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Abstract: Diamond-like B3C10N3 model was built by stacking monolayer B3C10N3 which has the lowest energy. The first-principle calculations were performed by CASTEP code based on density functional theory (DFT) and basal physical properties of the model were investigated. The diamond-like B3C10N3 is semiconducting with direct energy gap of 1.65eV. The results of elastic constants (C ij ) satisfy the Born criteria, showing the mechanical stability. Bulk modulus calculated is about 398GPa, with hardness about 60.6GPa calculated by hardness formula of the covalent crystal, which might be one of superhard materials. The transformation pressure from layered B3C10N3 to diamond-like was proposed at about 25.3GPa. [Copyright &y& Elsevier]

Carbon allotropes with distinct sp , sp 2 , and sp 3 hybridization possess various different properties. Here, a novel all- sp 3 hybridized tetragonal carbon, namely the P carbon, was predicted by the evolutionary particle swarm structural search. It demonstrated a low density among all- sp 3 carbons, due to the corresponding distinctive microporous structure. P carbon is thermodynamically stable than the known C 60 and could be formed through the single-walled carbon nanotubes (SWCNTs) compression. P carbon is a direct bandgap semiconductor displaying a strong and superhard nature. The unique combination of electrical and mechanical properties constitutes P carbon a potential superhard material for semiconductor industrial fields. [ABSTRACT FROM AUTHOR]

A lonsdaleite-like orthorhombic structure B 2 CO ( o P8-B 2 CO) with strong sp 3 covalent B‐C and B‐O bonds has been predicted theoretically by using the crystal structure prediction package CALYPSO. The mechanical, dynamical and thermodynamic stabilities of o P8-B 2 CO are verified by independent elastic constants, phonon dispersion spectrum and formation enthalpy, respectively. Pressure may promote synthesis of o P8-B 2 CO, which is demonstrated by calculating the relationship of formation enthalpies for o P8-B 2 CO via pressure. Based on bond resistance model for hardness, o P8-B 2 CO has a high hardness value of 47.70 GPa. Band structures calculation illustrates that all B 2 CO phases are semiconducting with indirect band gaps and o P8-B 2 CO has the widest band gap (3.540 eV) relative to t P4-B 2 CO (1.658 eV) and t I16-B 2 CO (2.988 eV). The superhardness and tunable band gap open an extensive industrial application and scientific research for B 2 CO compounds. [ABSTRACT FROM AUTHOR]

[Display omitted] • Detailed structural stability and properties of 4^3T175 were performed. • The Vickers hardness of 4^3T175 is 73 GPa. • 4^3T175 may be one of the products in the detonation soot. We present a systemical study on the stability, electronic and mechanical properties of a sp 3-bonded carbon allotrope with 4^3T175 topology. This carbon allotrope is composed of a C-centered orthorhombic unit cell with 20 atoms and is mechanically and dynamically stable over a pressure range from ambient conditions to 50 GPa. According to the calculations, 4^3T175 is an indirect band gap semiconductor with band gap of 2.84 eV and also a potential superhard material with Vickers hardness of 73 GPa. The simulated X-ray diffraction pattern can be matched to some of the diffraction peaks of those unidentified carbon phases in detonation soot samples, indicating its possible presence in these specimens. These findings yield a deeper understanding of 4^3T175 and provide a guidance to elucidate the unidentified carbon phase in the high-pressure synthesis products. [ABSTRACT FROM AUTHOR]

Carbon clathrate is a kind of typical carbon allotrope that has an open framework composed of various types of cages, displaying intriguing electronic and mechanical properties. This paper proposes a superhard superstrong carbon clathrate via first-principle calculations. This carbon clathrate, called C 60 clathrate, contains 60 carbon atoms buckled through sp 3 -hybridized bonds in a cubic unit cell with symmetry of Im-3m , and possesses the network topology of commonly called binodal net. C 60 clathrate is energetically more stable than fullerene C 60 at ambient pressure, and it is more favorable than graphite at a pressure above 50.1 GPa C 60 clathrate can be constructed by small C 24 cage and flat C 18 drum. It shows a high density of 3.34 g/cm 3 , which is the densest carbon clathrate to date. The estimated Vickers hardness, tensile strength, and shear strength of C 60 clathrate reach remarkably high values of 82.8, 90.7, and 76.4 GPa, respectively, indicating its superhard and superstrong characteristic. Band structure calculations indicate that C 60 clathrate is semiconductive with a direct band gap of 2.26 eV. C 60 clathrate can be potentially used in photovoltaic devices. [ABSTRACT FROM AUTHOR]

Four three-dimensional graphdiyne polymers, namely Tri-C 18 , Hex-C 36 , Tri-C 54 , and Orth-C 72 , are predicted with first principles calculations. These carbon allotropes are constituted with interlinked sp 3 -hybridized carbon pillars and sp 2 -hybridized graphene-like nanoribbons. Graphdiyne polymers are energetically more stable than C 60 and graphdiyne, and show lower phase transition energy barriers from AA-graphdiyne than that of graphite–diamond transition. Except for the semimetallic Tri-C 18 , these polymers show narrow direct (or quasi-direct) band gap of 1.55–1.74 eV. The porous structure and the high in-plane Young’s modulus of graphdiyne sheet are inherited by these polymers. Meanwhile, a greatly enhanced Young’s modulus as high as ca . 1 TPa along the direction perpendicular to graphdiyne sheet and high hardness are developed in these systems. [ABSTRACT FROM AUTHOR]

Exploration of novel hard metastable phases of silicon nitride was performed using a recently developed particle-swarm optimization method within the CALYPSO software package. Three potential hard metastable phases of t -Si 3 N 4 , m -Si 3 N 4 , and o -Si 3 N 4 were predicted. These phases are mechanically and dynamically stable at ambient pressure based on their elastic constants and phonon dispersions. t -Si 3 N 4 and m -Si 3 N 4 exhibit lower energies than γ -Si 3 N 4 at pressures below 2.5 GPa and 2.9 GPa, respectively, which promise that the formers could be obtained by quenching from γ -Si 3 N 4 . o -Si 3 N 4 is a better high-pressure metastable phase than CaTi 2 O 4 -type Si 3 N 4 proposed by Tatsumi et al. and it can come from the transition of γ -Si 3 N 4 under 198 GPa. The theoretical band gaps of t -Si 3 N 4 , m -Si 3 N 4 , and o -Si 3 N 4 at ambient pressure were 3.15 eV, 3.90 eV, and 3.36 eV, respectively. At ambient pressure, the Vickers hardness values of t -Si 3 N 4 (32.6 GPa), m -Si 3 N 4 (31.5 GPa), and o -Si 3 N 4 (36.1 GPa) are comparable to β -Si 3 N 4 and γ -Si 3 N 4 . With the pressure increasing, t -Si 3 N 4 , m -Si 3 N 4 , and o -Si 3 N 4 will change from the brittle to ductile state at about 15.7 GPa, 7.3 GPa and 28.9 GPa, respectively. [ABSTRACT FROM AUTHOR]

[Display omitted] • By compressing well-mixed CNT and BNNT arrays, nine novel BC 2 N phases were constructed. • 3D (6,6)-Ⅱ, 3D (4,4)-Ⅱ, 3D (8,8), 3D (6,6)-Ⅰ and 3D (8,0) are energetically more favorable than previously proposed diamond-like β -BC 2 N. • 3D (4,4)-I, 3D (4,4)-II, 3D (6,6)-I, 3D (6,6)-II, and 3D (8,8) are superhard phases with indirect band gap; 3D (6,0) and 3D (8,0) are superhard phases with direct band gap; 3D (10,0) is semi-metallic; 3D (12,0) is a low density phase. We constructed a series of novel 3D-BC 2 N structures that can be obtained by compressing well-mixed carbon nanotube and boron nitride nanotube arrays. Sequential theoretical calculations indicated a variety of electronic and mechanical properties, such as superhardness, tunable band gap, and ductility, for these novel 3D-BC 2 N polymorphs. In specific, 3D (6,6)-Ⅱ, 3D (4,4)-Ⅱ, 3D (8,8), 3D (6,6)-Ⅰ and 3D (8,0) are energetically more favorable than previously proposed diamond-like β -BC 2 N; the hardness of 3D (6,6)-Ⅱ can reach a remarkably high value of 72 GPa; 3D (6,0) and 3D (8,0) are direct gap superhard phases; and 3D (10,0) has a semimetallic nature. [ABSTRACT FROM AUTHOR]

Six metastable allotropes of silicon with direct or quasi-direct band gaps of 0.39-1.25 eV are predicted by ab initio calculations at ambient pressure. [ABSTRACT FROM AUTHOR]