N. Dubrovinskaia, L. Dubrovinsky, E. Yu. Zarechnaya, Y. Filinchuk, D. Chernyshov, V. Dmitriev, A. S. Mikhaylushkin, I. A. Abrikosov, S. I. Simak
Oganov et al report a discovery of an ionic high-pressure /hitherto unknown phase of boron/. We show that this phase has been known since 1965, and it is a covalent material.
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The allotropes of boron continue to challenge structural elucidation and solid-state theory. Here we use machine learning combined with random structure searching (RSS) algorithms to systematically construct an interatomic potential for boron. Starting from ensembles of randomized atomic configurations, we use alternating single-point quantum-mechanical energy and force computations, Gaussian approximation potential (GAP) fitting, and GAP-driven RSS to iteratively generate a representation of the element's potential-energy surface. Beyond the total energies of the very different boron allotrop…
The crystal structure of boron is unique among chemical elements, highly complex, and imperfectly known. Experimentalists report the beta-rhombohedral (black) form is stable over all temperatures from absolute zero to melting. However, early calculations found its energy to be greater than the energy of the alpha-rhombohedral (red) form, implying beta cannot be stable at low temperatures. Furthermore, beta exhibits partially occupied sites, seemingly in conflict with the thermodynamic requirement that entropy vanish at low temperature. Using electronic density functional theory methods and an …
The structure of amorphous materials has been debated since the 1930's as a binary question: amorphous materials are either Zachariasen continuous random networks (Z-CRNs) or Z-CRNs containing crystallites. It was recently demonstrated, however, that amorphous diamond can be synthesized in either form. Here we address the question of the structure of single-atom-thick amorphous monolayers. We reanalyze the results of prior simulations for amorphous graphene and report kinetic Monte Carlo simulations based on alternative algorithms. We find that crystallite-containing Z-CRN is the favored struc…
Boron is an element of fascinating chemical complexity. Controversies have shrouded this element since its discovery was announced in 1808: the new 'element' turned out to be a compound containing less than 60-70 percent of boron, and it was not until 1909 that 99-percent pure boron was obtained. And although we now know of at least 16 polymorphs, the stable phase of boron is not yet experimentally established even at ambient conditions. Boron's complexities arise from frustration: situated between metals and insulators in the periodic table, boron has only three valence electrons, which would…
The icosahedra boron chain and three icosahedra sheets (with α, δ4, and δ6 symmetries), constructed by the icosahedra B12, have been obtained as new members of boron family using a highly efficient molecular dynamics scheme based on a transferable and reliable semi-empirical Hamiltonian. The icosahedral B12 in the icosahedra chain is slightly elongated along the china direction and directly bonded each other with the two-center covalent bonds. A deformation of the icosahedra B12 was also found in the two-dimensional icosahedra sheets. In addition to the three-center bonding nature inside the i…
Boron bulk crystals are marked by exceptional structural complexity and unusual related physical phenomena. Recent reports of hydrogenated $α$-tetragonal and a new $δ$-orthorhombic boron B$_{52}$ phase have raised many fundamental questions. Using density functional theory calculations it is shown that hydrogenated $α$-tetragonal boron has at least two stable stoichiometric compositions, B$_{51}$H$_{7}$ and B$_{51}$H$_{3}$. Thermodynamic modeling was used to qualitatively reproduce the two-step phase transition reported by Ekimov et al. [J. Mater. Res. 31, 2773 (2016)] upon annealing, which co…
General purpose graphical processing units (GPU's) offer high processing speeds for certain classes of highly parallelizable computations, such as matrix operations and Fourier transforms, that lie at the heart of first-principles electronic structure calculations. Inclusion of exact-exchange increases the cost of density functional theory by orders of magnitude, motivating the use of GPU's. Porting the widely used electronic density functional code VASP to run on a GPU results in a 5-20 fold performance boost of exact-exchange compared with a traditional CPU. We analyze performance bottleneck…
Recently, a covalent-like quasi-bonding was unveiled for the inter-chain interaction in a promising semiconductor, few-layer Tellurium. Such quasi-bond offers comparable bond lengths and strengths with those of a typical Te-Te covalent bond, which may lead to much easier transformations between the quasi-bonds and the covalent bonds. Here, we show a few structural phase-transitions among four Te allotropes in few-layer Te under charge doping. In particular, the semiconducting α-phase can transform into a smaller-bandgap β-phase, even-smaller-bandgap γ-phase and then metallic δ-phase in a Te bi…
Bonding characteristics of liquid boron at 2500K are studied by using high resolution Compton scattering. An excellent agreement is found between the measurements and the corresponding Car-Parinello molecular dynamics simulations. Covalent bond pairs are clearly shown to dominate in liquid boron along with the coexistence of diffuse pairs. Our study reveals the complex bonding pattern of liquid boron, and gives insight into the unusual properties of this high temperature liquid.
Two boron-rich chalcogenides B12S and B12Se isostructural to α-rhombohedral boron were synthesized by chemical reaction of the elements at high-pressure - high-temperature conditions. The crystal structures and stoichiometries of both compounds were confirmed by Rietveld refinement and elemental analysis. The experimental Raman spectra of B12S and B12Se were investigated for the first time. All observed Raman bands have been attributed to the theoretically calculated phonon modes, and the mode assignment has been performed.
Boron nitride (BN) has been explored these days because of its extraordinary optical, chemical and mechanical properties. BN is sensitive to its crystal structure that slight change in lattice parameters enormously change its properties. Present study deals with synthesis, characterization as well as photocatalytic applications of BN-based composite. When boron nitride was mixed with SnO2 having tetragonal crystal structure, dissociation into smaller sheets occurred and the material oriented to (102) plane. SnO2 particles attached both sides of BN sheets provided high surface area which make t…
It has been widely accepted that planar boron structures, composed of triangular and hexagonal motifs are the most stable two dimensional (2D) phases and likely precursors for boron nanostructures. Here we predict, based on ab initio evolutionary structure search, novel 2D boron structure with non-zero thickness, which is considerably, by 50 meV/atom lower in energy than the recently proposed alpha-sheet structure and its analogues. In particular, this phase is identified for the first time to have a distorted Dirac cone, after graphene and silicene the third elemental material with massless D…
By means of variable-composition evolutionary algorithm coupled with density functional theory and in combination with aberration-corrected high-resolution transmission electron microscopy experiments, we have studied and characterized the composition, structure and hardness properties of WB$_{3+x}$ ($x$ $<$ 0.5). We provide robust evidence for the occurrence of stoichiometric WB$_3$ and non-stoichiometric WB$_{3+x}$ both crystallizing in the metastable $hP$16 ($P6_3/mmc$) structure. No signs for the formation of the highly debated WB$_4$ (both $hP$20 and $hP$10) phases were found. Our results…
The formation of boron carbide under high pressures and from elemental reactants has been studied and optimum parameters have been determined by varying the (P, T, reactants) conditions. To this end, stoichiometric mixtures of commercial beta rhombohedral boron and amorphous glassy carbon have been subjected to temperatures ranging from 1473 K to 2473 K at pressures of 2 GPa, 5 GPa and 13 GPa. Similar syntheses have been repeated for mixtures of beta boron and graphite, and amorphous boron and amorphous carbon at 2 GPa and 5 GPa. The carbon concentration of boron carbide is shown to be affecte…
The combination of high throughput computation and machine learning has led to a new paradigm in materials design by allowing for the direct screening of vast portions of structural, chemical, and property space. The use of these powerful techniques leads to the generation of enormous amounts of data, which in turn calls for new techniques to efficiently explore and visualize the materials space to help identify underlying patterns. In this work, we develop a unified framework to hierarchically visualize the compositional and structural similarities between materials in an arbitrary material s…
We report a high-throughput ab-initio study of the thermodynamic and superconducting proper- ties of the recently synthesized XB$_3$C$_3$ clathrates. These compounds, in which boron and carbon form a sponge-like network of interconnected cages each enclosing a central X atom, are attractive candidates to achieve high-Tc conventional superconductivity at ambient pressure, due to the simultaneous presence of a stiff B-C covalent network and a tunable charge reservoir, provided by the guest atom. Ternary compounds like CaB$_3$C$_3$, SrB$_3$C$_3$ and BaB$_3$C$_3$ are predicted to exhibit T$_c$ $\l…
Local structure, thermodynamics, and melting curve of boron phosphide at high pressures by deep learning-driven ab initio simulations
本文开发了磷化硼的深度机器学习势，在从头算精度下模拟其高压高温行为，揭示了熔化结构、配位演化及12–15 GPa的反常结构转变，为共价材料熔化研究提供了新方法和新认识。
Prediction of fully metallic σ-bonded boron framework induced high superconductivity above 100 K in thermodynamically stable Sr2B5 at 40 GPa
本研究通过第一性原理结构搜索，发现在40 GPa下热力学稳定的F43m Sr2B5因全金属σ键合硼框架与E声子模式强耦合，具有约100 K的高温超导性，为设计新型高超导硼化物提供了典型结构。
The complex crystal chemistry of elemental boron has led to numerous proposed structures with distinctive motifs as well as contradictory findings. Herein, evolutionary structure searches performed at 100 GPa have uncovered a series of potential new metastable phases of boron, and bonding analyses were carried out to elucidate their electronic structure. These polymorphs, dynamically stable at 100 GPa, were grouped into two families. The first was derived from the thermodynamic minimum at these conditions, $α$-Ga, whereas channels comprised the second. Two additional intergrowth structures wer…
Five single-phase WB2- and MoB2-containing high-entropy borides (HEBs) have been made via reactive spark plasma sintering of elemental boron and metals. A large reactive driving force enables the full dissolution of 10-20 mol. % WB2 to form dense, single-phase HEBs, including (Ti0.2Zr0.2Hf0.2Mo0.2W0.2)B2, (Ti0.2Ta0.2Cr0.2Mo0.2W0.2)B2, (Zr0.2Hf0.2Nb0.2Ta0.2W0.2)B2, and (Zr0.225Hf0.225Ta0.225Mo0.225W0.1)B2; the successful fabrication of such single-phase WB2-containing HEBs has not been reported before. In the processing science, this result serves perhaps the best example demonstrating that the…
Using projector augmented wave approach within the generalized gradient approximation, we have studied the structural property and electronic structure of ZrB12. The calculated lattice constants and bulk modulus are in good agreement with the available experimental values. A detailed study of the electronic structure and the charge-density redistribution reveals the features of strong covalent B-B and weak covalent Zr-B bondings in ZrB12. The states at the Fermi level mainly come from the $σ$-$p_{y}$ and $π$-$p_{z}$ orbitals of boron atoms, which are slightly hybridized with the $t_{2g}$-$d_{x…
Density Functional Theory calculations using the quasi-harmonic approximation have been used to calculate the solid Hugoniot of two polytypes of boron carbide up to 100 GPa. Under the assumption that segregation into the elemental phases occurs around the pressure that the B11Cp(CBC) polytype becomes thermodynamically unstable with respect to boron and carbon, two discontinuities in the Hugoniot, one at 50 GPa and one at 90-100 GPa, are predicted. The former is a result of phase segregation, and the latter a phase transition within boron. First principles molecular dynamics (FPMD) simulations …
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…
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…
Electrostatic, Luminescent, and Paramagnetic Responses of Fresh BN Nanopowders Synthesized under Concentrated Light
本研究探究浓光下合成的BN纳米粉体的静电、发光和顺磁响应，发现其初始因表面缺陷具相关活性，暴露空气后缺陷被钝化致活性消失，揭示表面化学对BN基纳米材料功能性质的关键调控作用。
Sublayers Editing of Covalent MAX Phase for Nanolaminated Early Transition Metal Compounds
针对 A、X 子层均为共价键的 MAX 相无法常规刻蚀制备 MXene 的问题，本文利用高温熔盐中的反应性差异进行子层编辑，获得兼具 MXene 与过渡金属硫族化合物特征的 TMXCs，并可剥离为单层。
For the first time, a group of CaB6-typed cubic rare earth high-entropy hexaborides have been successfully fabricated into dense bulk pellets (>98.5% in relative densities). The specimens are prepared from elemental precursors via in-situ metal-boron reactive spark plasma sintering. The sintered bulk pellets are determined to be single-phase without any detectable oxides or other secondary phases. The homogenous elemental distributions have been confirmed at both microscale and nanoscale. The Vickers microhardness are measured to be 16-18 GPa at a standard indentation load of 9.8 N. The nanoin…
Similar effects of metal and boron vacancies on phonon scattering and lattice thermal conductivity ($κ_l$) of ZrB$_2$ and HfB$_2$ are reported. These defects challenge the conventional understanding that associates larger impacts to bigger defects. We find the underlying reason to be a strong local perturbation caused by the boron vacancy that substantially changes the interatomic force constants. In contrast, a long ranged but weaker perturbation is seen in the case metal vacancies. We show that these behaviours originate from a mixed metallic and covalent bonding nature in the metal diboride…
High-Throughput Discovery of Chiral Carbon-Boron Semimetals with Weyl Topology and Phonon-Mediated Superconductivity
本研究通过高通量计算筛选出6种稳定的手性碳-硼拓扑半金属，其中两种分别具有电荷三外尔点及31.1K超导转变温度、电荷一外尔点及各向异性多带超导性，拓展了相关材料范围。
Cubic BeB$_2$: A metastable $p$-type conductive material from first principles
立方BeB₂通过第一性原理研究发现其具有高空穴浓度和迁移率，是高效p型导电材料的候选，同时在退化掺杂下表现出低温超导性。
We report the effect of carbon doping in Ti-stabilized nonstoichiometric molybdenum diboride (Mo$_{0.96}$Ti$_{0.04}$)$_{0.8}$B$_{2}$, which exhibits bulk superconductivity below $T_{\rm c}$ = 7.0 K. It is found that (Mo$_{0.96}$Ti$_{0.04}$)$_{0.8}$(B$_{1-x}$C$_{x}$)$_{2}$ maintains the AlB$_{2}$-type phase with a uniform elemental distribution for $x$ = 0.12 and 0.16. The substitution of carbon for boron leads to a slight increase in $a$-axis, a remarkable reduction in $c$-axis, the formation of planar defects along the (100) crystallographic planes, and a shift of the B 1$s$ peaks towards hig…
K-Cl is a simple system displaying all four main types of bonding, as it contains (i) metallic potassium, (ii) elemental chlorine made of covalently bonded Cl2 molecules held together by van der Waals forces, and (iii) an archetypal ionic compound KCl. Well-known octet rule (and its special case, the charge balance rule, assigning classical charges of +1 to K and -1 to Cl) predicts that no compounds other than KCl are possible. However, our quantum-mechanical variable-composition evolutionary simulations predict new stable compounds. The phase diagram of the K-Cl system turned out to be extrem…
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…
Compositional Tuning in NaxAlB14 via Diffusion Control
提出高压扩散控制结合后退火方法，在亚稳态 NaxAlB14 中实现均匀可调 Na 组成，并揭示其带隙、电导率及硼空位缺陷态的演化机制。
Prediction of an alternative route to polymeric carbon dioxide: A metastable energetic material
通过第一性原理计算预测，等温压缩CO+O₂混合物可将室温碳聚合起始压力从约118 GPa降至约7 GPa，并形成一种可回收至常温常压的亚稳态含能聚合物。
Phase stability and mechanical property trends for MAB phases by high-throughput ab initio calculations
本研究通过高通量从头计算筛选多种元素与化学计量比构成的MAB相，分析其相稳定性、相变增韧潜力与各向异性力学性能，识别出Nb₃AlB₄等多种高潜力材料，为MAB相的实验合成与应用提供理论指导。
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