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Diamond cubic Si is a widely used semiconductor; however, its optical properties still need to be improved to meet industrial demands. There are several Si metastable phases obtained from high-pressure synthesis, including the hexagonal diamond and bc8 phases. Nanostructured bc8-Si is considered as a promising solar-absorbing material owing to its direct band gap and multiple exciton generation but it is hard to produce through high-pressure methods due to amorphization during unloading. In this work, bulk nanocrystalline diamond cubic Si was compressed up to ∼20 GPa in a diamond anvil cell and decompressed at ambient conditions. The phase transitions during compression resemble those occured in previous nanostructured Si, while upon decompression, the phase transitions are similar to those observed in bulk Si, leading to the recovery of bulk nanocrystalline bc8/r8 phase. Annealing the bulk nanocrystalline bc8/r8-Si led to a gradual transformation from the r8 to bc8 phase. Grain boundaries are considered important for r8-Si nucleation in bulk nanocrystalline Si during decompression. The high-pressure behaviors of Si nanopowders under different pressure environments were also explored to verify the significance of grain boundaries in pressure-induced phase transitions. This study offers a new pathway toward the nanocrystalline bc8/r8-Si phase synthesis for Si-based optical applications.

Moiré superlattices formed by twisted stacking in van der Waals materials have emerged as a new platform for exploring the physics of strongly correlated materials and other emergent phenomena1–5. However, there remains a lack of research on the mechanical properties of twisted-layer van der Waals materials, owing to a lack of suitable strategies for making three-dimensional bulk materials. Here we report the successful synthesis of a polycrystalline boron nitride bulk ceramic with high room-temperature deformability and strength. This ceramic, synthesized from an onion-like boron nitride nanoprecursor with conventional spark plasma sintering and hot-pressing sintering, consists of interlocked laminated nanoplates in which parallel laminae are stacked with varying twist angles. The compressive strain of this bulk ceramic can reach 14% before fracture, about one order of magnitude higher compared with traditional ceramics (less than 1% in general), whereas the compressive strength is about six times that of ordinary hexagonal boron nitride layered ceramics. The exceptional mechanical properties are due to a combination of the elevated intrinsic deformability of the twisted layering in the nanoplates and the three-dimensional interlocked architecture that restricts deformation from propagating across individual nanoplates. The advent of this twisted-layer boron nitride bulk ceramic opens a gate to the fabrication of highly deformable bulk ceramics.

Boron-rich compounds with icosahedral-based structures possess rich, fascinating electronic and mechanical properties. Herein, the first comprehensive and systematic study of the crystal structures and properties of ternary B12CN and B13CN compounds with icosahedral structures has been performed by using particle swarm optimized structure prediction methods in combination with first-principles calculations. Compared with the widely studied variant structure of α-boron, the newly discovered Cmc21 structures are thermodynamically more stable for B12CN and B13CN. For structures with the same space group, B13CN possesses superior thermodynamic stability and mechanical properties than B12CN. Electronic structure calculations indicate that the boron-rich B-C-N system has abundant and tunable electronic properties, i.e., B13CN is a semiconductor, and B12CN possesses a hole-type conducting characteristic. The systematic study of structural ideal tensile strength indicates successive damage to icosahedra and successive bond breaks between icosahedra during tensile processes, leading to interesting deformation mechanisms, such as stress re-enhancement, structural multistep damage, and creep-like deformation.

A unique optimized of core–shell structural B4C nanopowder, sintering aid additive of Si, and high-pressure sintering technique has been used to process nanocrystalline B4C–SiC ceramics with enhanced mechanical properties. C-coated B4C nanopowder was initially uniformly mixed with micron Si of different content by ball-milling. B4C–SiC composites with a homogenous distribution of SiC in B4C matrix were subsequently obtained by sintering the mixed powders at 6 GPa and 1600 °C. The added Si reacted with submicron amorphous carbon layer and amorphous carbon nanoshell to form dispersed SiC nanocrystals and Si–C phase filled at B4C grain boundaries and pores, respectively. The prepared composite had the most outstanding mechanical properties when the Si content in the precursor was 15 wt%, with a hardness reaching 37.8 GPa and a fracture toughness reaching 7.3 MPa·m1/2. Microstructural characterizations indicated that the multi deflection of nanoscale crack caused by intergranular fracture, the covalent bonding of Si–C phase at the grain boundary, and the abundant nanotwin substructure were jointly responsible for the superior performance in hardness and fracture toughness.

The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential, yet relevant studies have been rarely reported. In this work, we report nanocrystalline (La0.25Ce0.25Nd0.25Eu0.25)B6high-entropy rare-earth hexaboride (HEReB6-1) ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time. The as-fabricated samples exhibited a highly dense (96.3%) nanocrystalline (94 nm) microstructure with major (001) fiber textures and good grain boundaries without any impurities, resulting in a remarkable mechanical, electrical, and thermionic emission performance. The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures; these were greater than the average values of corresponding binary rare-earth hexaborides, such as a Vickers hardness of 23.4 ± 0.6 GPa and a fracture toughness of 3.0 ± 0.4 MPa•m1/2at room temperature. More importantly, they showed high emission current densities at elevated temperatures, which were higher than the average values of the corresponding binary rare-earth hexaborides. For instance, the maximum emission current density reached 48.3 A•cm−2at 1873 K. Such superior performance makes the nanocrystalline HEReB6-1 ceramics highly suitable for potential applications in thermionic emission cathodes.

Understanding the direct transformation from graphite to diamond has been a long-standing challenge with great scientific and practical importance. Previously proposed transformation mechanisms1–3, based on traditional experimental observations that lacked atomistic resolution, cannot account for the complex nanostructures occurring at graphite−diamond interfaces during the transformation4,5. Here we report the identification of coherent graphite−diamond interfaces, which consist of four basic structural motifs, in partially transformed graphite samples recovered from static compression, using high-angle annular dark-field scanning transmission electron microscopy. These observations provide insight into possible pathways of the transformation. Theoretical calculations confirm that transformation through these coherent interfaces is energetically favoured compared with those through other paths previously proposed1–3. The graphite-to-diamond transformation is governed by the formation of nanoscale coherent interfaces (diamond nucleation), which, under static compression, advance to consume the remaining graphite (diamond growth). These results may also shed light on transformation mechanisms of other carbon materials and boron nitride under different synthetic conditions.

The traditional hardness–toughness tradeoff poses a substantial challenge for the development of superhard materials. Due to strong covalent bonds and intrinsic brittleness, the full advantage of microstructure engineering for enhanced mechanical properties requires further exploration in superhard materials. Here heterogeneous diamond–cBN composites were synthesized from a carefully prepared precursor (hBN microflakes uniformly wrapped by onion carbon nanoparticles) through phase transitions under high pressure and high temperature. The synthesized composites inherit the architecture of the precursors: cBN regions with an anisotropic profile that spans several micrometers laterally and several hundred nanometers in thickness are embedded in a nanograined diamond matrix with high-density nanotwins. A significantly high fracture toughness of 16.9 ± 0.8 MPa m1/2is achieved, far beyond those of single-crystal diamond and cBN, without sacrificing hardness. A detailed TEM analysis revealed multiple toughening mechanisms closely related to the microstructure. This work sheds light on microstructure engineering in superhard materials for excellent mechanical properties.

• The binder-free nanopolycrystalline WC ceramic with an average grain size of 103 nm was fabricated through high-pressure sintering. • The binder-free nanopolycrystalline WC ceramic exhibits excellent mechanical properties at both room temperature and high temperature up to 1000 °C. • Ultrahigh hardness at 1000 °C is mainly from the nanocrystalline microstructure and numerous low-energy, highly stable Σ2 grain boundaries. From the perspective of high-temperature applications, materials with excellent high-temperature mechanical properties are always desirable. The present work demonstrates that the binder-free nanopolycrystalline WC ceramic with an average grain size of 103 nm obtained by high-pressure and high-temperature sintering exhibits excellent mechanical properties at both room temperature and high temperature up to 1000 °C. Specifically, the binder-free nanopolycrystalline WC ceramic still maintains a considerably high Vicker hardness H V of 23.4 GPa at 1000 °C, which is only 22% lower than the room temperature H V. This outstanding thermo-mechanical stability is superior to that of typical technical ceramics, e.g. SiC, Si 3 N 4 , Al 2 O 3 , etc. Nanocrystalline grains with many dislocations, numerous low-energy, highly stable Σ2 grain boundaries, and a relatively low thermal expansion coefficient, are responsible for the observed outstanding high-temperature mechanical properties. [Display omitted] [ABSTRACT FROM AUTHOR]

Traditional ceramics or metals cannot simultaneously achieve ultrahigh strength and high electrical conductivity. The elemental carbon can form a variety of allotropes with entirely different physical properties, providing versatility for tuning mechanical and electrical properties in a wide range. Here, by precisely controlling the extent of transformation of amorphous carbon into diamond within a narrow temperature–pressure range, we synthesize an in situ composite consisting of ultrafine nanodiamond homogeneously dispersed in disordered multilayer graphene with incoherent interfaces, which demonstrates a Knoop hardness of up to ~53 GPa, a compressive strength of up to ~54 GPa and an electrical conductivity of 670–1,240 S m–1at room temperature. With atomically resolving interface structures and molecular dynamics simulations, we reveal that amorphous carbon transforms into diamond through a nucleation process via a local rearrangement of carbon atoms and diffusion-driven growth, different from the transformation of graphite into diamond. The complex bonding between the diamond-like and graphite-like components greatly improves the mechanical properties of the composite. This superhard, ultrastrong, conductive elemental carbon composite has comprehensive properties that are superior to those of the known conductive ceramics and C/C composites. The intermediate hybridization state at the interfaces also provides insights into the amorphous-to-crystalline phase transition of carbon.

A previous study reported an observed unidentified graphite/hexagonal boron nitride (hBN) superlattice structure in special multilayer heterojunction devices via cross-sectional transmission electron microscopy [Haigh S. J. et al., Nat. Mater. 2012, 11, 764–767]. In this letter, we designed and confirmed two possible graphite/hBN superlattice structures (AA and Ab), which were probably the structures observed by the aforementioned experiment. The formation enthalpies of both structures were negative, indicating that they could be successfully synthesized as the previous experiment reported. The results also showed that both structures possessed dynamic stability and elastic stability. Importantly, the theoretical interlayer distances of AA and Ab were 3.34 and 3.30 Å, respectively, which were consistent with the experimental value of 3.32 Å. The X-ray diffraction patterns and Raman spectra of both structures were simulated to aid in distinguishing them. This study on the atomic structure of the graphite/hBN superlattice lays a foundation for further research and application of this material.

Glassy carbon (GC) is a type of non-graphitizing disordered carbon material at ambient pressure and high temperatures, which has been widely used due to its excellent mechanical properties. Here we report the changes in the microstructure and mechanical properties of GC treated at high pressures (up to 5 GPa) and high temperatures. The formation of intermediate sp2–sp3phases is identified at moderate treatment temperatures before the complete graphitization of GC, by analyzing synchrotron X-ray diffraction, Raman spectra, and transmission electron microscopy images. The intermediate metastable carbon materials exhibit superior mechanical properties with hardness reaching up to 10 GPa and compressive strength reaching as high as 2.5 GPa, nearly doubling those of raw GC, and improving elasticity and thermal stability. The synthesis pressure used in this study can be achieved in the industry on a commercial scale, enabling the scalable synthesis of this type of strong, hard, and elastic carbon materials.

The design and synthesis of superhard carbon nitrides have always been a focus of attention. Recently, an experiment has revealed the existence of the previously predicted three-dimensional strongly covalent carbon-rich Pnnm-CN structure with high incompressibility. This discovery promotes the study of new carbon-rich C–N compounds with various stoichiometric ratios. In this paper, we investigated the electronic and mechanical properties of carbon-rich C–N compounds with the structural analogues of a very recently proposed pentadiamond. The carbon-rich C10N and C19N3were constructed by replacing the carbon atoms of the pentadiamond with nitride atoms at specific positions. The elastic and dynamic stabilities were confirmed by the calculations of the elastic constants and the phonon spectra. The C19N3possessed a bulk modulus of 257.6 GPa, a shear modulus of 168.4 GPa, a Young’s modulus of 414.8 GPa, and a hardness of 21.4 GPa, which were similar to the properties of the C10N. The analyses of the band structure and the density of states showed that the C10N and the C19N3were both metallic, completely different from the semiconducting pentadiamond. The detailed electron orbital analysis showed that the C10N and C19N3had different conductive behaviors due to the different positions of nitrogen atoms in the structures. In addition, our research expands the scope of C–N compounds and provides new ideas for the later theoretical design of carbon-rich C–N compounds.

Abstract

A novel tetragonal B2CO structure (tP16-B2CO), formed by strong covalent sp2–sp3B–C and B–O bonds, was predicted with aid of an unbiased structure searching method. With the energy lower than those of previously proposed candidates, except ol16-B2CO, tP16-B2CO was identified as the thermodynamic metastable phase for B2CO compound. The elastic matrix and phonon dispersion spectra declare that tP16-B2CO is mechanically and dynamically stable. The electronic band structure calculation at ambient pressure and a series of high pressure has manifested the indirect semiconducting and band gap increases first and then decreases with pressure increases. The calculation of mechanical properties such as hardness and stress–strain relations of tP16 structure revealed its common hard nature with high hardness of 23.19 GPa and anisotropy with the max stress along [001] is far higher than that along [100].

Nanotwinned cubic boron nitride (nt-cBN) with remarkable hardness, toughness, and stability has attracted widespread attention due to its distinct scientific and industrial importance. The key for nt-cBN synthesis is to adopt an onion-like BN (oBN) nano-precursor and induce phase transition under high pressure. Here, we found that the size change of oBN used greatly affected the mechanical performance of products. With the precursor size decreasing from ~320 to 90 nm, the Vickers hardness of nanostructured products improved from 61 to 108 GPa, due to the fact that large oBN nanoparticles possessed more flattened, orderly and graphite-like shell layers, in sharp contrast to the highly wrinkled and imperfect layers in small-diameter nanoparticles, thus resulting in the apparent reduction of ultrafine-twin substructure in the synthetic products. This study reveals that only small oBN precursor could produce complete ultrafine nt-cBN with outstanding performance. A practical route was proposed to further improve the performance of this important material. 拥有极高硬度、 韧性和稳定性的纳米孪晶立方氮化硼(nt-cBN)在材料领域备受关注. 前期研究表明, 在高温高压条件下以洋葱式氮化硼(oBN)为前驱物是合成nt-cBN的关键. 本文研究发现, 前驱物oBN的粒径变化会显著影响最终产物的组织结构和机械性能. 随着前驱物oBN的粒径从~320 nm减小到~90 nm, 合成的纳米结构块材的硬度从61 GPa逐渐提高到108 GPa. 表明大粒径的oBN拥有和普通六方氮化硼类似的平整、有序的外层结构, 这与小粒径的oBN纳米粒子中存在大量弯曲、褶皱的氮化硼原子层及高密度层错的结构特点形成鲜明对比; 大粒径oBN前驱物中的这些有序结构显著减少了最终产物中超细孪晶亚结构的含量, 从而导致相应产物硬度的下降. 本研究表明, 只有粒径足够小的oBN前驱物才能合成出拥有高密度超细纳米孪晶结构且性能优异的nt-cBN. 为进一步提高此类纳米孪晶材料的性能提供了一种切实可行的方案.

Despite an effective p-type dopant for PbTe, the low solubility of Na limits the fully optimization of thermoelectric properties of Na-doped PbTe. In this work, Na-doped PbTe was synthesized under high pressure. The formation of the desired rocksalt phase with substantially increased Na content leads to a high carrier concentration of 3.2×1020cm−3for Na0.03Pb0.97Te. Moreover, dense in-grain dislocations are identified from the microstructure analysis. Benefited from the improved power factor and greatly suppressed lattice thermal conductivity, the maximal ZTof 1.7 is achieved in the optimal Na0.03Pb0.97Te. Current work thus designates the advantage of high pressure in synthesizing PbTe-based thermoelectric materials. 尽管钠可以对碲化铅进行有效的p型掺杂, 但其较低的固溶度限制了对掺杂样品热电性能的全面优化. 本工作采用高压合成方法合 成钠掺杂的碲化铅样品. 结构及成分分析表明样品具有典型的岩盐矿结构, 且钠的含量显著提高. 相应的, Na0.03Pb0.97Te样品的载流子浓度 也提高至3.2×1020cm−3. 此外, 显微结构分析确认在高压合成样品的晶粒中形成了高密度的位错. 受益于增强的功率因数和大大抑制晶格 热导率, Na0.03Pb0.97Te样品的热电优值达到1.7. 该工作展示了压力在合成碲化铅基热电材料中的优势.

Nanotwinned diamond (nt-diamond), which demonstrates unprecedented hardness and stability, is synthesized through the martensitic transformation of onion carbons at high pressure and high temperature (HPHT). Its hardness and stability increase with decreasing twin thickness at the nanoscale. However, the formation mechanism of nanotwinning substructures within diamond nanograins is not well established. Here, we characterize the nanotwins in nt-diamonds synthesized under different HPHT conditions. Our observation shows that the nanotwin thickness reaches a minimum at ~ 20 GPa, below which phase-transformation twins and deformation twins coexist. Then, we use the density-functional-based tight-binding method and kinetic dislocation theory to investigate the subsequent plastic deformation mechanism in these pre-existing phase-transformation diamond twins. Our results suggest that pressure-dependent conversion of the plastic deformation mechanism occurs at a critical synthetic pressure for nt-diamond, which explains the existence of the minimum twin thickness. Our findings provide guidance on optimizing the synthetic conditions for fabricating nt-diamond with higher hardness and stability. 在高温高压条件下以洋葱碳为原料合成的纳米孪晶金刚石具有前所未有的硬度和稳定性, 且二者随纳米孪晶厚度的减小而提高. 目前为止, 在金刚石纳米晶中纳米孪晶的形成机制尚不明确. 本研究通过分析在不同条件下合成的纳米孪晶金刚石块材中的孪晶厚度, 发现在合成压力约为20 GPa时孪晶厚度达到一个极小值(~5 nm). TEM结果表明在合成压力低于20 GPa时, 纳米孪晶金刚石中同时存在因马氏体相变而形成的相变孪晶和塑性形变所导致的形变孪晶. 针对马氏体相变后形成的相变孪晶内部塑性变形, 基于密度泛函理论的紧束缚方法和位错运动学理论的分析表明: 纳米孪晶金刚石的塑性变形存在一个依赖于压力的机制转变, 而机制转变的临界压力能够解释孪晶厚度随压力变化时出现的极小值. 本研究对通过优化合成条件制备出更高硬度和稳定性的纳米孪晶金刚石具有指导意义.

Millimeter-sized FeB4bulks were synthesized under high pressure of 15 GPa and 1600 °C. Multiple analysis methodologies including XRD, SEM, TEM, and Raman spectroscopy verified this new phase with an oP10 crystal structure. A relatively low hardness of 15.4 GPa, which is consistent with hardness estimated from both macroscopic and microscopic hardness models, excludes FeB4as a superhard material. Magnetization and resistance measurements do not indicate superconductivity in FeB4for temperature as low as 2.5 K. Previously reported superconductivity in FeB4needs a further investigation, especially for structural imperfections, unidentified phases, and/or contaminations.

The potential structures of magnesium nitride with a chemical composition of Mg3N2are examined by utilizing a widely adopted evolutionary methodology for crystal structure prediction. In addition to the previously proposed phases (α-, β- and γ-Mg3N2), we find five high-pressure phases for Mg3N2: (1) a Cmc21 symmetric structure (ε-Mg3N2) at 27 GPa, (2) a R3c̅symmetric structure (τ-Mg3N2) at 30 GPa, (3) a Cmcmsymmetric structure (ω-Mg3N2) at 53 GPa, (4) a Ima2 symmetric structure (λ-Mg3N2) at 68 GPa, and (5) a Ibamsymmetric structure (μ-Mg3N2) at 115 GPa. All these phases are mechanically and dynamically stable by checking the elastic constants and phonon dispersion. All phases are direct band gap semiconductors except that the ω-Mg3N2phase has a nontrivial indirect band gap. Mechanical properties calculations reveal that the γ-Mg3N2phase has superior ductility than other phases. The Vickers hardness of each phase has been evaluated to be about 15 GPa based on an empirical relation.

A stable ambient pressure phase of t-SiCN was determined by extensive forecasting through crystal structure analysis by particle swarm optimization algorithm. The energy of t-SiCN was much lower than that of c-SiCN, a structure proposed 40 years ago. Two high-pressure phases of o-SiCN and h-SiCN were also predicted. Transformations from ambient pressure phase t-SiCN to o-SiCN and h-SiCN occurred at 21.6 and 21.9 GPa, respectively. It is likely to be quenched to ambient conditions for h-SiCN and o-SiCN, as the energy values of h-SiCN and o-SiCN are very close at the range of pressures we calculated. The three novel phases of t-SiCN, o-SiCN, and h-SiCN are all mechanically and dynamically stable at ambient pressure as determined by their elastic constants and phonon dispersions. At ambient pressure, the densities of t-SiCN, o-SiCN, and h-SiCN crystals were calculated to be 3.20, 3.71, and 3.75 g/cm3, respectively. On the basis of the density of states of these compounds, t-SiCN was a narrow gap semiconductor with a band gap of 0.89 eV, whereas o-SiCN and h-SiCN were of hole-type conductivity. The hardness of t-SiCN was 41.5 GPa, which indicates that it is a superhard material. At ambient pressure, o-SiCN and h-SiCN exhibited hardness values of 30.0 and 30.2 GPa, respectively.

Elemental carbon exists in various aesthetically pleasing architectures. These forms include a group of synthesized allotropes with cubic modifications that have taken controversial or even unidentified crystal structures, which makes determining their physical properties difficult. In this study, four novel cubic carbon polymorphs (fcc-C10, fcc-C12, bcc-C20, and fcc-C32) that exhibit lattice parameters within the same range as those of undetermined cubic carbon allotropes are proposed by employing a newly developed ab initio particle-swarm optimization methodology for crystal structure prediction. The four structures are all three-dimensional polymers consisting of unique, small C10, C12, C20, and C32cages with quite low density. Investigation of their electronic and mechanical properties illustrate that the cage-like cubic carbons are all semiconductors with excellent mechanical performance, specifically superhardness and high ductility. Moreover, we readily explain a long-standing controversial experimentally synthesized cubic carbon (viz., the so-called “superdense” carbon) using the previously proposed bcc C6based on the coincident lattice constant and electron diffraction data between the theoretical and experimental results.

Zirconia (ZrO2) nanocrystals with average size of 4 nm are fabricated by oxidation of the nonstoichiometric ZrC0.6with ordered carbon vacancies at 450 °C under atmosphere. The nanocrystals are predominantly tetragonal (t) phase and spherical in shape, and their exposed surfaces are constructed by the {011} and {001} facets. After annealing at 700 °C under atmosphere, the coalescence of adjacent t-ZrO2nanocrystals is observed, and most of the annealed t-ZrO2nanoparticles are found to exhibit the {011}-specific twins. The dominant cyclic twins as well as a small number of the single and lamellar twins are recognized in the twinned nanoparticles. The cyclic-twinned nanoparticles are identified to have the 5-fold symmetry of either decahedron or icosahedron. In contrast to the single and lamellar twins which are formed via the coalescence of adjacent nanocrystals on the well-developed {011} surfaces, the cyclic-twinned nanoparticles are developed from the coalescence on the disoriented contact surfaces, in which the emission of partial dislocations and induced deformation are recognized to play the key role.

Abstract

A new high-pressure phase of RhB has been predicted using a newly developed particle swarm optimization algorithm based on first-principles calculations. The new phase belongs to the orthorhombic Pnmaspace group (FeB type); it transforms from hexagonal RhB (anti-NiAs type) when the pressure exceeds 22 GPa. The high-pressure phase is both mechanically and dynamically stable, as verified by the calculations of its elastic stiffness constants and phonon dispersion. Further calculations predicate this high-pressure phase to be semimetallic, with excellent ductility (B/G= 3.56) and high Vickers hardness (25 GPa).

Eight fascinating sp2- and sp3-hybridized carbon allotropes have been uncovered using a newly developed ab initioparticle-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 C60fullerene, and some are energetically more stable than the van der Waalspacking 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.

The ruthenium monocarbide (RuC) structure was investigated through a density functional theory based on the 10 structures of known transition-metal compounds. The calculations indicated that zinc blend structured RuC (ZB-RuC) is the most energetically stable at the ground state, and the calculated lattice parameters are more consistent with the experimental values than those recently reported of rock salt structured RuC (RS-RuC). The structural stability of RuC was studied by calculating total energy, elastic constants, and phonon frequencies. Only ZB-RuC showed dynamical stability. It can be concluded that the most likely structure of the synthesized RuC is ZB-RuC rather than RS-RuC. From the calculated band structure and density of states (DOS), it was found that ZB-RuC is a semiconductor while other RuC structures are metallic. The estimated hardness indicates that ZB-RuC is a new superhard material with a Vickers hardness value of 42.8 GPa, compared with B4C. Even though it is a superhard material, the high B/G value (3.95) indicates that ZB-RuC is still a ductile material.

Nanostructured Nb-doped SrTiO3(001) surfaces were investigated with STM, and the surface patterns for observed nanostructures were assigned. Sequential C60deposition onto these nanostructured templates reveals distinct growth modes, including discrete small C60islands on the c(4 × 2) reconstruction surface, parallel one-dimensional C60chains on (6 × 2) dilines, C60double chains on (8 × 2) trilines, epitaxial C60close packed adlayers over (11 × 2) tetralines, and two-dimensional ordered C60dimer arrays on (7 × 6) waffles. These structural diversities mainly stem from the relatively strong adsorbate−substrate interactions as well as the surface topography demands. The nanostructured oxide surfaces as templates thus have great potential in molecular nanoarchitecture.

Carbon is one of the most fascinating elements due to its structurally diverse allotropic forms stemming from its bonding varieties (sp, sp2and sp3). Exploring new forms of carbon has been the eternal theme of scientific research. Herein, we report on amorphous (AM) carbon materials with a high fraction of sp3bonding recovered from compression of fullerene C60under high pressure and high temperature, previously unexplored. Analysis of photoluminescence and absorption spectra demonstrates that they are semiconducting with a bandgap range of 1.5–2.2 eV, comparable to that of widely used AM silicon. Comprehensive mechanical tests demonstrate that synthesized AM-III carbon is the hardest and strongest AM material known to date, and can scratch diamond crystal and approach its strength. The produced AM carbon materials combine outstanding mechanical and electronic properties, and may potentially be used in photovoltaic applications that require ultrahigh strength and wear resistance.The currently synthesized semiconducting amorphous carbon form is the hardest and strongest amorphous material known to date, and has a bandgap comparable to that of amorphous silicon.

Synthesized cubic BC 4N with Vickers hardness 10% higher than that of cubic BC 2N exhibits a strange X-ray diffraction spectrum, assigned as a zinc-blende cubic structure. However, its crystal structure cannot be constructed with a zinc-blende structure as viewed from the stoichiometric formula. We found that this structure can be described by 3 C-BC 4N in Ramsdell notation, with the space group of P3 m1. First-principles calculations show that this so-called “cubic” BC 4N is a large gap insulator and has outstanding mechanical properties such as an ideal tensile strength of 77.1 GPa and a theoretical Vickers hardness of 84.3 GPa. The simulated Raman spectra of five 3 C-BC 4N configurations show totally different features, providing a valid method to conclusively determine the atomic arrangement of cubic BC 4N.

Five dense B4C3structures are established via substitution of N atoms with B atoms in the five hypothetical dense C3N4phases. Lattice parameters and elastic and electronic properties of these five B4C3polymorphs are investigated by first-principles calculations. Our calculations show that -B4C3is energetically favorable relative to other structures. The pseudocubic B4C3is conductive, and the other four structures are semiconductive. Vickers hardness of the four semiconductive B4C3crystals is calculated using the microscopic model of hardness. These four B4C3polymorphs are found to be superhard materials, with hardness values higher than 55 GPa. Both the cubic and the cubic spinel phases of B4C3exhibit higher hardness, which is comparable with the hardness of cubic boron nitride. The relationship between hardness and crystal structure is also discussed.

A tetragonal BC2(t-BC2) phase originating from the cubic diamond structure was predicted by first-principles calculations. The t-BC2structure has a tetragonal lattice (space group I41/amd, No. 141) and possesses the simple stacking sequence BC2BC2... along its caxis. The structural stability of the proposed t-BC2was confirmed by calculations of the elastic constants and phonon frequencies. The electronic densities of states of t-BC2show that all the B−C and C−C bonds in the crystal are metallic. Thus, t-BC2possesses three-dimensional conductivity, which is different from the two-dimensional conductivity of the diamond-like t-BC3. The theoretical Vickers hardness of t-BC2is 56.0 GPa, which is close to that of cubic boron nitride. The B/Gratio of t-BC2is 1.09, which is larger than that of diamond (0.83), indicating that the t-BC2phase is more ductile than diamond.

Compressive strength, like tensile and shear strength, is one of the intrinsic mechanical properties of solid materials. We investigated the ideal compressive strength and corresponding compressive deformation of diamond using pseudopotential density functional theory. The calculated uniaxial compressive strength of diamond is −223.1, −469.0, and −470.4 GPa along the ⟨100⟩, ⟨110⟩, and ⟨111⟩ directions, respectively. The elastic deformation of diamond in compressive strain below the proportional limit of 17% has been determined from the perfect linear response between strain and stress along the ⟨100⟩ direction. The present work should be helpful to the understanding of the compressive property of diamond and to the explanation of the limit strength of diamond anvil cell.

B4C is widely known by a series of unique advantages, such as low density, high hardness, good chemical stability and excellent environmental stability, as a hard ceramic material. However, the study of B4C as the electrode material on micro-electrochemical energy storage devices has not yet been reported. To some extent, the poor conductivity of B4C is an important reason to limit its extensive application in energy storage field. Here, we first report an elaborated design B4C@C of core-shell structure by simple vacuum heating synthesis strategy as electrodes for flexible all-solid-state micro-supercapacitors (MSCs) with the method of mask assisted-vacuum filtration. Notably, the B4C@C-MSCs exhibit outstanding electrochemical performances, such as high areal capacitance of 7.33 mF cm−2, remarkable cycling stability with 100.94% capacitance retention after 50000 cycles, wide operating temperature range (−25 to 75 °C), excellent mechanical flexibility without obviously performance degradation under different bending states, distinguished environmental stability and superior integration. These results suggest that the hard ceramic materials B4C have a promising prospect in micro-electrochemical energy storage devices.