A. R. Oganov, V. L. Solozhenko, O. O. Kurakevych, C. Gatti, Y. Ma, J. Chen, Z. Liu, R. J. Hemley
This Comment points out a number of errors in the recent paper by Zarechnaya, Dubrovinskaia, Dubrovinsky, et al. (Phys. Rev. Lett. 102, 185501 (2009)). Results and conclusions presented by Zarechnaya et al. (2009) are either incorrect or have been presented before.
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Microstructure and mechanical properties of bulk polycristalline rhombohedral boron subnitride B13N2 synthesized by crystallization from the B-BN melt at 7 GPa have been systematically studied by micro- and nanoindentation, atomic force microscopy and scanning electron microscopy. The obtained data on hardness, elastic properties and fracture toughness clearly indicate that B13N2 belongs to a family of superhard phases and can be considered as a promising superabrasive or binder for cubic boron nitride.
Boron is a unique element, being the only element, all known polymorphs of which are superhard, and all of its crystal structures are distinct from any other element. The electron-deficient bonding in boron explains its remarkable sensitivity to even small concentrations of impurity atoms and allows boron to form peculiar chemical compounds with very different elements. These complications made the study of boron a great challenge, creating also a unique and instructive chapter in the history of science. Strange though it may sound, the discovery of boron in 1808 was ambiguous, with pure boron…
A superhard boron nitride phase dubbed as Z-BN is proposed as possible intermediate phase between h-BN and zinc blende BN (c-BN), and investigated using first-principles calculations within the framework of the density functional theory. Although the structure of Z-BN is similar to that of bct-BN containing four-eight BN rings, it is more energy favorable than bct-BN. Our study reveals that Z-BN, with a considerable structural stability and high density comparable to c-BN, is a transparent insulator with an indirect band gap about 5.27 eV. Amazingly, its Vickers hardness is 55.88 Gpa which is …
New carbon forms exhibiting extraordinary physico-chemical properties can be generated from nanostructured precursors under extreme pressure. Nevertheless, synthesis of such fascinating materials is often not well understood that results, as is the case of C60 precursor, in irreproducibility of the results and impeding further progress in the materials design. Here the semiconducting amorphous carbon having bandgaps of 0.1-0.3 eV and the advantages of isotropic superhardness and superior toughness over single-crystal diamond and inorganic glasses are produced from transformation of fullerene a…
In the present paper we performed the analysis of available data on structural, thermodynamic and mechanical properties of B6O. Although the compound is known for half a century and has been extensively studied, many properties of this boron-rich solid remain unknown or doubtful. Semi-empirical analysis of our experimental and literature data allowed us to choose the best values of main thermodynamic and mechanical characteristics among previously reported data, to predict the thermoelastic equation of state of B6O, and dependence of its hardness on non-stoichiometry and temperature.
In the present work we have proposed the method that allows one to easily estimate hardness and bulk modulus of known or hypothetical solid phases from the data on Gibbs energy of atomization of the elements and corresponding covalent radii. It has been shown that hardness and bulk moduli of compounds strongly correlate with their thermodynamic and structural properties. The proposed method may be used for a large number of compounds with various types of chemical bonding and structures; moreover, the temperature dependence of hardness may be calculated, that has been performed for diamond and…
The basic known and hypothetic one- and two-element phases of the B-C-N-O system (both superhard phases having diamond and boron structures and precursors to synthesize them) are described. The attention has been given to the structure, basic mechanical properties, and methods to identify and characterize the materials. For some phases that have been recently described in the literature the synthesis conditions at high pressures and temperatures are indicated.
Boron suboxide B6O, the hardest known oxide, has an R-3m crystal structure (α-B6O) that can be described as an oxygen-intercalated structure of α-boron, or, equivalently, as a cubic close packing of B12 icosahedra with two oxygen atoms occupying all octahedral voids in it. Here we show a new ground state of this compound at ambient conditions, Cmcm-B6O (\b{eta}-B6O), which in all quantum-mechanical treatments that we tested (GGA, LDA, and hybrid functional HSE06) comes out to be slightly but consistently more stable. Increasing pressure and temperature further stabilize it with respect to the …
The search of new superhard materials has received a strong impulse by industrial demands for low-cost alternatives to diamond and $c$-BN, such as metal borides. In this Letter we introduce a new family of superhard materials, "fused borophenes", containing 2D boron layers which are interlinked to form a 3D network. These materials, identified through a high-throughput scan of BxC1-x structures, exhibit Vicker's hardnesses comparable to those of the best commercial metal borides. Due to their low formation enthalpies, fused borophenes could be synthesized by high-temperature methods, starting …
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-3m 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 B24 clusters with stuffing of C2 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 ne…
The pressure-induced transformation of diatomic nitrogen into non-molecular polymeric phases may produce potentially useful high-energy-density materials. We combine first-principles calculations with structure searching to predict a new class of nitrogen-rich boron nitrides with a stoichiometry of B3N5 that are stable or metastable relative to solid N2 and h-BN at ambient pressure. The most stable phase at ambient pressure has a layered structure (h-B3N5) containing hexagonal B3N3 layers sandwiched with intercalated freely rotating N2 molecules. At 15 GPa, a three-dimensional C2221 structure …
Hexagonal BC$_{2}$N is a superhard material recently identified to be comparable to or even harder than cubic boron nitride (c-BN) due the full $sp^3$ bonding character and the higher number of C-C and B-N bonds compared to C-N and B-C.Using a first-principles approach to calculate force constants and an exact numerical solution to the phonon Boltzmann equation, we show that BC$_{2}$N has a high lattice thermal conductivity exceeding that of c-BN owing to the strong C-C and B-N bonds, which produce high phonon frequencies as well as high acoustic velocities. The existence of large group veloci…
Elementary semiconductors are rare and attractive, especially for low-dimensional materials. Unfortunately, most of boron nanostructures were found to be metallic, despite of their typical semiconducting bulk structure. Herein, we propose a general recipe to realize low-dimensional semiconducting boron. This unusual semiconducting behavior is attributed to charge transfer and electron localization, induced by the symmetry breaking that divides boron atoms into cations and anions. In addition, it is feasible to accomplish band gap engineering by rationally designing various structures. Importan…
The two-dimensional boron monolayers were reported to be metallic both in previous theoretical predictions and experimental observations, however, we have firstly found a family of boron monolayers with the novel semiconducting property as confirmed by the first-principles calculations with the quasi-particle G0W0 approach. We demonstrate that the vanished metallicity characterized by the pz-derived bands cross the Fermi level is attributed to the motif of a triple-hexagonal-vacancy, with which various semiconducting boron monolayers are designed to realize the band-gap engineering for the pot…
The present review covers the production of new materials under high pressures. A primary limitation on the use of pressures higher than 1 GPa is a small volume and mass of a produced material. Therefore, despite an extremely wide range of new high-pressure synthesized substances with unique properties, synthesis on an commercial scale is applied up to now only to obtain superhard materials, this real treasure of today's industry. At the same time, high-pressure experiments often give material scientists a hint at what new intriguing substances can exist in principle. This is true for new supe…
The typical two-dimensional semiconductors, group \uppercase\expandafter{\romannumeral3A} chalcogenides, have garnered tremendous interest for their outstanding electronic, mechanical, and chemical properties. However, so far, there have been almost no reports on boron monosulfides (BS) binary material. Here, four two-dimensional BS sheets, namely the $α$-, $β$-, $γ$-, and $δ$-BS sheets, are proposed and discussed from $\emph{ab initio}$ calculations. State-of-the-art first-principles calculations reveal all these structures are thermally and dynamically stable, indicating the potential for su…
New candidate ground states at 1:4, 1:2, and 1:1 compositions are identified in the well-known Fe-B system via a combination of ab initio high-throughput and evolutionary searches. We show that the proposed oP12-FeB2 stabilizes by a break up of 2D boron layers into 1D chains while oP10-FeB4 stabilizes by a distortion of a 3D boron network. The uniqueness of these configurations gives rise to a set of remarkable properties: oP12-FeB2 is expected to be the first semiconducting metal diboride and oP10-FeB4 is shown to have the potential for phonon-mediated superconductivity with a Tc of 15-20 K.
We investigate the influence of impurities and vacancies in the formation of magnetic moment in CaB6 using full potential ab initio band structure calculations. CaB6 is found to be a band insulator with a band gap of about 0.2 eV. The calculated results indicate that carbon and oxygen substitution in boron sublattice, usually expected as impurity in low purity borons, do not play significant role in the local moment formation. Boron vacancy in the boron sublattice, on the other hand, leads to the formation of an impurity band in the vicinity of the Fermi level, which exhibit finite exchange sp…
We present {\it ab initio} and self-consistent tight-binding calculations on the band structure of single wall semiconducting carbon nanotubes with high degrees (up to 25 %) of boron substitution. Besides a lowering of the Fermi energy into the valence band, a regular, periodic distribution of the p-dopants leads to the formation of a dispersive ``acceptor''-like band in the band gap of the undoped tube. This comes from the superposition of acceptor levels at the boron atoms with the delocalized carbon $π$-orbitals. Irregular (random) boron-doping leads to a high concentration of hybrids of ac…
Ultrananocrystalline diamond/hydrogenated amorphous carbon composite thin films synthesized via coaxial arc plasma possess a marked structural feature of diamond grains embedded in an amorphous carbon and a hydrogenated amorphous carbon matrix which are the largest constituents of the films. Since the amorphous nature yields much larger light absorption coefficients as well as a generation source of photo-induced carriers with UV rays, these films can be potential candidates for deep-UV photodetector applications. From some previous studies p-type conduction of the films has been realized by d…
Molybdenum borides were studied theoretically using first-principles calculations, empirical total energy model and global optimization techniques to determine stable crystal structures. Our calculations reveal the structures of known Mo-B phases, attaining close agreement with experiment. Following our developed lattice model, we describe in detail the crystal structure of boron-rich $MoB_x$ phases with 3<x<9 as the hexagonal $P6_3/mmc$-$MoB_3$ structure with Mo atoms partially replaced by triangular boron units. The most energetically stable arrangement of these $B_3$ units corresponds to th…
Phase relations in the B-BN system have been studied ex situ and in situ at pressures 2-20 GPa and temperatures up to 2800 K. The evolution of topology of the B-BN phase diagram has been investigated up to 24 GPa using models of phenomenological thermodynamics with interaction parameters derived from our experimental data on phase equilibria at high pressures and high temperatures. There are two thermodynamically stable boron subnitrides in the system i.e. B13N2 and B50N2. Above 16.5 GPa the B50N2 = L + B13N2 peritectic reaction transforms to the solid-phase reaction of B50N2 decomposition int…
Through the use of perturbation theory, in this work we develop a method which allows for a substantial reduction in the size of the plane-wave basis used in density-functional calculations. This method may be used for both pseudopotentials and all-electron calculations and is particularly beneficial in the latter case. In all cases, the approach has the advantage of allowing accurate predictions of transferability errors for any environment. Finally, this method can be easily implemented into conjugate gradient techniques and it is therefore computationally efficient. In this work, we apply t…
Molecular Beam Epitaxy growth of MoTe$_2$ on Hexagonal Boron Nitride
本文首次在六方氮化硼衬底上实现MoTe$_2$的分子束外延生长，并通过AFM和拉曼光谱揭示了2H与1T'相共存及退火对扩散常数的显著调控作用。
Two-dimensional materials offer a versatile platform to study high-harmonic generation (HHG), encompassing as limiting cases bulk-like and atomic-like harmonic generation [Tancogne-Dejean and Rubio, Science Advance \textbf{4}, eaao5207 (2018)]. Understanding the high-harmonic response of few-layer semiconducting systems is important, and might open up possible technological applications. Using extensive first-principle calculations within a time-dependent density functional theory framework, we show how the in-plane and out-of-plane nonlinear non-perturbative response of two-dimensional materi…
In the present study, the structural and hitherto uninvestigated mechanical (elastic stiffness constants, machinability index, Cauchy pressure, anisotropy indices, brittleness/ductility, Poissons ratio), electronic, optical, and thermodynamic properties of novel boron-rich compounds B6X (X = S, Se) have been explored using density functional theory. The estimated structural lattice parameters were consistent with the prior report. The mechanical and dynamical stability of these compounds have been established theoretically. The materials are brittle in nature and elastically anisotropic. The v…
Theoretical Predictions of MB5N5: Atom-Stuffed Boronitride Clathrate Cages Derived from the High-Pressure Superhydride
该研究通过高通量DFT、声子计算、AIMD和机器学习预测，筛选源自MH10超氢化物的MX5Y5笼状结构，发现34种常压动力学稳定MB5N5相，但超导相均在300 K、1 atm下分解。
We investigate the adsorption of the nucleic acid bases, adenine (A), guanine (G), cytosine (C), thymine (T) and uracil (U) on the outer wall of a high curvature semiconducting single-walled boron nitride nanotube (BNNT) by first principles density functional theory calculations. The calculated binding energy shows the order: G>A\approxC\approxT\approxU implying that the interaction strength of the (high-curvature) BNNT with the nucleobases, G being an exception, is nearly the same. A higher binding energy for the G-BNNT conjugate appears to result from a stronger hybridization of the molecula…
Triple points and nexus points are two interesting topological phases, which have been reported in some three-dimensional (3D) materials. Here, we propose that triple points, nexus points and related phases, such as topological tangle nodal lines, can be obtained by alternatively stacking two types of monolayers. Two conditions for the stacking monolayers are required: the first condition is that they have a three-fold (C3) rotation symmetry and three mirror planes along the C3 axis; the second condition is that one of the monolayers should be insulating while the other one should be metallic …
In the search for MgB2-like phonon-mediated superconductors we have carried out a systematic density functional theory study of the Ca-B system, isoelectronic to Mg-B, at ambient and gigapascal pressures. A remarkable variety of candidate high-pressure ground states have been identified with an evolutionary crystal structure search, including a stable alkaline-earth monoboride oI8-CaB, a superconductor with an expected critical temperature (Tc) of 5.5 K. We have extended our previous study of CaB6 [Phys. Rev. Lett. 108, 102501 (2012)] to nearby stoichiometries of CaB[6+x], finding that extra b…
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