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Wehave investigated the chiroptical, linear, and second-ordernonlinear optical (NLO) properties of five 1,1,4,4-tetracyanobuta-1,3-diene(TCBD) derivatives and elucidated structure–property relationshipsfrom the micromechanism. The experimental UV–vis absorptionand circular dichroism (CD) spectra were well reproduced by our calculationsat TDB3LYP/6-31+G* level of theory. The electron transition propertyand chiroptical origin have been assigned and analyzed. The resultsshow that the studied compounds possess large molecular first hyperpolarizabilities,especially for compound 5which has a value of 35 ×10–30esu, which is comparable with the measuredvalue for highly π-delocalized phenyliminomethyl ferrocene complexand about 200 times larger than the average first hyperpolarizabilityof the organic urea molecule. Despite the nonplanarity of these compounds,efficient intramolecular charge transfer (CT) from electron donorto electron acceptor moieties was observed, which plays the key rolein determining the NLO response. The intramolecular charge transfercooperativity was also probed. In view of the first hyperpolarizabilityvalues, intrinsic noncentrosymmetric electronic structure, and highstability, the studied compounds have the possibility to be excellentsecond-order NLO materials. [ABSTRACT FROM AUTHOR]

An electride, characterized by unique interstitial anionic electrons (IAEs), offers promising avenues for modulating its superconductivity. The pressure-dependent coupling between IAEs and orbital electrons significantly affects the superconducting transition temperature ( T c ). However, existing research has predominantly concentrated on pressures within 300 GPa. To advance the understanding, we propose investigating the Li-S system under ultrahigh pressure to unveil novel electride superconductors. Five stable Li-rich electrides with diverse IAE topologies, including one Li 7 S, three Li 9 S, and one Li 12 S phases, are identified through structural search calculations. Among the Li 9 S phases, in the C 2/ c phase (600 GPa), the IAEs are connected to the S atomic extra-nuclear electrons with the unconventional d orbital attribute due to the extreme pressure, while two low-pressure R -3 (25 GPa) and C 2/ m (400 GPa) phases have interconnected IAEs. Due to its unique IAE attributes, C 2/ c Li 9 S exhibits the highest T c of 53.29 K at 600 GPa. Its superconductivity results from the coupling of the S d , Li p electrons, and IAEs with the low-frequency phonons associated with the attraction between IAEs and the Li-S framework. Our work enhances insights into IAEs within electrides and their role in facilitating superconductivity.

Advancing the compositional space of a compound class can result in intriguing superconductors, such as LaH 10 . Herein, we performed a comprehensive first-principles structural search on a binary B-C system with various chemical compositions. The identified diamond-like BC 15 , named d -BC 15 , is thermodynamically superior to the synthesized BC 3 and BC 5 . Interestingly, d -BC 15 shows anisotropic superconductivity resulting from three distinct Fermi surfaces. Its predicted critical temperature ( T c ) is 43.6 K at ambient pressure, beyond the McMillan limit. d -BC 15 reaches a maximum of around 75 K at 0.43% hole doping due to the substantially enhanced density of states at the Fermi level. Additionally, d -BC 15 demonstrates superhard characteristics with a Vickers hardness of 75 GPa. The calculated tensile and shear stresses are 72 and 73 GPa, respectively. The combination of high superconductivity and superhardness in d -BC 15 offers new insights into the design of multifunctional materials.

For hydride superconductors, each significant advance is built upon the discovery of novel H-based structural units, which in turn push the understanding of the superconducting mechanism to new heights. Based on first-principles calculations, we propose a metastable LiH 4 with a wavy H layer composed of the edge-sharing pea-like H 18 rings at high pressures. Unexpectedly, it exhibits pressure-insensitive superconductivity manifested by an extremely small pressure coefficient (d T c /d P ) of 0.04 K/GPa. This feature is attributed to the slightly weakened electron-phonon coupling with pressure, caused by the reduced charge transfer from Li atoms to wavy H layers, significantly suppressing the substantial increase in the contribution of phonons to T c . Its superconductivity originates from the strong coupling between the H 1s electrons and the high-frequency phonons associated with the H layer. Our study extends the list of H-based structural units and enhances the in-depth understanding of pressure-related superconductivity.

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 (BSi 5 , BSi 4 , and BSi 3 ) 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 BSi 5 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 2p z 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 2p z 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.

TeO 2 glass has been studied by Raman spectroscopy up to the record pressure of 70 GPa. The boson peak frequency ω b exhibits a decrease of the ∂ω b /∂ P slope at 5-6 GPa and saturates above 30 GPa with a practically constant value up to 70 GPa. Experiment and theory indicate that pressures up to 20 GPa induce the transformation of single Te-O-Te bridges to double Te-O 2 -Te bridges, leading to a more compact structure, while Raman activity developing at higher pressures around 580 cm -1 signals the increase of Te coordination from 4- to 6-fold. Natural bond orbital analysis shows that double Te-O 2 -Te bridges favor the s → d transition and promote the increase of Te coordination through d 2 sp 3 hybridization. This transition leads to the formation of TeO 6 octahedra, in strict difference with crystalline TeO 2 at the same pressure range, and to the development of a 3D network that freezes the medium range order.

K-ion batteries (KIBs) have been considered as appealing alternatives to Li ion batteries due to the high abundance of K, their high working voltages, and allowing the use of mature LIB technology. Thus far, anode materials that can meet the rigorous requirements of KIBs are still rather rare. Here, we have identified a desirable anode material, a metallic C 3 NS monolayer with high stability, a high storage capacity of 980 mAh/g, a low diffusion barrier of 0.24 eV, and a low open-circuit voltage of 0.36 V, through first-principles calculations. Metallic C 3 NSK n ( n = 1-3) can ensure a high electron conductivity during the charge/discharge process. Valence electrons of the N atom in a triangular bipyramid configuration favor the formation of a planar edge-sharing hexagonal C 4 N 2 unit and delocalized π bonding with C 2p electrons. The lone pair electrons of the S atom induce strong interactions with K atoms, facilitating storage capacity. These interesting properties make the C 3 NS monolayer a promising anode for KIBs.

The rational design of ferromagnetic materials is crucial for the development of spintronic devices. Using first-principles structural search calculations, we have identified 73 two-dimensional transition metal hydrides. Some of them show interesting magnetic properties, even when combined with the characteristics of the electrides. In particular, the P 3̅ m 1 Fe 2 H monolayer is stabilized in a 1T-MoS 2 -type structure with a local magnetic moment of 3 μ B per Fe atom, whose robust ferromagnetism is attributed to the exchange interaction between neighboring Fe atoms within and between sublayers, leading to a remarkably high Curie temperature of 340 K. On the other hand, it has a large magnetic anisotropic energy and spin-polarization ratio. Interestingly, the above room-temperature ferromagnetism of the Fe 2 H monolayer is well preserved within a biaxial strain of 5%. The structure and electron property of surface-functionalized Fe 2 H are also explored. All of these interesting properties make the Fe 2 H monolayer an attractive candidate for spintronic nanodevices.

Photocatalytic water splitting is a promising method that uses sunlight to generate hydrogen from water to provide clean and renewable energy resources. Two-dimensional materials with abundant active sites are ideal candidates for achieving this goal; however, few of the known ones can meet the rigorous requirement of photocatalytic water splitting. By using first-principles swarm-intelligence search calculations, we have successfully identified two new semiconducting SiCP 2 and SiCP 4 monolayers. Their band-edge heights evidently straddle the redox potentials of water. For the more prominent SiCP 4 monolayer, additional external biases of 0.32 V for water oxidation and 0.03 V for the hydrogen reduction half-reaction would be enough to drive its reaction sequences at pH 0, and it can spontaneously proceed to the water oxidation half-reaction in a neutral solution. Interestingly, the excellent optical absorbance ability (∼10 4 cm -1 ) and high carrier mobility (∼10 5 cm 2 V -1 s -1 ) of SiCP 2 and SiCP 4 facilitate the utilization of sunlight and the fast transportation of photogenerated carriers. All of these properties make SiCP 2 and SiCP 4 monolayers promising candidates for applications in photocatalytic water splitting.

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 Li 5 AuP 2 , identified by a swarm intelligence-based structural prediction, becomes the first example of Au with sp 3 hybridization. The most remarkable feature of Li 5 AuP 2 is that it contains various frameworks made by AuP 4 , AuLi 4 , LiP 4 , 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 Li 5 AuP 2 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.

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 Mo 2 P 3 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 × 10 15 site/cm 2 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 Mo 2 P 3 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 Mo 2 P 3 having high stability. All these excellent properties make the Mo 2 P 3 monolayer a promising candidate for electrocatalytic water splitting.

Phosphorene has offered an additional advantage for developing new optoelectronic devices due to its anisotropic and high carrier mobility. However, its instability in air causes a rapid degradation of the performance of the device. Thus, improving the stability of phosphorene while maintaining its original properties has become the key to the development of high-performance electronic devices. Herein, we propose that the formation of two-dimensional (2D) P-rich P-S compounds could achieve this goal. First-principles swarm-structural searches revealed two previously unkonwn P 3 S and P 2 S monolayers. The P 3 S monolayer, consisting of n -bicyclo-P 6 units along the armchair direction, exhibits anisotropic and wide band gap characteristics. Interestingly, its carrier mobility reaches 1.11 × 10 4 cm 2 V -1 s -1 and is much higher than in phosphorene. Its electronic band gap and optical absorption coefficients in the ultraviolet region reach 2.71 eV and 10 5 cm -1 , respectively. Additionally, the P 3 S monolayer has a high structural stability and resistance to air oxidation.

Taking into account the high conductivity and stability of carbon materials, such as graphene, and the strong polar covalent bonding character of main-group compounds, we explore potential 2D materials in the C-S binary system through first-principles structure search calculations. Herein, a hitherto unknown semiconducting C 3 S monolayer is identified, consisting of well-known n -biphenyl and S atom linked benzenes, exhibiting an obvious direction-dependent atomic arrangement. Thus, it exhibits anisotropic mechanical properties and carrier mobility. Its electron mobility reaches 2.14 × 10 4 cm 2  V -1  s -1 in the b direction, along which n -biphenyl units are arranged, and is much higher than that in the well-used MoS 2 monolayer and black phosphorus. Meanwhile, the C 3 S monolayer has high optical absorption coefficients (10 5 cm -1 ), high thermal and dynamical stabilities, and a moderate ability to split water. All these desirable properties make the C 3 S monolayer a promising candidate for applications in novel optoelectronic devices.

The preparation of compounds with novel atomic oxidation states and emergent properties is of fundamental interest in chemistry. As s-block elements, alkali-earth metals invariably show a +2 formal oxidation state at normal conditions, and among them, barium (Ba) presents the strongest chemical reactivity. Herein, we propose that novel valence states of Ba can be achieved in pressure-induced chalcogenides, where it also shows a feature of 5d-elements. First-principles swarm-intelligence structural search calculations identify three novel stoichiometric compounds: BaCh 4 (Ch = O, S) containing Ba 2+ , Ba 3 Ch 2 (Ch = S, Se, Te) with Ba + and Ba 2+ , and Ba 2 Ch (Ch = Se, Te) with Ba + cations. The pressure-induced drop of the Ba 5d level relative to Ba 6s is responsible for this unusual oxidation state. These compounds display captivating structural characters, such as Ba-centered polyhedra and chain-shaped Ch units. More interestingly still, the interaction between two Ba + ions ensures their structural stability.

The design of materials meeting the rigorous requirements of photocatalytic water splitting is still a challenge. Anisotropic Janus 2D materials exhibit great potential due to outstandingly high photocatalytic efficiency. Unfortunately, these materials are scarce. By means of ab initio swarm-intelligence search calculations, we identify a SiP 2 monolayer with Janus structure (i.e., out-of-plane asymmetry). The material turns out to be semiconducting with an indirect band gap of 2.39 eV enclosing the redox potentials of water. Notably, the oxygen and hydrogen evolution half reactions can happen simultaneously at the Si and P atoms, respectively, driven merely by the radiation-induced electrons and holes. The carrier mobility is found to be anisotropic and high, up to 10 -4 cm 2 V -1 s -1 , facilitating fast transport of the photogenerated carriers. The SiP 2 monolayer shows remarkably strong optical absorption in the visible-to-ultraviolet range of the solar spectrum, ensuring efficient utilization of the solar energy.

Electrides, accommodating excess electrons in lattice voids as anions, have attracted considerable attention in both fundamental research and application development because of their interesting properties, such as ultralow work functions, high electronic mobility, high catalytic activity, and anisotropic electronic and optical properties. Recently, much research progress has been made in both types and applications of inorganic electrides because of the high stability. In this Perspective, we aim to summarize the recent development of inorganic electrides discovered and proposed by experiment and theoretical calculations, highlighting the main applications, including catalysis, metal-ion batteries, superconductivity, magnetism, and organic light-emitting diodes. We provide insights into the role of anionic electrons in electrides playing in the stability and properties. Finally, the problems, challenges, and opportunities are presented, which provide an outlook for future research.

For the development of high-performance spintronic nanodevices, one of the most urgent and challenging tasks is the preparation of two-dimensional materials with room-temperature ferromagnetism and a large magnetic anisotropic energy (MAE). Through first-principles swarm-intelligence structural search calculations, we identify an ideal ferromagnetic Fe 3 P monolayer, in which Fe atoms show a perfect Kagome lattice, leading to strong in-plane Fe-Fe coupling. The predicted Curie temperature of Fe 3 P reaches ∼420 K, and its MAE is comparable to those of ferromagnetic materials, such as Fe and Fe 2 Si. Moreover, the Fe 3 P monolayer remains as an above room-temperature ferromagnet under biaxial strains as large as 10%. Its lattice can be retained at temperatures of ≤1000 K, exhibiting a high thermodynamic stability. All of these desirable properties make the Fe 3 P monolayer a promising candidate for applications in spintronic nanodevices.

An important goal in chemistry is to prepare F-rich transition metal fluorides due to the high oxidation states and potential applications such as oxidating and fluorinating agents. Thus far, the highest F stoichiometry in the neutral transition metal fluorides is 7. Here, we identify a hitherto unknown IrF 8 compound through first-principles swarm-intelligence structure search calculations under high pressure. The three identified IrF 8 phases exhibit typical molecular crystal characters, showing +8 oxidation state in Ir. The spatial symmetry of the basic building block in the three IrF 8 phases gradually increases with pressure (e.g., dodecahedron [Formula: see text] square antiprism [Formula: see text] quasicube). The pressure-induced faster increase of Ir 5d orbital energy level with respect to F 2p provides a strong charge transfer driving force from Ir 5d to F 2p, facilitating the formation of F-rich compounds. More interestingly, the predicted electron affinities of the three predicted IrF 8 phases are comparable/larger than that of PtF 6 , the strongest oxidation agent in the third row transition metal hexafluorides. The built high-pressure phase diagram of Ir-F binary compounds provides useful information for experimental synthesis.

MoS 2 is well-known for its catalytic properties, mainly to adsorb hydrogenous or carbonaceous materials. However, the effect of MoS 2 on the oxygen adsorption has been investigated only a few times thus far. In this work, we first studied the adsorbability of O 2 by MoS 2 through the analysis of Li 2 O 2 growth on the surface of flower-like MoS 2 microspheres with different concentrations of sulfur vacancies, which can be applied as the highly active electrocatalysts for Li-O 2 batteries. The enhancement of battery performance for the Def-MoS 2 @CTs (CTs = carbon textile substrates) with a larger concentration of sulfur vacancies (S/Mo = 1.61) can be achieved. The experimental and theoretical results confirm that the sulfur vacancies play a crucial role in the adsorption process and thus affect the morphology and nucleation of Li 2 O 2 . In addition, a fundamental catalytic mechanism for this adsorption process is also proposed. These results provide a new insight into the development of a highly active electrocatalyst by introducing a large concentration of defects for Li-O 2 batteries.

Graphene and phosphorene are two major types of atomically thin two-dimensional materials under extensive investigation. However, the zero band gap of graphene and the instability of phosphorene greatly restrict their applications. Here, we make first-principle unbiased structure search calculations to identify a new buckled graphene-like PC 6 monolayer with a number of desirable functional properties. The PC 6 monolayer is a direct-gap semiconductor with a band gap of 0.84 eV, and it has an extremely high intrinsic conductivity with anisotropic character (i.e., its electron mobility is 2.94 × 10 5 cm 2 V -1 s -1 along the armchair direction, whereas the hole mobility reaches 1.64 × 10 5 cm 2 V -1 s -1 along the zigzag direction), which is comparable to that of graphene. On the other hand, PC 6 shows a high absorption coefficient (10 5 cm -1 ) in a broad band, from 300 to 2000 nm. Additionally, its direct band gap character can remain within a biaxial strain of 5%. All these appealing properties make the predicted PC 6 monolayer a promising candidate for applications in electronic and photovoltaic devices.

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, AuF 4 and AuF 6 , 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.

Sodium-ion batteries (SIBs) have attracted considerable attention due to the intrinsic safety and high abundance of sodium. However, the lack of high-performance anode materials becomes a main obstacle for the development of SIBs. Here, we identify an ideal anode material, a metallic TiC 3 monolayer with not only remarkably high storage capacity of 1278 mA h g -1 but also low barrier energy and open-circuit voltage, through first-principles swarm-intelligence structure calculations. TiC 3 still keeps metallic after adsorbing two-layer Na atoms, ensuring good electrical conductivity during the battery cycle. Besides, high melting point and superior dynamical stability are in favor of practical application. Its excellent performance can be mainly attributed to the presence of an unusual n-biphenyl unit in the TiC 3 monolayer. High cohesive energy, originating from multibonding coexistence (e.g., covalent, ionic, and metal bonds) in the TiC 3 monolayer, provides strong feasibility for experimental synthesis. In comparison with TiC 3 , functionalized TiC 3 with oxygen shows a higher storage capacity; meanwhile, it keeps nearly the same barrier energy. This is in sharp contrast with metal-rich MXenes. These intriguing properties make the TiC 3 monolayer a promising anode material for SIBs.