optimization algorithm with first-principles calculation on the phase stabilities of the various TM-F compounds, a collection of new TMF₆ species with the space group Pnma that have a +6 oxidation state. Subsequently, we develop an understanding of the high +6 oxidation state for the TM elements. These findings are expected to play a crucial role in the predictive discoveries of new fluorides with high oxidation states of +6.

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.

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.

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.

Formation of twelve-fold iodine coordination at high pressure

Halogen compounds have been studied widely due to their unique hypercoordinated and hypervalent features. Generally, in halogen compounds, the maximal coordination number of halogens is smaller than eight. Here, based on the particle swarm optimization method and first-principles calculations, we report an exotically icosahedral cage-like hypercoordinated IN6 compound composed of N6 rings and an unusual iodine-nitrogen covalent bond network. To the best of our knowledge, this is the first halogen compound showing twelve-fold coordination of halogen. High pressure and the presence of N6 rings reduce the energy level of the 5d orbitals of iodine, making them part of the valence orbital. Highly symmetrical covalent bonding networks contribute to the formation of twelve-fold iodine hypercoordination. Moreover, our theoretical analysis suggests that a halogen element with a lower atomic number has a weaker propensity for valence expansion in halogen nitrides.

Au with sp3 Hybridization in Li5AuP2

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

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.

Predicted Stable Electrides in Mg-Al System under High Pressure

Magnesium and aluminum, as the adjacent light metal elements, are difficult to form the stable stoichiometries compounds under ambient conditions. In this work, using evolutionary ab initio structural prediction approaches,...

High-Pressure Phase Diagram of the Ti-O System

Titanium oxides are technologically important compounds. The chemistry of the Ti-O system is quite rich, largely because of the multiple oxidation states that titanium atoms can take. In this work, using a combination of variable-composition evolutionary crystal structure prediction (USPEX code) and data mining (Materials Project), we predicted all of the stable titanium oxides in the pressure range 0-200 GPa and found that 27 compounds can be stable at different pressures. We resolved contradictions between previous works and predicted four hitherto-unknown stable phases: P2₁/c-TiO₃, I4/mmm-Ti₃O₂, Imm2-Ti₅O₂, and R3̅-Ti₁₂O₅. We also showed that the high-pressure P6̅m2-TiO phase is an electride.

Electron-Deficient-Type Electride Ca5Pb3: Extension of Electride Chemical Space

Electrides have been identified so far by two major routes: one is conversion of elemental metals and stoichiometric compounds by high pressure; the other is to search for electron-rich compounds, and this approach is more general. In contrast, few electron-deficient structures in existing databases have been revealed as potential electride candidates. In this work, we found an electron-deficient compound Ca₅Pb₃ could be transformed into electrides upon applying external pressure or strain along the c-axis, which induces the electron immigration from Pb to interstitial sites. Furthermore, the electron doping via Hf substitution of Ca atoms for Ca₅Pb₃ was found to be capable of tuning the interstitial electron density under ambient pressure, resulting in a new stable ternary electride Ca₃Hf₂Pb₃, Hf-substituted Ca₅Pb₃. The electron-deficient electride discovered here is of novel type and can largely expand the research scope of electrides. Considering a recently reported neutral electride Na₃N and the present finding, it is now clarified that electrides can be identified irrespective of stoichiometry (electron-rich, -neutral, or -poor) for compounds.

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.

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.

Predicted stable Li5P2 and Li4P at ambient pressure: novel high-performance anodes for lithium-ion batteries

Lithium-rich phosphides have recently attracted considerable attention due to their potential application as high-capacity and high-rate anodes for lithium-ion batteries (LIBs). However, there is still short of the promising candidate thus far because of the poor electrical conductivity or huge volume change in the already known Li-P compounds. In this work, we report two novel Li-P states, Li₅P₂ and Li₄P, stabilized under high pressures that are predicted to be quenchable down to ambient conditions by first-principles swarm structure calculations. The predicted P3m1 Li₅P₂ shows interesting features as a p-type semiconductor with an indirect band gap of 0.787 eV, possessing significant anisotropy properties in electrical transport, while R3[combining macron]m Li₄P acts as a typical electride with metallic behavior at pressures of 0-82 GPa. More importantly, our calculations reveal that the theoretical capacities of Li₅P₂ and Li₄P are predicted to reach 2164 and 3462 mA h g^-1, respectively. Combined with the good electrical transport properties, the calculated volume expansion of Li₅P₂ (130%) is found to be much smaller than those of the previously reported Li-P compounds, indicating its potential as a high performance anode material for LIBs.

Pressure-Stabilized Zinc Trifluoride

By combining the particle swarm optimization algorithm with first-principles calculation, the high-pressure phase diagram of Zn-F binary compounds was established. An unexpected stoichiometry of ZnF₃ with space group Cccm is thermodynamically stable above 183 GPa. The new structure is fascinating with the appearance of Zn²⁺[F₃]^2- units. The stability of the new phase stems from the mixed ionic and covalent chemical bonding in ZnF₃. The electronic properties indicate that Zn has a tendency to form high oxidation states under higher pressure. Our work is an important step in understanding the bonding behavior of Zn under extreme conditions and provides a valuable reference for experimental synthesis and identification of ZnF₃.

Pressure-induced unexpected -2 oxidation states of bromine and superconductivity in magnesium bromide

Finding novel compounds with unusual crystal structures and physical properties is always an important goal for the materials and chemistry community. Pressure becomes attractive due to its unique ability to break down many fundamental rules by modifying the chemical properties of elements, overcoming reaction barriers and shortening interatomic distances, leading to the formation of some novel materials with unexpected properties. In this work, for the first time we have analyzed the high-pressure phase diagram, crystal structures and electron properties of the Mg-Br system up to 200 GPa using unbiased structure searching techniques. Besides the already known MgBr₂, here we report that three unusual stoichiometries of Mg-Br compounds can be stabilized at high pressures as MgBr₃, MgBr and Mg₄Br. Firstly, among the predicted stable compounds, we find that the Mg₄Br in the I4/mmm structure stabilized at 178 GPa behaves as a typical electride, indicating that the formation pressure of an electride for Mg can be significantly reduced by bonding with Br atoms. Secondly, it is surprising that the unexpected oxidation states of Br approaching -2 are observed in the predicted I4/mmm Mg₄Br and Pm3[combining macron]m MgBr compounds. Furthermore, P2₁/m MgBr₃ and I4[combining macron]2m MgBr₃ phases are predicted as superconductors with an estimated T_c of 23.2 and 0.49 K, respectively. Our work represents a significant step toward understanding the high pressure behaviors of alkaline earth halides and searching for novel high temperature superconductors.

OBCN isomerization and noble gas insertion compounds of identical valence electron number species: stability and bonding

A series of new noble gas (Ng) insertion compounds of the general type XNgX, XNgY and XNgY+ has been theoretically studied using ab initio and DFT methods herein. We first studied the isomerization process of the OBCN compound, and then investigated the bonding properties and stability of the compounds formed by inserting Ng into the single bond of the three low energy isomers by high-level ab initio calculations. The OBNgCN compounds are thermochemically stable with respect to all dissociation channels except for the processes of releasing OBCN/OBNC and free Ng. Furthermore, the two dissociation processes OBNgCN → Ng + OBNC and OBNgNC → Ng + OBCN are kinetically prohibited by the relatively high free energy barrier ranging from 22.7 to 31.7 kcal mol-1 except for the OBKrCN and OBKrNC analogues. And the adaptive natural density partitioning (AdNDP) analysis indicated that chemical bonding in OBNgCN compounds is realized via a delocalized 3-center 2-electron (3c-2e) σ-bond in the B-Ng-C moiety and a totally delocalized 5-center 2-electron (5c-2e) σ-bond in the whole O-B-Ng-C-N. Natural bond orbital (NBO) theory, atoms-in-molecules (AIM) and energy decomposition analysis (EDA) based on the molecular wavefunction revealed that the B-Ng bond and Ng-C bond have some covalent character in OBNgCN. In addition, the calculation and detailed bonding analysis on a large number of neutral and monocationic compounds with identical valence electron numbers to OBNgCN demonstrate that the two bonds directly linked to the Ng atoms have covalent properties in neutral compounds, whereas Ng forms one typical covalent bond and one partial covalent and partial ionic bond with the neighboring atoms in the monocationic compounds.

Prediction of the Reactivity of Argon with Xenon under High Pressures

Pressure significantly modifies the microscopic interactions in the condense phase, leading to new patterns of bonding and unconventional chemistry. Using unbiased structure searching techniques combined with first-principles calculations, we demonstrate the reaction of argon with xenon at a pressure as low as 1.1 GPa, producing a novel van der Waals compound XeAr₂. This compound is a wide-gap insulator and crystallizes in a MgCu₂-type Laves phase structure. The calculations of phonon spectra and formation enthalpy indicate that XeAr₂ would be stable without any phase transition or decomposition at least up to 500 GPa.

Squeezing All Elements in the Periodic Table: Electron Configuration and Electronegativity of the Atoms under Compression

We present a quantum mechanical model capable of describing isotropic compression of single atoms in a non-reactive neon-like environment. Studies of 93 atoms predict drastic changes to ground-state electronic configurations and electronegativity in the pressure range of 0-300 GPa. This extension of atomic reference data assists in the working of chemical intuition at extreme pressure and can act as a guide to both experiments and computational efforts. For example, we can speculate on the existence of pressure-induced polarity (red-ox) inversions in various alloys. Our study confirms that the filling of energy levels in compressed atoms more closely follows the hydrogenic aufbau principle, where the ordering is determined by the principal quantum number. In contrast, the Madelung energy ordering rule is not predictive for atoms under compression. Magnetism may increase or decrease with pressure, depending on which atom is considered. However, Hund's rule is never violated for single atoms in the considered pressure range. Important (and understandable) electron shifts, s→p, s→d, s→f, and d→f are essential chemical and physical consequences of compression. Among the specific intriguing changes predicted are an increase in the range between the most and least electronegative elements with compression; a rearrangement of electronegativities of the alkali metals with pressure, with Na becoming the most electropositive s¹ element (while Li becomes a p group element and K and heavier become transition metals); phase transitions in Ca, Sr, and Ba correlating well with s→d transitions; spin-reduction in all d-block atoms for which the valence d-shell occupation is d ⁿ (4 ≤ n ≤ 8); d→f transitions in Ce, Dy, and Cm causing Ce to become the most electropositive element of the f-block; f→d transitions in Ho, Dy, and Tb and a s→f transition in Pu. At high pressure Sc and Ti become the most electropositive elements, while Ne, He, and F remain the most electronegative ones.

Predicted Pressure-Induced Superconducting Transition in Electride Li_{6}P

Electrides are unique compounds where most of the electrons reside at interstitial regions of the crystal behaving as anions, which strongly determines its physical properties. Interestingly, the magnitude and distribution of interstitial electrons can be effectively modified either by modulating its chemical composition or external conditions (e.g., pressure). Most of the electrides under high pressure are nonmetallic, and superconducting electrides are very rare. Here we report that a pressure-induced stable Li_{6}P electride, identified by first-principles swarm structure calculations, becomes a superconductor with a predicted superconducting transition temperature T_{c} of 39.3 K, which is the highest among the already known electrides. The interstitial electrons in Li_{6}P, with dumbbell-like connected electride states, play a dominant role in the superconducting transition. Other Li-rich phosphides, Li_{5}P, Li_{11}P_{2}, Li_{15}P_{2}, and Li_{8}P, are also predicted to be superconducting electrides, but with a lower T_{c}. Superconductivity in all these compounds can be attributed to a combination of a weak electronegativity of phosphorus (P) with a strong electropositivity of lithium (Li), and opens up the interest to explore high-temperature superconductivity in similar binary compounds.

How and Why Does Helium Permeate Nonporous Arsenolite Under High Pressure?

Investigations into the helium permeation of arsenolite, the cubic, molecular arsenic(III) oxide polymorph As₄ O₆ , were carried out to understand how and why arsenolite helium clathrate As₄ O₆ ⋅2 He is formed. High-pressure synchrotron X-ray diffraction experiments on arsenolite single crystals revealed that the permeation of helium into nonporous arsenolite depends on the time for which the crystal is subjected to high pressure and on the crystal history. The single crystal was totally transformed into As₄ O₆ ⋅2 He within 45 h under 5 GPa. After release of the pressure, arsenolite was recovered and a repeated increase in pressure up to 3 GPa led to practically instant As₄ O₆ ⋅2 He formation. However, when a pristine arsenolite single crystal was quickly subjected to a pressure of 13 GPa, no helium permeation was observed at all. No neon permeation was observed in analogous experiments. Quantum mechanical computations indicate that there are no specific attractive interactions between He atoms and As₄ O₆ molecules at the distances observed in the As₄ O₆ ⋅2 He crystal structure. Detailed analysis of As₄ O₆ molecular structure changes has shown that the introduction of He into the arsenolite crystal lattice significantly reduces molecular deformations by decreasing the anisotropy of stress exerted on the As₄ O₆ molecules. This effect and the pΔV term, rather than any specific As⋅⋅⋅He binding, are the driving forces for the formation As₄ O₆ ⋅2 He.

Crystal Structures and Electronic Properties of Oxygen-rich Titanium Oxides at High Pressure

Pressure is well-known to significantly change the bonding patterns of materials and lift the reactivity of elements, leading to the synthesis of unconventional compounds with fascinating properties. Titanium-oxygen (Ti-O) compounds (e.g., TiO₂) are attracting increasing attention due to their attractive electronic properties and extensive industrial applications (e.g., photocatalysis and solar cells). Using the effective CALYPSO structure searching method combined with first-principles calculations, we theoretically explored various oxygen-rich Ti-O compounds at pressures ranging from 0 to 200 GPa. Our results revealed, unexpectedly, that pressure stabilizes two hitherto unknown stoichiometric oxygen-rich Ti₂O₅ and TiO₃ compounds. Ti₂O₅ crystallized in P-42₁ c structure, whose remarkable feature is that it contains a peroxide group (O2^2-) with an O-O distance of 1.38 Å at 150 GPa. The trioxide TiO₃ is an ionic metal and is the oxygen-richest compound known thus far in the Ti-O system. It adopts a high symmetry (space group Pm-3 n) structure consisting of a 12-fold coordinated face-sharing TiO₁₂ icosahedron, where Ti has the highest coordination number with O among all Ti-O structures. The underlying mechanisms for the stabilization of Ti₂O₅ and TiO₃ lie in the higher coordination number and denser structure packing. Our current results unravel the unusual oxygen-rich stoichiometry of Ti-O compounds and provide further insight into the diverse electronic properties of Ti oxides under high pressure.

Reactivity of He with ionic compounds under high pressure

Until very recently, helium had remained the last naturally occurring element that was known not to form stable solid compounds. Here we propose and demonstrate that there is a general driving force for helium to react with ionic compounds that contain an unequal number of cations and anions. The corresponding reaction products are stabilized not by local chemical bonds but by long-range Coulomb interactions that are significantly modified by the insertion of helium atoms, especially under high pressure. This mechanism also explains the recently discovered reactivity of He and Na under pressure. Our work reveals that helium has the propensity to react with a broad range of ionic compounds at pressures as low as 30 GPa. Since most of the Earth’s minerals contain unequal numbers of positively and negatively charged atoms, our work suggests that large quantities of He might be stored in the Earth’s lower mantle.

Helium was long thought to be unable to form stable solid compounds, until a recent discovery that helium reacts with sodium at high pressure. Here, the authors demonstrate the driving force for helium reactivity, showing that it can form new compounds under pressure without forming any local chemical bonds.

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

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

Cesium's Off-the-Map Valence Orbital

The Td -symmetric [CsO4 ]+ ion, featuring Cs in an oxidation state of 9, is computed to be a minimum. Cs uses outer core 5s and 5p orbitals to bind the oxygen atoms. The valence Cs 6s orbital lies too high to be involved in bonding, and contributes to Rydberg levels only. From a molecular orbital perspective, the bonding scheme is reminiscent of XeO4 : an octet of electrons to bind electronegative ligands, and no low-lying acceptor orbitals on the central atom. In this sense, Cs+ resembles hypervalent Xe.

Phase diagram, stability and electronic properties of an Fe–P system under high pressure: a first principles study

The phase diagram and relative stabilities of Fe–P binary compounds are reliably determined at pressures of up to 400 GPa.

Noble gas bond and the behaviour of XeO3under pressure

The covalent Xe–O bond lengths in XeO₃are elongated upon increasing the pressure, which is similar to the change observed with hydrogen bonds under pressure. Moreover, XeO₃rearranges in a highly-ordered manner by O hopping at about 2 GPa, which is analogous to the proton hopping observed among hydrogen bonds.

Unexpected Trend in Stability of Xe-F Compounds under Pressure Driven by Xe-Xe Covalent Bonds

Xenon difluoride is the first and the most stable of hundreds of noble-gas (Ng) compounds. These compounds reveal the rich chemistry of Ng's. No stable compound that contains a Ng-Ng bond has been reported previously. Recent experiments have shown intriguing behaviors of this exemplar compound under high pressure, including increased coordination numbers and an insulator-to-metal transition. None of the behaviors can be explained by electronic-structure calculations with fixed stoichiometry. We therefore conducted a structure search of xenon-fluorine compounds with various stoichiometries and studied their stabilities under pressure using first-principles calculations. Our results revealed, unexpectedly, that pressure stabilizes xenon-fluorine compounds selectively, including xenon tetrafluoride, xenon hexafluoride, and the xenon-rich compound Xe₂F. Xenon difluoride becomes unstable above 81 GPa and yields metallic products. These compounds contain xenon-xenon covalent bonds and may form intercalated graphitic xenon lattices, which stabilize xenon-rich compounds and promote the decomposition of xenon difluoride.

Formation of xenon-nitrogen compounds at high pressure

Molecular nitrogen exhibits one of the strongest known interatomic bonds, while xenon possesses a closed-shell electronic structure: a direct consequence of which renders both chemically unreactive. Through a series of optical spectroscopy and x-ray diffraction experiments, we demonstrate the formation of a novel van der Waals compound formed from binary Xe-N₂ mixtures at pressures as low as 5 GPa. At 300 K and 5 GPa Xe(N₂)₂-I is synthesised, and if further compressed, undergoes a transition to a tetragonal Xe(N₂)₂-II phase at 14 GPa; this phase appears to be unexpectedly stable at least up to 180 GPa even after heating to above 2000 K. Raman spectroscopy measurements indicate a distinct weakening of the intramolecular bond of the nitrogen molecule above 60 GPa, while transmission measurements in the visible and mid-infrared regime suggest the metallisation of the compound at ~100 GPa.

Iodine Anions beyond -1: Formation of LinI (n = 2-5) and Its Interaction with Quasiatoms

Novel phases of LinI (n = 2, 3, 4, 5) compounds are predicted to form under high pressure using first-principles density functional theory and an unbiased crystal structure search algorithm. All of the phases identified are thermodynamically stable with respect to decomposition into elemental Li and the binary LiI at a relatively low pressure (≈20 GPa). Increasing the pressure to 100 GPa yields the formation of a high pressure electride where electrons occupy interstitial quasiatom (ISQ) orbitals. Under these extreme pressures, the calculated charge on iodine suggests the oxidation state goes beyond the conventional and expected -1 charge for the halogens. This strange oxidative behavior stems from an electron transfer going from the ISQ to I(-) and Li(+) ions as high pressure collapses the void space. The resulting interplay between chemical bonding and the quantum chemical nature of enclosed interstitial space allows this first report of a halogen anion beyond a -1 oxidation state.