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

Tough while Recyclable Plastics Enabled by Monothiodilactone Monomers

The current scale of plastics production and the attendant waste disposal issues represent an underexplored opportunity for chemically recyclable polymers. Typical recyclable polymers are subject to the trade-off between the monomer’s polymerizability and the polymer’s depolymerizability as well as insufficient performance for practical applications. Herein, we demonstrate that a single atom oxygen-by-sulfur substitution of relatively highly strained dilactone is an effective and robust strategy for converting the “non-recyclable” polyester into a chemically recyclable polymer by lowering the ring strain energy in the monomer (from 16.0 kcal mol–1 in dilactone to 9.1 kcal mol–1 in monothiodilactone). These monothio-modification monomers enable both high/selective polymerizability and recyclability, otherwise conflicting features in a typical monomer, as evidenced by regioselective ring-opening, minimal transthioesterifications, and quantitative recovery of the pristine monomer. Computational and experimental studies demonstrate that an n→π* interaction between the adjacent ester and thioester in the polymer backbone has been implicated in the high selectivity for propagation over transthioesterification. The resulting polymer demonstrates high performance with its mechanical properties being comparable to some commodity polyolefins. Thio-modification is a powerful strategy for enabling conversion of six-membered dilactones into chemically recyclable and tough thermoplastics that exhibit promise as next-generation sustainable polymers

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

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

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

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

Comparison between the SPEI_AJ reconstruction and dryness/wetness indices of Lanzhou.

<p>Comparison between the SPEI_AJ reconstruction and dryness/wetness indices of Lanzhou.</p

Monthly mean temperature and total precipitation values at Jingtai (1957–2013) and Jingyuan (1951–2013) meteorological stations.

<p>Monthly mean temperature and total precipitation values at Jingtai (1957–2013) and Jingyuan (1951–2013) meteorological stations.</p

Computer-Assisted Design of a Superior Be₂BO₃F Deep-Ultraviolet Nonlinear-Optical Material

Deep-ultraviolet (DUV) nonlinear-optical (NLO) materials generating coherent DUV light by a direct second-harmonic-generation (SHG) process have long been pursued as industrially useful lasers. For several decades, KBe₂BO₃F₂ (KBBF) has been regarded as the best DUV-NLO material; it is characterized by a short DUV phase-matching edge of 161 nm and a large SHG coefficient of 0.47 pm/V. However, it suffers a strong layering tendency, hindering the growth of large crystals for commercial use. Here, we use a computer-aided swarm structure searching technique to design an alternative DUV-NLO material with a new atmospheric-pressure phase Be₂BO₃F₂ with a <i>P</i>6̅2<i>c</i> space group (γ-BBF) that outperforms the DUV-NLO properties of KBBF. The predicted DUV phase-matching edge and SHG coefficient of γ-BBF are 152 nm and 0.70 pm/V, respectively. The structure of γ-BBF reduces the layering tendency compared with KBBF because of the absence of K atoms in the γ-BBF crystal. Our work paves the way for superior DUV-NLO materials that can be grown as large crystals for commercial applications