Typical Dirac cones in graphene induce the absence of electronic density around the Fermi energy level (Ef), prohibiting intrinsic superconductivity. Here, we tested the theoretical superconducting properties of graphene after introducing pentagonal and heptagonal carbon rings into the structure. Generally, 5-7 polygons of a metastable Ψ-graphene monolayer break the hexagonal symmetry to form type-II Dirac cones by band crossings. The polyhedral structure maintains integrity under high temperatures. The large specific surface area of the Ψ-graphene monolayer facilitates the physical adsorption of NO molecules. Weak interactions of atomic bonding and antibonding features coexist with the Bader charge transfer in the carbon monolayer in close proximities to one another. The collective vibrations of carbon, nitrogen, and oxygen atoms provide good dynamic stability of the Ψ-graphene-NO adsorption system. In the Ψ-graphene monolayer, the shift of Dirac cones leads to the formation of visible Fermi surfaces, which motivates further investigation into their influences on the superconducting properties. Out-of-plane and in-plane carbon vibrations are attributed to phonon modes in mediation with electron couplings. After computing the Eliashberg function, we evaluated strong electron-phonon coupling, with the superconducting transition temperature reaching 22 K. These theoretical predictions can stimulate interests in exploring topological graphene allotropes of intrinsic superconductivity.

High-temperature conventional superconductivity at ambient conditions beyond MgB2 presents key challenges in the dynamic stabilization of compositional structures. However, the isostructural beryllium diboride (BeB2) phase lacks ambient-pressure dynamics stability, thus prohibiting superconductivity. Here, we predict a novel superconducting BeB2 phase with P63cm symmetry featuring distorted hexagonal boron layers. The Wurtzitic gauche boron framework favors thermodynamic stability at ambient pressure by modulating anharmonic beryllium displacements. Interestingly, mirror symmetry is observed to protect topological nodal lines in electronic states. Eliashberg function computations estimate strong electron-phonon coupling strength for the BeB2 phase beyond MgB2. High superconducting transition temperature achieves enhancement by mediating with broadened phonon modes attributed to atomic displacements. Our work can inspire wide interest in designing and modulating high-temperature superconducting structures with topology, especially offering potential exploration within meta-stable crystallographic databases.

In recent decades, pentazolate salts have gained considerable attention as high energy density materials (HEDMs). Using the machine-learning accelerated structure searching method, we predicted four pentazolate salts stabilized with tetravalent metals (Ti-N and Zr-N). Specifically, the ground state MN20 (M = Ti, Zr) adopts the space-group P4/mcc under ambient conditions, transforming into the I-4 phase at higher pressure. Moreover, the I-4-MN20 becomes energetically stable at moderate pressure (46.8 GPa for TiN20, 38.7 GPa for ZrN20). Anharmonic phonon spectrum calculations demonstrate the dynamic stabilities of these MN20 phases. Among them, the P4/mcc phase can be quenched to 0 GPa. Further ab-initio molecular dynamic simulations suggest that the N5 rings within these MN20 systems can still maintain integrity at finite temperatures. Calculations of the projected crystal orbital Hamilton population and reduced density gradient revealed their covalent and noncovalent interactions, respectively. The aromaticity of the N5 ring was investigated by molecular orbital theory. Finally, we predicted that these MN20 compounds have very high energy densities and exhibit good detonation velocities and pressures, compared to the HMX explosive. These calculations enrich the family of pentazolate compounds and may also guide future experiments.

Intrinsic superconductivity is rarely discovered in sp2-hybridized monolayer carbon allotropes. Here we design a carbon monolayer configured of pentagon, heptagon, and hexagon rings with p2 plane group symmetry. Full-sp2 hybridization is proposed to favor thermal metastability on a low Gibbs free energy. The extremely small thermal expansion coefficient is predicted to the turn negative value to positive with elevating temperature. Carbon polygon structures remain intact at a high thermal temperature of 3,000 K. The high specific surface area is found to approach 2,700 m2/g, with O2-adsorption being advantageous over pristine graphene. We reveal electronic Fermi surfaces mediated by phonon modes of carbon out-of-plane vibrations. By calculating the Eliashberg equation, we evaluate intrinsic superconductivity with a large electron-phonon coupling coefficient. The superconducting transition temperature is estimated to reach 20 K under a high logarithmic average frequency. These first-principles calculations shall stimulate experimentalists' interest in exploring low-dimensional carbon superconductors with gas sensitivity.

The understanding of the lattice dynamics is essential for engineering the thermal transport properties in quantum materials. Based on the canonical point of view, acoustic phonons are believed to be the principal thermal carriers in heat flow. Here, in this work, optical phonons are elucidated to play a pivotal role in determining the lattice thermal conductivity in thermoelectric material SnS by using the state-of-the-art inelastic neutron scattering technique combined with first-principles calculations. Additionally, in contrast to acoustic phonons, optical phonons are observed to exhibit pronounced softening and broadening with temperature. Our observations not only shed light on the significance of the optical phonons in thermal transport but also provide a vital clue to suppress the propagation of optical phonons to optimize the thermoelectric performance of SnS.

Pentagraphene frameworks with sp2 carbon atoms have significance in fundamental research studies and material science. Here, we find pentagon ribbons stacked in an AB sequence at the atomic scale within alkali metal atoms. A Pnma phase is favored on the Gibbs free energy landscape at moderate pressures and finite temperatures. Strong electron localization, covalent interactions, weak van der Waals interactions, and electronic repulsive interactions coexist in this ionic structure. Electronic bands with narrow direct gaps are flattened with double degeneracy to produce a small effective mass and van Hove singularity for the density of states, which enhances visible-light absorption and produces a thermoelectric power factor on crystals. Alkali metal atoms strongly scatter acoustic-optical phonons to reduce the lattice thermal conductivity to an ultralow level. These character-istics introduce potential thermoelectric effects into Pnma crystals. In addition, the alkali metal ions exhibit high delocalizations with superionic properties at high temperatures.

Polymeric nitrogen has attracted much attention owing to its possible application as an environmentally safe high-energy-density material. Based on a crystal structure search method accelerated by the use of machine learning and graph theory and on first-principles calculations, we predict a series of metal nitrides with chain-like polynitrogen (P21-AlN6, P21-GaN6, P-1-YN6, and P4/mnc-TiN8), all of which are estimated to be energetically stable below 40.8 GPa. Phonon calculations and ab initio molecular dynamics simulations at finite temperature suggest that these nitrides are dynamically stable. We find that the nitrogen in these metal nitrides can polymerize into two types of poly-N42− chains, in which the π electrons are either extended or localized. Owing to the presence of the polymerized N4 chains, these metal nitrides can store a large amount of chemical energy, which is estimated to range from 4.50 to 2.71 kJ/g. Moreover, these compounds have high detonation pressures and detonation velocities, exceeding those of conventional explosives such as TNT and HMX.

Through first-principles calculations, we employed anisotropic compression to T-carbon and identified several carbon allotropes. Among them, we predict a novel body-centered tetragonal carbon allotrope with space group I4(1)/amd (No. 141), in which sixteen carbon atoms comprise four-and five-and eight-membered rings within a unit cell; we name it as bct-C-16-II. Phonon spectrum calculations exhibit that it is dynamically stable under pressure up to at least 100 GPa. This bct-C-16-II is energetically more stable than T-carbon and the previously proposed bct-C-16-I, although it has higher energy than graphite or diamond. Our calculations also show that bct-C-16-II may be super-hard with Vickers hardness around 55 GPa, comparable to cubic boron nitride. Its ideal tensile and shear strength is calculated to be around 75 and 43 GPa, respectively. We simulated the trans-formation pathway of importance from T-carbon to this novel bct-C-16-II phase. The energy barrier is estimated to be around 0.187 eV/atom, which provides a possible approach to synthesize it. The achievement of other carbon allotropes indicates that the tetrahedral units in T-carbon are not very strong under anisotropic strains that T-carbon might be considered as a good precursor to synthesize other carbon materials. The discovery of this new phase may enrich our knowledge of carbon materials and their formation mechanisms.

WN_{6} phase discovered at 126-165 GPa after heating of W in nitrogen. XRD refinements reveal a unit cell in space group R3[over ¯]m which is consistent with the WN_{6} structure with armchairlike hexazine (N_{6}) rings, while strong A_{1g} Raman mode confirms its N─N single bonds. Density functional theory (DFT) calculations reveal balanced contributions of attractive interactions between W and covalent N_{6} rings, and repulsions between N_{6} rings that make WN_{6} ultrastiff and tough. The WN_{6} phase displays long bond lengths in the nearest N-N and pressure-enhanced electronic band gap, which pave the way for finding novel nitrides.

Recently, SnSe has been found to possess a remarkable thermoelectric performance. As an isostructural compound to SnSe, SnS contains a more environmentally compatible and earth abundant element, which enables SnS a promising thermoelectric material for commercial application. In this work, we have performed systematic studies of the crystal structures and the lattice vibrations by means of neutron total scattering, Raman scattering measurements, and first-principle calculations. A structural transition has been observed by both temperature dependent neutron diffraction and pressure dependent Raman scattering. The lattice anharmonicity has been revealed by the local distortion induced by the Sn $5{s}^{2}$ lone pair and is also evidenced from the temperature dependent atomic displacement parameter of the Sn atom. By separating the intrinsic anharmonicity and lattice thermal expansion effects, we found that the former plays the primary role in the softening of the Raman active modes. Such anharmonicity has also been observed experimentally by the linewidth broadening of the high-energy optical modes and confirmed by frozen phonon calculations. Furthermore, our frozen phonon calculation reveals the presence of the quartic anharmonicity potential of those high-energy optical modes vibrating along the $b$ axis. Our data will contribute to a better understanding of the thermal conduction in SnS, which will be beneficial for the enhancement of the thermoelectric performance of this material.

Boron-rich compounds have attracted significant attention due to their promising and diverse physical properties, which include ultrahardness, resistance to oxidation and corrosion, and even superconductivity. Here, using a crystal structure search method based on first-principles calculations, we find a boron-rich silicon compound SiB12 that is stable under moderate pressure of around 20 GPa, and which we predict is recoverable to ambient pressure. This silicon boride, with space group Pnnm, is structurally related to the gamma-B-28 boron phase. Specifically, the SiB12 structure is formed by replacing the B-2 pairs in gamma-B-28 with silicon atoms. Our calculations show that this Pnnm SiB12 phase exhibits good thermal stability at moderate pressures above 20 GPa and temperatures to 900 K. We suggest this structure has dynamic stability at ambient pressure and remains stable to temperatures as high as 2000 K. Impressively, this SiB12 phase possesses good light absorption and thermoelectrical properties, which are enhanced by its small and indirect band gap, doubly degenerate bands, and low lattice thermal conductivity. Our predictions should stimulate further investigations of this class of boron-rich semiconductors, especially in view of their superior photovoltaic and thermoelectric properties which may be beneficial in energy applications.

Recently discovered massive Dirac fermions and room temperature skyrmions in the ferromagnetic kagome metal Fe3Sn2 have attracted extensive attention due to the potential applications in topological and spintronic devices. Because of its centrosymmetric lattice, the topological spin texture in Fe3Sn2 arises primarily from the competition between anisotropy and exchange interaction. In this Letter, we have identified the spin-reorientation at around 80 K by the magnetization and AC-susceptibility measurements. The evolution of the anisotropy is revealed by neutron powder diffraction data from 18 to 700 K, where the "breathing"-like kagome structure evolves into an ideal kagome lattice at higher temperature. Meanwhile, the deviation between the experimental and calculated results on heat capacity at high temperature and the softening of optical phonon modes in Raman spectra suggest the presence of anharmonic phonons in Fe3Sn2, which is responsible for the degeneracy of lattice thermal conductivity at high temperature. Our study indicates that Fe3Sn2 possesses a promising future in the design and development of topological, spintronic as well as thermoelectric devices.

One-dimensional ferromagnetic semiconductor is important for fundamental research and nanoscale spintronic applications. In this article, we systematically studied the vanadium-trihalide VX3 (X= F, Cl, Br, I) nanowires. Among the VX3 nanowires, we found that a finite-length single-chain VI3 nanowire is a ferromagnetic semiconductor with a blocking temperature of around 64 K using Monte Carlo simulations. In addition, we constructed a 2D nanowire array and found that the interchain exchange interactions of VI3 nanowires favor ferromagnetic states. The VI3 nanowire array has a Curie temperature of around 113 K, which is higher than the liquidus temperature of nitrogen. Moreover, the ferromagnetic VI3 nanowire keeps stable during deformation and doping. The intrinsic semiconductivity found in ferromagnetic VI3 nanowire allows it to have significant applications in nanoscale spintronic.

The search for high energy density materials (HEDMs) in polymeric nitrogen compounds has gained considerable attention. Previous theoretical predictions and experiments have revealed that metal ions can be used to stabilize the pentazolate (N5(-)) anion. In this work, by employing a machine learning-accelerated crystal structure searching method and first-principles calculations, we found that the new pentazolate salts, CaN(10)and BaN10, are energetically favorable at high pressures. Phonon dispersion calculations reveal that they are quenchable at ambient pressure.Ab initiomolecular dynamics simulations verify their dynamic stability at finite temperature. Bader charge and electron localization function illustrates that alkaline earth atoms serve as electron donors, contributing to the stability of N(5)rings. Bonding calculations reveal covalent bonds between nitrogen atoms and weak interactions between N(5)rings. Similar to other pentazolate salts, these polymeric nitrides have high energy densities of approximately 2.35 kJ/g for CaN(10)and 1.32 kJ/g for BaN10. The predictions of CaN(10)and BaN(10)structures indicate that these salts are potential candidates for green nitrogen-rich HEDMs.

High-energy-density materials (HEDMs) have been intensively studied for their significance in fundamental sciences and practical applications. Here, using the molecular crystal structure search method based on first-principles calculations, we have predicted a series of metastable energetic trivalent metal pentazolate salts MN15 (M= Al, Ga, Sc, and Y). These compounds have high energy densities, with the highest nitrogen content among the studied nitrides so far. Pentazolate N-5(-) molecules stack up face-to-face and form wave-like patterns in the C222(1) and Cc symmetries. The strong covalent bonding and very weak noncovalent interactions with nonbonded overlaps coexist in these ionic-like structures. We find MN15 molecular structures are mechanically stable up to high temperature (similar to 1000 K) and ambient pressure. More importantly, these trivalent metal pentazolate salts have high detonation pressure (similar to 80 GPa) and velocity (similar to 12 km/s). Their detonation pressures exceeding that of TNT and HMX make them good candidates for high-brisance green energetic materials.

Polynitrogen compounds especially pentazolate anion complexes recently have attracted substantial attention due to their promising potential as highenergy -density materials. Here, using a machine-learning-accelerated crystal structure search method and first-principles calculations, we predict a new hybrid compound by inserting a large fraction of nitrogen into alkaline-earth metals. It is a new stoichiometric type MN10 (M = Be, Mg), which possesses a metal-centering octahedral pentazolate framework with the space group Fdd2. This type of ionic-like molecular crystal is found to be energetically more favorable than the mixtures of M3N2 or MN4 compounds and pure nitrogen and is possibly synthesized at relatively low pressures (around 12 GPa for MgN10). The ab initio molecular dynamics simulations show that they are metastable and can be quenched to ambient conditions once synthesized at high pressure. Moreover, decomposition of this polymeric MN10 structure can release a large amount of energy and shows high performance in detonation. The detonation velocity and pressure of BeN10 are about twice and 4 times that of trinitrotoluene, respectively.

Received 17 December 2017DOI:https://doi.org/10.1103/PhysRevB.97.019902©2018 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCrystal structureFirst order phase transitionsPolymerizationPressure effectsPhysical SystemsEnergy materialsMolecular solidsTechniquesDensity functional theoryHigh-throughput calculationsCondensed Matter, Materials & Applied Physics

Transition metal nitrides have been suggested to have both high hardness and good thermal stability with large potential application value, but so far stable superhard transition metal nitrides have not been synthesized. Here, with our newly developed machine-learning accelerated crystal structure searching method, we designed a superhard tungsten nitride, h-WN6, which can be synthesized at pressure around 65 GPa and quenchable to ambient pressure. This h-WN6 is constructed with single-bonded N6 rings and presents ionic-like features, which can be formulated as W2.4+N62.4-. It has a band gap of 1.6 eV at 0 GPa and exhibits an abnormal gap broadening behavior under pressure. Excitingly, this h-WN6 is found to be the hardest among transition metal nitrides known so far (Vickers hardness around 57 GPa) and also has a very high melting temperature (around 1900 K). These predictions support the designing rules and may stimulate future experiments to synthesize superhard material.

Crystal structure searching and ab initio calculations have been used here to explore low-energy structures of boron carbides under high pressure. Under pressures of 85-110 GPa, a metastable B6C with R (3) over barm symmetry is found to be energetically more stable than the mixture of previous B4C and elemental boron. This B6C is a rhombohedral structure and contains mooncake-like B-24 clusters with stuffing of C-2 pairs. The mechanical and dynamical stabilities of this structure at ambient pressure are confirmed by its elastic constants and phonon dispersions. The bulk modulus and shear modulus of this newly predicted B6C at ambient pressure reach high values of 291 GPa and 272 GPa, respectively. The Vickers hardness is calculated to be around 48 GPa, and its melting temperature at ambient pressure is estimated to be around 2500 K, indicating that this metastable B6C is a potential superhard material with very good thermal stability. Interestingly, this superhard B6C structure is also found to be superconducting and its superconducting critical temperature (T-c) is evaluated to be around 12.5 K at ambient pressure by electron-phonon coupling. (c) 2017 Published by Elsevier Ltd.