Ionicities of boron-boron bonds in B(12) icosahedra

Interfacial polarization-induced tribological behavior in MoS2/β-Te and G/β-Te heterostructures

Two-dimensional (2D) heterostructures have opened up new avenues for controlling nanoscale friction; however, the relationship between their interfacial characteristics and frictional behavior remains to be thoroughly explored. In this work, we synthesized β-tellurene nanosheets via a hydrothermal method and systematically investigated the interfacial properties of their heterostructures with graphene and MoS2. By combining Kelvin probe force microscopy, second-harmonic generation, and atomic force microscopy, it was revealed that interface charge transfer and polarization effects are key sources of friction behavior. Compared with those at the MoS2/β-Te interface, the friction and adhesion forces of G/β-Te are significantly lower. Density functional theory calculations further quantified the interfacial charge redistribution and sliding barriers. Notably, the G/β-Te heterostructure exhibited an ultralow friction coefficient (μ ≈ 0.005) and maintained stable superlubricity over 2300 sliding cycles. The ionic difference between 2D material friction pairs serves as an effective indicator for evaluating interlayer friction performance, with larger ionic differences often corresponding to lower friction coefficients. Our work not only provides crucial insights into the friction mechanisms of 2D heterostructures but also offers a powerful tool for designing ultralow friction interfaces in nanoelectromechanical systems. These findings pave the way for advanced tribological applications and contribute to a fundamental understanding of nanoscale friction in layered materials.

Structural and Stress Response of Nanotwinned B13CN under Large Strains

Boron-rich carbides with icosahedral cages as pivotal structural units, which exhibit high hardness and low density, have promising industrial applications. However, the insufficient fracture toughness of these materials hinders their engineering applications. A recent first-principles study revealed that single-crystal B13CN (sc-B13CN) exhibits interesting structural deformation modes and superior mechanical properties to boron-rich carbides, prompting us to further explore this intriguing material. Herein, we adopted sc-B13CN as an archetypal system owing to its excellent structural and mechanical properties to construct nanotwinned B13CN (nt-B13CN) and explore its mechanical properties and structural deformation modes under large strains. We unraveled the specific stress-strain relationship of nt-B13CN and the considerable effect of twinning on its structural deformation modes under diverse loading conditions. Our results indicate that twinning leads to interesting structural deformation patterns and is extremely beneficial to improving the structural stability and mechanical properties of boron-rich materials. The current results provide an improved understanding of the theoretical design for various nanotwinned boron-rich materials with intricate bonding configurations.

Hardness and electronic properties of Si-C-N structures

Three novel hexagonal Si-C-N structures, namely SiC3N3, SiC7N6, and SiC13N14, were constructed on the basis of the α-Si3N4 crystal structure. The stability of the three structures is demonstrated by analyzing their elastic constants and phonon dispersion spectra and by calculating their formation energies. The calculated band structures and partial densities of states suggest that the SiC3N3 and SiC7N6 structures possess hole conductivity. The electron orbital analyses indicate that the SiC3N3 and SiC7N6 crystals possess three-dimensional and one-dimensional conductivity, respectively. SiC13N14 is a semiconductor with a wide bandgap of 4.39 eV. Based on two different hardness models and indentation shear stress calculations, the Vickers hardness values of SiC3N3, SiC7N6, and SiC13N14 are estimated to be 28.04/28.45/16.18 GPa, 31.17/34.19/20.24 GPa, and 40.60/41.59/36.40 GPa. This result indicates that SiC3N3 and SiC7N6 are conductive hard materials while SiC13N14 is a quasi superhard material.

Novel Boron-rich Phosphides with High Hardness, Large Strain, and Magnetism

Boron-rich compounds have attracted much attention due to their interesting structures and excellent properties. Here, we performed an extensive study on the different B-P stoichiometries under pressure by combining a particle swarm optimization method with first-principles calculations. At 1 atm, BP and B₆P are thermodynamically stable, while other stoichiometries are metastable. Under pressure, BP and B₆P remain stable relative to constituent pure solids up to 80 GPa, while other stoichiometries become unstable at relatively low pressures. A new Cmca B₆P is predicted with the lowest energy at 1 atm and shows higher shear strain than the R3̅m structure, which is known to be more resistant to brittle fracture than B₄C. Moreover, the predicted Pm B₈P is a magnetic semiconductor with a magnetic moment of 1 μ_B. All these boron-rich phosphides are hard materials. The present results enrich the B-P phase diagram and promote extensive research on their excellent properties.

Exploring Hydrogen Incorporation into the Nb4AlC3 MAX Phases: Ab Initio Calculations

The Nb4AlC3 MAX phase can be regarded as a TMC structure with stacking faults, which has great potential as a novel solid hydrogen storage material. Herein, we used ab initio calculations for understanding the hydrogen incorporation into Nb4AlC3 MAX phases, including equilibrium structural characteristics, energy changes, electronic structures, bonding characteristics, and diffusion paths. According to the calculated results, H has thermal stability in the interstice of the Nb-Al layer, and the most probable insertion site is an octahedron (3-site) composed of three Nb atoms and three Al atoms. When C vacancies are introduced, the Nb-C layer has a specific storage capacity for H. In addition, Al vacancies can also be used as possible sites for H incorporation. Moreover, the introduction of vacancies significantly increase the hydrogen storage capacity of the MAX phase. According to the electronic structure and bonding characteristics, the excellent hydrogen storage ability of the Nb4AlC3 structure may be due to the formation of ionic bonds between H and Nb/Al. It is worth noting that the H-Al bond in the 1-site is a covalent bond and an ionic bond key mixture. The linear synchronous transit optimization study shows that only H diffusion in Al vacancies is not feasible. In conclusion, the Nb-Al layer in Nb4AlC3 can provide favorable conditions for the continuous insertion and subsequent extraction of H, while the vacancy structure is more suitable for H storage. Our work provides solid theoretical results for understanding the hydrogen incorporation into Nb4AlC3 MAX phases that can be helpful for the design of advanced hydrogen storage materials.

Surface Magnetism in Pristine α Rhombohedral Boron and Intersurface Exchange Coupling Mechanism of Boron Icosahedra

We report intrinsic surface magnetism in pristine α rhombohedral boron (α-boron) using first-principles calculations. Semiconducting α-boron has been cleaved along the (001), (102̅), and (101) planes to produce icosahedral-based non-van der Waals face-boron, t-face-boron, and edge-boron structures, respectively. Face-boron is found to be metallic, while t-face-boron and edge-boron show semiconducting features. In particular, edge-boron exhibits layer-dependent magnetism with a transition from an overall antiferromagnetic (AFM) state with AFM surfaces to either an AFM or a ferromagnetic (FM) state with FM surfaces as the number of layers increases. The magnetism in edge-boron arises from the spin polarization of boron atoms with unsaturated bonds at the edge sites in the upper and lower surfaces, and magnetic exchange coupling can be mediated via adjacent boron icosahedra by up to a maximum of 8.4 Å. These findings deepen our understanding of icosahedral-based boron and boron-rich materials, which may be useful in potential spintronics applications.

The Effects of Chemical Bonding at Subatomic Resolution: A Case Study on α-Boron

Similar to classical asphericity shifts, aspherical deformations of the electron density in the atomic core region can result in core asphericity shifts in refinements using a Hansen-Coppens multipolar model (HCM), especially when highly precise experimental datasets with resolutions far beyond sin(θ)/λ ≤ 1.0 Å⁻¹ are employed. These shifts are about two orders of magnitude smaller than their counterparts caused by valence shell deformations, and their underlying deformations are mainly of dipolar character for 1st row atoms. Here, we analyze the resolution dependence of core asphericity shifts in α-boron. Based on theoretical structure factors, an appropriate Extended HCM (EHCM) is developed, which is tested against experimental high-resolution (sin(θ)/λ ≤ 1.6 Å⁻¹) single-crystal diffraction data. Bond length deviations due to core asphericity shifts of α-boron in the order of 4–6·10⁻⁴ Å are small but significant at this resolution and can be effectively compensated by an EHCM, although the correlation of the additional model parameters with positional parameters prevented a free refinement of all core model parameters. For high quality, high resolution data, a proper treatment with an EHCM or other equivalent methods is therefore highly recommended.

Prediction of unexpected B n P n structures: promising materials for non-linear optical devices and photocatalytic activities

In the present work, a modern method of crystal structure prediction, namely USPEX conjugated with density functional theory (DFT) calculations, was used to predict the new stable structures of B n P n (n = 12, 24) clusters. Since B12N12 and B24N24 fullerenes have been synthesized experimentally, it motivated us to explore the structural prediction of B12P12 and B24P24 clusters. All new structures were predicted to be energetically favorable with negative binding energy in the range from -4.7 to -4.8 eV per atom, suggesting good experimental feasibility for the synthesis of these structures. Our search for the most stable structure of B n P n clusters led us to classify the predicted structures into two completely distinct structures such as α-B n P n and β-B n P n phases. In α-B n P n , each phosphorus atom is doped into a boron atom, whereas B atoms form a B n unit. On the other hand, each boron atom in the β-phase was bonded to a phosphorus atom to make a fullerene-like cage structure. Besides, theoretical simulations determined that α-B n P n structures, especially α-B24P24, show superior oxidation resistance and also, both α-B n P n and β-B n P n exhibit better thermal stability; the upper limit temperature that structures can tolerance is 900 K. The electronic properties of new compounds illustrate a higher degree of absorption in the UV and visible-region with the absorption coefficient larger than 105 cm-1, which suggests a wide range of opportunities for advanced optoelectronic applications. The β-B n P n phase has suitable band alignments in the visible-light excitation region, which will produce enhanced photocatalytic activities. On the other hand, α-B n P n structures with modest band gap exhibit large second hyperpolarizability, which are anticipated to have excellent potential as second-order non-linear optical (NLO) materials.

Three metallic BN polymorphs: 1D multi-threaded conduction in a 3D network

In this paper, three novel metallic sp2/sp3-hybridized boron nitride (BN) polymorphs are proposed by first-principles calculations. One of them, denoted as tP-BN, is predicted based on the evolutionary particle swarm structural search. tP-BN is composed of two interlocked rings forming a tube-like 3D network. The stability and band structure calculations show that tP-BN is metastable and metallic at zero pressure. Calculations for the density of states and electron orbitals confirm that the metallicity originates from the sp2-hybridized B and N atoms, forming 1D linear conductive channels in the 3D network. According to the relationship between the atomic structure and electronic properties, another two 3D metastable metallic sp2/sp3-hybridized BN structures are constructed manually. Electronic property calculations show that both of these structures have 1D conductive channels along different axes. The polymorphs predicted in this study enrich the structures and provide a different picture of the conductive mechanism of BN compounds.

Study on spheroidization and the growth mechanism of eutectic boron in Cu–B alloys

The formation process of a solid boron sphere, hollow boron sphere and cladding structure.

Elastic, electronic, chemical bonding and thermodynamic properties of the ternary nitride Ca4TiN4: Ab initio predictions

In order to shed light on the unexplored properties of the ternary nitride Ca₄TiN₄, we report for the first time the results of an ab initio study of its structural, electronic, elastic, chemical bonding and thermodynamic properties. Calculated equilibrium structural parameters are in excellent concordance with available experimental data. Electronic properties were explored through the calculation of the energy band dispersions and density of states. It is found that Ca₄TiN₄ has an indirect band gap (Z-Γ) of 1.625 (1.701) eV using LDA (GGA). Nature of the chemical bonding was studied via Mulliken population analysis and charge density distribution map. It is found that the Ca-N bond is dominantly ionic, whereas the Ti-N one is dominantly covalent. Elastic properties of both single-crystal and polycrystalline phases of the title compound were explored in details using the stain-stress approach. Analysis of the calculated elastic moduli reveals that the title compound is mechanically stable, ductile and elastically anisotropic. Temperature and pressure dependencies of the unit-cell volume, bulk modulus, heat capacities, volume thermal expansion coefficient, Grüneisen parameter and Debye temperature were investigated based on the quasiharmonic Debye model.

Boron Nitride Nanostructures: Fabrication, Functionalization and Applications

Boron nitride (BN) structures are featured by their excellent thermal and chemical stability and unique electronic and optical properties. However, the lack of controlled synthesis of quality samples and the electrically insulating property largely prevent realizing the full potential of BN nanostructures. A comprehensive overview of the current status of the synthesis of two-dimensional hexagonal BN sheets, three dimensional porous hexagonal BN materials and BN-involved heterostructures is provided, highlighting the advantages of different synthetic methods. In addition, structural characterization, functionalizations and prospective applications of hexagonal BN sheets are intensively discussed. One-dimensional BN nanoribbons and nanotubes are then discussed in terms of structure, fabrication and functionality. In particular, the existing routes in pursuit of tunable electronic and magnetic properties in various BN structures are surveyed, calling upon synergetic experimental and theoretical efforts to address the challenges for pioneering the applications of BN into functional devices. Finally, the progress in BN superstructures and novel B/N nanostructures is also briefly introduced.

A first-principles study of orthorhombic CN as a potential superhard material

Using first-principles calculations, we have investigated the structural, electronic, dynamical and mechanical properties of a recently synthesized Pnnm-CN. Phonon dispersion and elastic constant calculations were carried out to demonstrate the dynamical and mechanical stabilities of the Pnnm structure of CN at ambient pressure. The electronic band structure suggests that Pnnm-CN is an insulator with an indirect band gap of about 3.7 eV. First-principles strain-stress relationships at large strains were also simulated to examine the structural and mechanical properties of Pnnm-CN. The established ideal tensile strength of ∼41 GPa in the 〈100〉 direction suggests that CN is a potential superhard material. The present results provide deep insights for understanding the mechanical properties of CN and thus are helpful to explore the potential industrial applications of CN.

Insight into the mechanical, thermodynamics and superconductor properties of NbRuB via first-principles calculation

Using the first-principles calculations, the electronic structure, chemical bonding, mechanical, thermodynamics and superconductor properties of NbRuB are investigated. The optimized lattice parameters were in good agreement with the experimental data. The analysis of the density of states and chemical bonding implies that the metallic behavior of NbRuB originates from the Ru and Nb, and the bonding behaviors are a mixture of covalent-ionic bonds. The bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio and hardness of NbRuB were calculated. The results reveal that the NbRuB is ductility and the Vickers hardness is 15.06 GPa. Moreover, the 3D dependences of reciprocals of Young’s modulus is also calculated and discussed, showing strong anisotropic character for NbRuB. Finally, the Debye temperature and superconducting transition temperature are obtained.

Effective masses, electronic and optical properties of (111)-layered B-site deficient hexagonal perovskite Ba5M4O15 (M = Ta, Nb): a DFT study using HSE06

Density functional theory has been used to investigate crystal structure, effective masses, electronic and optical properties of (111)-layered B-site deficient hexagonal perovskite Ba₅M₄O₁₅ (M = Ta, Nb) as UV-light-responsive photocatalysts.

The low coordination number of nitrogen in hard tungsten nitrides: a first-principles study

Tungsten-nitrogen (W-N) compounds are studied via a combination of first-principles calculations and variable-composition evolutionary structure searches. New candidate ground states and high-pressure phases at 3 : 2, 1 : 1, and 5 : 6 compositions are uncovered and established for possible synthesis. We found that the structures in 4/5-fold N coordination (i.e., NbO-WN and W5N6) are more favoured for the W-N system at low-pressures compared with the conventional 6-fold phases (rs-WN and δ-WN). We attribute the low N coordination feature of W-N ground states to the enhanced W 5d-N 2p orbital hybridization and strong covalent W-N bonding, which involves the full-filling of W-N bonding and antibonding states and can remarkably improve the mechanical strength and hardness. These findings not only clarify the phase diagram of the W-N system, but also shed light on the correlations of hardness with microscopic crystal and electronic structures.

Hardness of FeB4: density functional theory investigation

A recent experimental study reported the successful synthesis of an orthorhombic FeB4 with a high hardness of 62(5) GPa [H. Gou et al., Phys. Rev. Lett. 111, 157002 (2013)], which has reignited extensive interests on whether transition-metal borides compounds will become superhard materials. However, it is contradicted with some theoretical studies suggesting transition-metal boron compounds are unlikely to become superhard materials. Here, we examined structural and electronic properties of FeB4 using density functional theory. The electronic calculations show the good metallicity and covalent Fe-B bonding. Meanwhile, we extensively investigated stress-strain relations of FeB4 under various tensile and shear loading directions. The calculated weakest tensile and shear stresses are 40 GPa and 25 GPa, respectively. Further simulations (e.g., electron localization function and bond length along the weakest loading direction) on FeB4 show the weak Fe-B bonding is responsible for this low hardness. Moreover, these results are consistent with the value of Vickers hardness (11.7-32.3 GPa) by employing different empirical hardness models and below the superhardness threshold of 40 GPa. Our current results suggest FeB4 is a hard material and unlikely to become superhard (>40 GPa).

A metallic superhard boron carbide: first-principles calculations

This work proposed a new metallic, superhard phase of BC₃ which might exist in experimentally synthesized BC₃.

Electronic, optical and photocatalytic properties of three-layer perovskite Dion–Jacobson phase CsBa2M3O10(M = Ta, Nb): a DFT study

This work discloses structural, electronic and optical properties of three-layer perovskite Dion–Jacobson phase CsBa₂M₃O₁₀(M = Ta, Nb) using the DFT method. These semiconductors are UV-light-responsive photocatalysts.

Phase transitions, mechanical properties and electronic structures of novel boron phases under high-pressure: a first-principles study

We have explored the mechanical properties, electronic structures and phase transition behaviors of three designed new phases for element boron from ambient condition to high-pressure of 120 GPa including (1) a C2/c symmetric structure (m-B₁₆); (2) a symmetric structure (c-B₅₆) and (3) a Pmna symmetric structure (o-B₂₄ ). The calculation of the elastic constants and phonon dispersions shows that the phases are of mechanical and dynamic stability. The m-B₁₆ phase is found to transform into another new phase (the o-B₁₆ phase) when pressure exceeds 68 GPa. This might offer a new synthesis strategy for o-B₁₆ from the metastable m-B₁₆ at low temperature under high pressure, bypassing the thermodynamically stable γ-B₂₈. The enthalpies of the c-B₅₆ and o-B₂₄ phases are observed to increase with pressure. The hardness of m-B₁₆ and o-B₁₆ is calculated to be about 56 GPa and 61 GPa, approaching to the highest value of 61 GPa recorded for α-Ga-B among all available Boron phases. The electronic structures and bonding characters are analyzed according to the difference charge-density and crystal orbital Hamilton population (COHP), revealing the metallic nature of the three phases.

Unexpected reconstruction of the α-boron (111) surface

We report a novel reconstruction of the α-boron (111) surface, discovered using ab initio evolutionary structure prediction, and show that this unexpected neat structure has a much lower energy than the recently proposed (111)-I(R,(a)) surface. In this reconstruction, all single interstitial boron atoms bridge neighboring B(12) icosahedra by polar covalent bonds, and this satisfies the electron counting rule, leading to the reconstruction-induced metal-semiconductor transition. The peculiar charge transfer between the interstitial atoms and the icosahedra plays an important role in stabilizing the surface.

Band gap engineering of early transition-metal-doped anatase TiO₂: first principles calculations

The thermal stability and electronic structures of anatase TiO2 doped with early transition metals (TM) (group III-B = Sc, Y and La; group IV-B = Zr and Hf; group V-B = V, Nb and Ta) have been studied using first principles calculations. It was found that all doped systems are thermodynamically stable, and their band gaps were reduced by 1-1.3 eV compared to pure TiO2. Doping with transition metals affects the strength of the hybrid orbital of TM-O bonding, and the band gap increases approximately linearly with the MP value of TM-O bonding.

Nitrogen concentration driving the hardness of rhenium nitrides

The structures and properties of rhenium nitrides are studied with density function based first principle method. New candidate ground states or high-pressure phases at Re:N ratios of 3:2, 1:3, and 1:4 are identified via a series of evolutionary structure searches. We find that the 3D polyhedral stacking with strong covalent N-N and Re-N bonding could stabilize Re nitrides to form nitrogen rich phases, meanwhile, remarkably improve the mechanical performance than that of sub-nitrides, as Re3N, Re2N, and Re3N2. By evaluating the trends of the crystal configuration, electronic structure, elastic properties, and hardness as a function of the N concentration, we proves that the N content is the key factor affecting the metallicity and hardness of Re nitrides.

A computational study of carbon dioxide adsorption on solid boron

The study demonstrates these “electron deficient” boron solids can capture CO₂ on their basic sites due to Lewis acid–base interactions.

First-principle calculations on the structural stability and electronic properties of superhard BxCy compounds

With first-principle calculations, we studied the structural stability and electronic properties of the BxCy compounds based on three kinds of phases including diamond-like, C20-like and B15-like phases. The C20-like structure B8C12 is found to be a new stable structure with relatively low formation energy in middle boron concentration and is expected to be synthesized experimentally. Combined with a microscopic model, the Vickers hardness of the different configurations of BxCy compounds is analyzed with the change of boron concentration. It is found that the hardness of the B-C system has a decreasing trend with the increase of boron concentration. In addition, all the structures have metallic properties, except B12C3 and B14C. With the analysis of Mulliken bond population and charge distribution, the bonds with high electron density and short bond length have an important contribution to the hardness in the B-C system, while the effect of metallicity to hardness can be ignored.

Compressed carbon nanotubes: a family of new multifunctional carbon allotropes

The exploration of novel functional carbon polymorphs is an enduring topic of scientific investigations. In this paper, we present simulations demonstrating metastable carbon phases as the result of pressure induced carbon nanotube polymerization. The configuration, bonding, electronic, and mechanical characteristics of carbon polymers strongly depend on the imposed hydrostatic/non-hydrostatic pressure, as well as on the geometry of the raw carbon nanotubes including diameter, chirality, stacking manner, and wall number. Especially, transition processes under hydrostatic/non-hydrostatic pressure are investigated, revealing unexpectedly low transition barriers and demonstrating sp(2)→sp(3) bonding changes as well as peculiar oscillations of electronic property (e.g., semiconducting→metallic→semiconducting transitions). These polymerized nanotubes show versatile and superior physical properties, such as superhardness, high tensile strength and ductility, and tunable electronic properties (semiconducting or metallic).

A novel low compressible and superhard carbon nitride: body-centered tetragonal CN2

A novel body-centered tetragonal CN(2) (4 units per cell), named as bct-CN(2), has been predicted here using our newly developed particle swarm optimization algorithm for crystal structure prediction. Bct-CN(2) is energetically much superior (3.022 eV per f.u.) to previously proposed pyrite structure and stable against decomposition into a mixture of diamond + N(2) or 1/3(C(3)N(4) + N(2)) above 45.4 GPa. No imaginary phonon frequencies in the whole Brillouin zone indicate bct-CN(2) is dynamically stable. The electronic calculations indicate that bct-CN(2) is a wide gap dielectric material with an indirect band gap of 3.6 eV. The ideal tensile, shear, and compressive strength at large strains of bct-CN(2) are examined to understand further the microscopic mechanism of the structural deformation. Strikingly, it is found that bct-CN(2) has high calculated ideal strength, bulk modulus, shear modulus, and simulated hardness, indicating its very incompressible and superhard nature. The results provide new thoughts for designing and synthesizing novel superhard carbon nitrides, and insights for understanding the mechanical properties.

Extra-electron induced covalent strengthening and generalization of intrinsic ductile-to-brittle criterion

Traditional strengthening ways, such as strain, precipitation, and solid-solution, come into effect by pinning the motion of dislocation. Here, through first-principles calculations we report on an extra-electron induced covalent strengthening mechanism, which alters chemical bonding upon the introduction of extra-valence electrons in the matrix of parent materials. It is responsible for the brittle and high-strength properties of Al(12)W-type compounds featured by the typical fivefold icosahedral cages, which are common for quasicrystals and bulk metallic glasses (BMGs). In combination with this mechanism, we generalize ductile-to-brittle criterion in a universal hyperbolic form by integrating the classical Pettifor's Cauchy pressure with Pugh's modulus ratio for a wide variety of materials with cubic lattices. This study provides compelling evidence to correlate Pugh's modulus ratio with hardness of materials and may have implication for understanding the intrinsic brittleness of quasicrystals and BMGs.

Superhard F-carbon predicted by ab initio particle-swarm optimization methodology

A simple (5 + 6 + 7)-sp(3) carbon (denoted as F-carbon) with eight atoms per unit cell predicted by a newly developed ab initio particle-swarm optimization methodology on crystal structure prediction is proposed. F-carbon can be seen as the reconstruction of AA-stacked or 3R-graphite, and is energetically more stable than 2H-graphite beyond 13.9 GPa. Band structure and hardness calculations indicate that F-carbon is a transparent superhard carbon with a gap of 4.55 eV at 15 GPa and a hardness of 93.9 GPa at zero pressure. Compared with the previously proposed Bct-, M- and W-carbons, the simulative x-ray diffraction pattern of F-carbon also well matches the superhard intermediate phase of the experimentally cold-compressed graphite. The possible transition route and energy barrier were observed using the variable cell nudged elastic band method. Our simulations show that the cold compression of graphite can produce some reversible metastable carbons (e.g. M- and F-carbons) with energy barriers close to diamond or lonsdaleite.

Novel superhard carbon: C-centered orthorhombic C8

A novel carbon allotrope of C-centered orthorhombic C(8) (Cco-C(8)) is predicted by using a recently developed particle-swarm optimization method on structural search. Cco-C(8) adopts a sp(3) three-dimensional bonding network that can be viewed as interconnected (2,2) carbon nanotubes through 4- and 6-member rings and is energetically more favorable than earlier proposed carbon polymorphs (e.g., M carbon, bct-C(4), W carbon, and chiral C(6)) over a wide range of pressures studied (0-100 GPa). The simulated x-ray diffraction pattern, density, and bulk modulus of Cco-C(8) are in good accordance with the experimental data on structurally undetermined superhard carbon recovered from cold compression of carbon nanotube bundles. The simulated hardness of Cco-C(8) can reach a remarkably high value of 95.1 GPa, such that it is capable of cracking diamond.

Three dimensional carbon-nanotube polymers

Eight fascinating sp(2)- and sp(3)-hybridized carbon allotropes have been uncovered using a newly developed ab initio particle-swarm optimization methodology for crystal structure prediction. These crystalline allotropes can be viewed respectively as three-dimensional (3D) polymers of (4,0), (5,0), (7,0), (8,0), (9,0), (3,3), (4,4), and (6,6) carbon nanotubes, termed 3D-(n, 0) or 3D-(n, n) carbons. The ground-state energy calculations show that the carbons all have lower energies than C(60) fullerene, and some are energetically more stable than the van der Waals packing configurations of their nanotube parents. Owing to their unique configurations, they have distinctive electronic properties, high Young's moduli, high tensile strength, ultrahigh hardness, good ductility, and low density, and may be potentially applied to a variety of needs.

Electron-deficient and polycenter bonds in the high-pressure γ-B28 phase of boron

The peculiar bonding situation in γ boron is characterized on the basis of an experimental electron-density distribution which is obtained by multipole refinement against low-temperature single-crystal x-ray diffraction data. A topological analysis of the electron-density distribution reveals one-electron-two-center bonds connecting neighboring icosahedral B(12) clusters. A unique polar-covalent two-electron-three-center bond between a pair of atoms of an icosahedral cluster and one atom of the interstitial B(12) dumbbell explains the observed charge separation in this high-pressure high-temperature polymorph of boron.