A Stable, Magnetic, and Metallic Li₃O₄ Compound as a Discharge Product in a Li–Air Battery

The Li–air battery with the specific energy exceeding that of a Li ion battery has been aimed as the next-generation battery. The improvement of the performance of the Li–air battery needs a full resolution of the actual discharge products. Li₂O₂ has been long recognized as the main discharge product, with which, however, there are obvious failures on the understanding of various experimental observations (e.g., magnetism, oxygen K-edge spectrum, etc.) on discharge products. There is a possibility of the existence of other Li–O compounds unknown thus far. Here, a hitherto unknown Li₃O₄ compound as a discharge product of the Li–air battery was predicted through first-principles swarm structure searching calculations. The new compound has a unique structure featuring the mixture of superoxide O₂^– and peroxide O₂^2–, the first such example in the Li–O system. The existence of superoxide O₂^– creates magnetism and hole-doped metallicity. Findings of Li₃O₄ gave rise to direct explanations of the unresolved experimental magnetism, triple peaks of oxygen K-edge spectra, and the Raman peak at 1125 cm^–1 of the discharge products. Our work enables an opportunity for the performance of capacity, charge overpotential, and round-trip efficiency of the Li–air battery

Robust Diffusive Proton Motions in Phase IV of Solid Hydrogen

Systematic first-principles molecular dynamics (MD) simulations with long simulation times (7–13 ps) for phase IV of solid hydrogen using different supercell sizes of 96, 288, 576, and 768 atoms established that the diffusive proton motions process in the graphene-like layer is an intrinsic property and independent of the simulation cell sizes. The present study highlights an often overlook issue in first-principles calculations that long time MD is essential to achieve ergodicity, which is mandatory for a proper description of dynamics of a system. It is inappropriate to make arguments on the analysis of MD results, which are far from ergodic. Furthermore, we have simulated the vibrational density of states of phase IV based on our proton diffuse model at a pressure range of 225–300 GPa, which is qualitatively in agreement with experimental data

Barium in High Oxidation States in Pressure-Stabilized Barium Fluorides

The oxidation state of an element influences its chemical behavior of reactivity and bonding. Finding unusual oxidation state of elements is a theme of eternal pursuit. As labeled by an alkali-earth metal, barium (Ba) invariably exhibits an oxidation state of +2 by a loss of two 6s valence electrons while its inner 5p closed shell is known to remain intact. Here, we show through the reaction with fluorine (F) at high pressure that Ba exhibits a hitherto unexpected high oxidation state greater than +2 in three pressure-stabilized F-rich compounds BaF₃, BaF₄, and BaF₅, where Ba takes on the role of a 5p element by opening up its inert 5p shell. Interestingly enough, these pressure-stabilized Ba fluorides share common structural features of Ba-centered polyhedrons but exhibit a diverse variety of electronic properties showing semiconducting, metallic, and even magnetic behaviors. Our work modifies the traditional knowledge on the chemistry of alkali-earth Ba element established at ambient pressure and highlights the major role of pressure played in tuning the oxidation state of elements

Silicon Framework-Based Lithium Silicides at High Pressures

The bandgap and optical properties of diamond silicon (Si) are not suitable for many advanced applications such as thin-film photovoltaic devices and light-emitting diodes. Thus, finding new Si allotropes with better bandgap and optical properties is desirable. Recently, a Si allotrope with a desirable bandgap of ∼1.3 eV was obtained by leaching Na from NaSi₆ that was synthesized under high pressure [<i>Nat. Mater.</i> <b>2015</b>, <i>14</i>, 169], paving the way to finding new Si allotropes. Li is isoelectronic with Na, with a smaller atomic core and comparable electronegativity. It is unknown whether Li silicides share similar properties, but it is of considerable interest. Here, a swarm intelligence-based structural prediction is used in combination with first-principles calculations to investigate the chemical reactions between Si and Li at high pressures, where seven new compositions (LiSi₄, LiSi₃, LiSi₂, Li₂Si₃, Li₂Si, Li₃Si, and Li₄Si) become stable above 8.4 GPa. The SiSi bonding patterns in these compounds evolve with increasing Li content sequentially from frameworks to layers, linear chains, and eventually isolated Si ions. Nearest-neighbor Si atoms, in <i>Cmmm</i>-structured LiSi₄, form covalent open channels hosting one-dimensional Li atom chains, which have similar structural features to NaSi₆. The analysis of integrated crystal orbital Hamilton populations reveals that the SiSi interactions are mainly responsible for the structural stability. Moreover, this structure is dynamically stable even at ambient pressure. Our results are also important for understanding the structures and electronic properties of LiSi binary compounds at high pressures

Gold as a 6p-Element in Dense Lithium Aurides

The negative oxidation state of gold (Au) has drawn a great attention due to its unusual valence state that induces exotic properties in its compounds, including ferroelectricity and electronic polarization. Although monatomic anionic gold (Au^–) has been reported, a higher negative oxidation state of Au has not been observed yet. Here we propose that high pressure becomes a controllable method for preparing high negative oxidation state of Au through its reaction with lithium. First-principles calculations in combination with swarm structural searches disclosed chemical reactions between Au and Li at high pressure, where stable Li-rich aurides with unexpected stoichiometries (e.g., Li₄Au and Li₅Au) emerge. These compounds exhibit intriguing structural features like Au-centered polyhedrons and a graphene-like Li sublattice, where each Au gains more than one electron donated by Li and acts as a 6p-element. The high negative oxidation state of Au has also been achieved through its reactions with other alkali metals (e.g., Cs) under pressures. Our work provides a useful strategy for achieving diverse Au anions

Gold with +4 and +6 Oxidation States in AuF₄ and AuF₆

An important goal in chemistry is to prepare compounds with unusual oxidation states showing exciting properties. For gold (Au), the relativistic expansion of its 5d orbitals makes it form high oxidation state compounds. Thus far, the highest oxidation state of Au known is +5. Here, we propose high pressure as a controllable method for preparing +4 and +6 oxidation states in Au via its reaction with fluorine. First-principles swarm-intelligence structure search identifies two hitherto unknown stoichiometric compounds, AuF₄ and AuF₆, exhibiting typical molecular crystal character. The high-pressure phase diagram of Au fluorides is rather different from Cu or Ag fluorides, which is indicated by stable chemical compositions and the pressures needed for the synthesis of these compounds. This difference can be associated with the stronger relativistic effects in Au relative to Cu or Ag. Our work represents a significant step forward in a more complete understanding of the oxidation states of Au

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

Pressure-Stabilized Semiconducting Electrides in Alkaline-Earth-Metal Subnitrides

High pressure is able to modify profoundly the chemical bonding and generate new phase structures of materials with chemical and physical properties not accessible at ambient conditions. We here report an unprecedented phenomenon on the pressure-induced formation of semiconducting electrides via compression of layered alkaline-earth subnitrides Ca₂N, Sr₂N, and Ba₂N that are conducting electrides with loosely confined electrons in the interlayer voids at ambient pressure. Our extensive first-principles swarm structure searches identified the high-pressure semiconducting electride phases of a tetragonal <i>I</i>4̅2<i>d</i> structure for Ca₂N and a monoclinic <i>Cc</i> structure shared by Sr₂N and Ba₂N, both of which contain atomic-size cavities with paring electrons distributed within. These electride structures are validated by the excellent agreement between the simulated X-ray diffraction patterns and the experimental data available. We attribute the emergence of the semiconducting electride phases to the p<i>–</i>d hybridization on alkaline-earth-metal atoms under compression as well as the filling of the p<i>–</i>d hybridized band due to the interaction between Ca and N. Our work provides a unique example of pressure-induced metal-to-semiconductor transition in compound materials and reveals unambiguously the electron-confinement topology change between different types of electrides

Predicted Lithium–Boron Compounds under High Pressure

High pressure can fundamentally alter the bonding patterns of light elements and their compounds, leading to the unexpected formation of materials with unusual chemical and physical properties. Using an unbiased structure search method based on particle-swarm optimization algorithms in combination with density functional theory calculations, we investigate the phase stabilities and structural changes of various Li–B systems on the Li-rich regime under high pressures. We identify the formation of four stoichiometric lithium borides (Li₃B₂, Li₂B, Li₄B, and Li₆B) having unforeseen structural features that might be experimentally synthesizable over a wide range of pressures. Strikingly, it is found that the B–B bonding patterns of these lithium borides evolve from graphite-like sheets in turn to zigzag chains, dimers, and eventually isolated B ions with increasing Li content. These intriguing B–B bonding features are chemically rationalized by the elevated B anionic charges as a result of Li→B charge transfer

Nonmetallic FeH₆ under High Pressure

High pressure induces unexpected chemical and physical properties in materials. For example, hydrogen-rich compounds under pressure have recently gained much attention as potential room-temperature superconductors, and iron hydrides have also gained significant interest as potential candidates for being the main constituents of the Earth’s core. It is well-known that pressure induces insulator-to-metal transitions, whereas pressure-induced metal-to-insulator transitions are rare, especially for transition metal hydrides. In this article, we have extensively explored the structural phase diagram of iron hydrides by using ab initio particle swarm optimization. We have found a new stable stoichiometry, FeH₆, above 213.7 GPa with <i>C</i>2/<i>c</i> symmetry. Interestingly, <i>C</i>2/<i>c</i> FeH₆ presents an unexpected nonmetallicity, and its band gap becomes larger with increasing pressure. This is in sharp contrast with <i>P</i>2₁/<i>m</i> FeH₄. The nonmetallicity of <i>C</i>2/<i>c</i> FeH₆ mainly originates from the pressure-induced hybridization between the Fe and H orbitals. This new compound shows a unique structure with a mixture of nonbonded hydrogen atoms in a helical iron framework. The strong Fe–Fe interaction and ionic Fe–H bonds are responsible for its structural stability. In addition, we have also found a more stable tetragonal FeH₂ structure with the same <i>I</i>4/<i>mmm</i> symmetry as the previously proposed one, the X-ray diffraction pattern of which perfectly agrees with that of the experiment