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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.

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 HV of 23.4 GPa at 1000 °C, which is only 22% lower than the room temperature HV. This outstanding thermo-mechanical stability is superior to that of typical technical ceramics, e.g. SiC, Si3N4, Al2O3, 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.

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–1 at 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.

Columbite-rich multiphase TiO2 nanoceramics with outstanding mechanical and dielectric properties were successfully prepared through high-pressure sintering by using anatase-type TiO2 as precursor. High-pressure sintering combined with phase transformation assisted consolidation can effectively refine the grain size of the recovered samples. This process is conducive to obtain nanocrystalline columbite-rich TiO2 ceramics with excellent performance. The highest hardness is approximately 12.76 GPa, which is 2.5 times higher than that of ordinary coarse-grained ceramics. The effect of columbite phase on the hardness of multiphase ceramics is discussed. The columbite-rich TiO2 ceramic shows a colossal permittivity (∼8 × 103) and low loss (∼0.2) at 1 kHz and room temperature, which are superior to that of undoped rutile polycrystalline ceramics. This ceramic shows a steadier frequency-dependent dielectric permittivity and loss than rutile TiO2 crystal. These results enrich the fundamental knowledge of columbite-rich TiO2, thereby enabling the exploitation of new applications.

Nanostructured silicon carbide (SiC) materials are expected to have bright prospect in application as high-performance electrode materials with excellent charge–discharge cycling stability. However, the exploration of SiC-based micro-supercapacitors (MSCs) still remains a grand challenge seriously hampered by low areal capacities and complicated multistep production process. Herein, we report that rationally designed SiC/C nanocomposite with hierarchical porous structure and improved electrical conductivity has been realized by a facile and rapid carbothermic reduction using silica sol and sucrose as silicon and carbon source. The amorphous carbon between SiC nanoparticles (NPs) contributes to enlarged surface areas and excellent conductivity, not only ensuring intimate contact between the electrolyte and the electrode but also providing an effective ion highway for electrolyte ions. As a result, MSCs based on SiC/C nanocomposite (Si/C mass ratio of 1:1.5) demonstrate an optimal specific areal capacitance of 11.8 mF cm−2 at 2 mV s−1, outstanding flexibility (104.5% retention of initial capacitance at 180° bending), and superior integration. Most notably, the capacitance remains at 97.3% of the initial value after 50000 charging/discharging cycles, superior to that of most advanced SiC-based MSCs ever reported. This work demonstrates an effective design for hierarchical porous SiC/C nanocomposite for energy storage, which gives significant inspirations on the exploration of high-performance SiC-based MSCs.

Boron carbide (B4C) is an important structural ceramic material, however, the use of this material is limited by its poor sinterability and low fracture toughness. Nano-sized powders have small particle size, high chemical activity and great sintering ability, which can promote the fabrication of the dense B4C ceramics. Herein, by spark plasma sintering (SPS) homemade high-purity nanopowders, single phase B4C ceramics with density close to the theoretical value were prepared. Results showed that the best SPS condition was holding at temperature of 1850 °C for 4 min. Under this condition the sample exhibited enhanced mechanical properties including Vickers hardness of 33.9 GPa, fracture toughness of 3.81 MPa m1/2 and flexure strength of 554.7 MPa. Transmission electron microscopy (TEM) analysis shows that non-coherent twins exist at the grain boundary instead of amorphous phase or second phase, which further confirms the high purity and high densification of the obtained B4C ceramics.

Here a series of sp2-sp3 BxNx+1 (x = 1, 2, 3, 4, 5, 6) structures was constructed. These structures can be viewed as diamond-like BN blocks connected by single N-N bonds. Elastic constants and phonon dispersion curves confirm that all of the proposed structures are mechanically and dynamically stable. These structures all possess metallicity originating from the conductive channels formed by sp2-hybridized N atoms and adjacent sp3-hybridized B and N atoms. These structures exhibit tunable mechanical properties with a regular change in the sp2/sp3 ratio. The theoretical Vickers hardness increases and the ductility decreases as the number of diamond-like BN blocks increases, gradually approaching those of c-BN. Moreover, the convex hull at ambient pressure and 50 GPa indicates that high pressure is beneficial in the synthesis of these B-N phases. The simulated X-ray diffraction patterns of these structures were also calculated to provide more information for further experiments.

Ultrahard nanotwinned diamond (nt-D) is an ideal material for next-generation high-precision, high-efficiency cutting tools, due to its high hardness, enhanced fracture toughness, and increased oxidation resistance temperature, when compared with natural diamond. However, a critical problem that limits the application of such material is the grinding and polishing of nt-D material into suitable shapes. In this study, we confirmed the feasibility of classical mechanical polishing for nt-D through amorphization transition. The effect of mechanical loading and the catalysis of Fe nano-particles work together, promoting the transformation of nt-D into a hard sp2-sp3 amorphous carbon. The surface of the amorphous carbon layer and the interface between amorphous carbon and nt-D were both smooth after polishing. The results are valuable for the mechanical processing and further applications of ultrahard nanotwinned diamond.

Summary New carbon forms that exhibit extraordinary physicochemical properties can be generated from nanostructured precursors under extreme pressure. Nevertheless, synthesis of such fascinating materials is often not well understood. That is the case of the C60 precursor, with irreproducible results that impede further progress in the materials design. Here, the semiconducting amorphous carbon, having band gaps of 0.1–0.3 eV and the advantages of isotropic superhardness and superior toughness over single-crystal diamond and inorganic glasses, is produced from fullerene at high pressure and moderate temperatures. A systematic investigation of the structure and bonding evolution is carried out with complementary characterization methods, which helps to build a model of the transformation that can be used in further high-pressure/high-temperature (high p,T) synthesis of novel nano-carbon systems for advanced applications. The amorphous carbon materials produced have the potential of accomplishing the demanding optoelectronic applications that diamond and graphene cannot achieve.

Ternary boron-carbon-nitrogen (B-C-N) compounds are considered to possess hardness comparable to diamond and thermal stability comparable to c-BN. Explorations for desirable B-C-N phases have been continuous. However, the nonconductive properties of most B-C-N compounds narrow the applications of these compounds. Herein, we propose a sp2-sp3 hybridized phase of t-B2C3N2, which consists of diamond-like BC blocks connected with single N-N bonds. Elastic constants and phonon dispersion curves confirm that t-B2C3N2 is mechanically and dynamically stable. The structure processes 2D metallicity in a strong 3D network. Furthermore, hardness and electron-phonon calculations reveal that t-B2C3N2 is superhard and superconductive with a superconducting critical temperature reaching 2.3 K.

Composites that consist of ceramic and C materials have attracted considerable attention in the microwave absorption field. In this work, we applied a facile and scalable synthesis strategy to prepare B4C nanoparticles with core–shell structures. XRD and Raman results confirmed the existence of B4C and amorphous C in the core–shell B4C nanoparticles. Morphological analysis revealed that the B4C nanoparticles exhibited regular morphologies with average diameter of approximately 100 nm and were well encapsulated by amorphous C shells. The C-coated B4C nanoparticles fabricated with a B/C ratio of 1:1.25 and with thicknesses of 1.5 mm provided the lowest reflection loss value of −60.8 dB at 15.5 GHz. The C coated B4C nanoparticles have considerable potential uses as microwave absorbers given their facile preparation and excellent microwave absorption performance.

By a first-principles approach combined with an evolutionary method, four novel cubic AlN phases (cP16, cF40, cI16, and cI24 AlN) are proposed as promising phases after detailed stability analysis. These four cubic AlN phases exhibit a hardness range of 7.8–17.0 GPa. The anisotropy of their mechanical properties, including Young's modulus, shear modulus, and Poisson's ratio, is systematically studied. Moreover, they are all semiconductors with band gaps smaller than those of experimental phases, and consist of polar covalent Al–N bonds with sp3 hybridization. Their thermodynamic functions, including enthalpy, Gibbs free energy, and vibrational entropy, are calculated in the temperature range from 0 to 2000 K. Their thermodynamic properties, including Debye temperature and constant-volume specific heat, are investigated. The anisotropy of their minimum thermal conductivities is discussed on the basis of Clarke's model. We hope that this study of the mechanical, electronic, and thermodynamic properties of novel cubic AlN phases will provide guidance for the industrial applications of these four cubic AlN phases.

Boron carbide is an extremely hard material extensively used in the industry. In this study, carbon-coated B4C nanoparticles were synthesized through a one-step synthesis method using high-temperature firing cheap and readily available raw materials, namely, boric acid and sucrose. X-ray diffraction and Raman spectra analysis indicated the formation of B4C, and high-resolution transmission electron spectroscopy revealed that the carbon-coated B4C nanoparticles exhibited regular B4C-C core-hell structure with an average particle diameter of around 100 nm. This method is simple and time-saving, and can be adopted for the large-scale production of the core-shell nanoparticle. Directly using these core-shell structural nanoparticles as precursor, high-performance B4C/C ceramics with hardness reaching 34 GPa, fracture toughness reaching 3.3 MPa m1/2 were synthesized. This study has important implications in developing high-performance B4C ceramics.

A glassy carbon phosphonitride material with bulk chemical composition roughly approximating C3N3P was synthesized through a high-pressure, high-temperature process using a pure P(CN)3 molecular precursor. The resulting material (hereafter referred to as “HPHT-C3N3P”) was characterized using a variety of techniques, including X-ray scattering, pair distribution function analysis, 31P, 13C, 15N magic-angle spinning nuclear magnetic resonance spectroscopies; X-ray photoelectron spectroscopy, and Raman and IR spectroscopies. The measurements indicate that HPHT-C3N3P lacks long-range structural order with a local structure predominantly composed of a sp2, s-triazine-like network in which phosphorus atoms substitute for bridging nitrogen sites found in related C3N4 materials. The HPHT-C3N3P sample exhibits semiconducting properties, with electrical transport dominated by variable-range hopping. The high phosphorus content of HPHT-C3N3P (approaching 13 at. %) is associated with a major decrease in the optical a...

Carbon is likely the most fascinating element of the periodic table because of the diversity of its allotropes stemming from its variable (sp, sp2, and sp3) bonding motifs. Exploration of new forms of carbon has been an eternal theme of contemporary scientific research. Here we report on novel amorphous carbon phases containing high fraction of sp3 bonded atoms recovered after compressing fullerene C60 to previously unexplored high pressure and temperature. The synthesized carbons are the hardest and strongest amorphous materials known to date, capable of scratching diamond crystal and approaching its strength which is evidenced by complimentary mechanical tests. Photoluminescence and absorption spectra of the materials demonstrate they are semiconductors with tunable bandgaps in the range of 1.5-2.2 eV, comparable to that of amorphous silicon. A remarkable combination of the outstanding mechanical and electronic properties makes this class of amorphous carbons an excellent candidate for photovoltaic applications demanding ultrahigh strength and wear resistance., 40 pages, 17 figures

Here a series of sp

Ternary boron-carbon-nitrogen (B-C-N) compounds are considered to possess hardness comparable to diamond and thermal stability comparable to c-BN. Explorations for desirable B-C-N phases have been continuous. However, the nonconductive properties of most B-C-N compounds narrow the applications of these compounds. Herein, we propose a sp2-sp3 hybridized phase of t-B2C3N2, which consists of diamond-like BC blocks connected with single N-N bonds. Elastic constants and phonon dispersion curves confirm that t-B2C3N2 is mechanically and dynamically stable. The structure processes 2D metallicity in a strong 3D network. Furthermore, hardness and electron-phonon calculations reveal that t-B2C3N2 is superhard and superconductive with a superconducting critical temperature reaching 2.3 K.

Three novel sp2-sp3 hybridized BC6N phases with a sandwich structure, including a type of orthorhombic BC6N (o-BC6N) and two types of tetragonal BC6N (t-BC6N-1 and t-BC6N-2), are investigated through first principles calculations. The structural stabilities are confirmed by the calculated elastic constants and phonon dispersions. Calculated electronic band structures, density of states (DOS), and partial DOS show that the o-BC6N, t-BC6N-1, and t-BC6N-2 crystals may possess the metallicity with the conducting electrons from the p orbits of sp2-hybridized C atoms. Calculations of electron orbits indicate that the electrons in o-BC6N and t-BC6N-1 structures can conduct through the π bonds along the orientation parallel to the [1 0 0] and [0 1 0] directions in different layers. Moreover, the electrons in t-BC6N-2 structure can conduct along the orientation parallel to the [1 1 0] and [ 1 ¯ 1 0] directions in different layers. The behavior of the linear electron conductivity in the layer and vertical direction of conduction between the adjacent layers imply that the three kinds of crystals have potential applications in the field-effect devices. Calculation results using the semi-empirical microscopic model show that o-BC6N, t-BC6N-1, and t-BC6N-2 are potential superhard materials with Vickers hardness of 52.4, 45.3, and 40.1 GPa, respectively.

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

Carbon possesses various allotropes with diverse electronic properties from insulation to conduction due to the sp, sp2, and sp3 bonding types. Here, a new sp2-sp3 carbon allotrope, called C48, is proposed. C48 presents a lilac-like structure with a cage-like center and four petal-like tubes around, and can be formed through compressing single-walled carbon nanotubes. C48 is not only strong and hard in 3D but also has unexpected 1D conduction along eight divided linear pathways in the unit cell. This multithreaded 1D conduction in 3D framework is peculiar in carbon materials, and would be helpful to design singular electronic nanodevices.

A lonsdaleite-like orthorhombic structure B2CO (oP8-B2CO) with strong sp3 covalent B‐C and B‐O bonds has been predicted theoretically by using the crystal structure prediction package CALYPSO. The mechanical, dynamical and thermodynamic stabilities of oP8-B2CO are verified by independent elastic constants, phonon dispersion spectrum and formation enthalpy, respectively. Pressure may promote synthesis of oP8-B2CO, which is demonstrated by calculating the relationship of formation enthalpies for oP8-B2CO via pressure. Based on bond resistance model for hardness, oP8-B2CO has a high hardness value of 47.70 GPa. Band structures calculation illustrates that all B2CO phases are semiconducting with indirect band gaps and oP8-B2CO has the widest band gap (3.540 eV) relative to tP4-B2CO (1.658 eV) and tI16-B2CO (2.988 eV). The superhardness and tunable band gap open an extensive industrial application and scientific research for B2CO compounds.

We constructed a series of novel 3D-BC2N structures that can be obtained by compressing well-mixed carbon nanotube and boron nitride nanotube arrays. Sequential theoretical calculations indicated a variety of electronic and mechanical properties, such as superhardness, tunable band gap, and ductility, for these novel 3D-BC2N polymorphs. In specific, 3D (6,6)-Ⅱ, 3D (4,4)-Ⅱ, 3D (8,8), 3D (6,6)-Ⅰ and 3D (8,0) are energetically more favorable than previously proposed diamond-like β-BC2N; the hardness of 3D (6,6)-Ⅱ can reach a remarkably high value of 72 GPa; 3D (6,0) and 3D (8,0) are direct gap superhard phases; and 3D (10,0) has a semimetallic nature.

By utilizing an evolutionary methodology on crystal structure search, we propose three new metastable phases for aluminum arsenide (AlAs) as follows: (1) a P6422 symmetric structure (hP6-AlAs), (2) a C222 symmetric structure (oC12-AlAs), and (3) a I 4 ¯ 3d symmetric structure (cI24-AlAs). By controlling the unloading pressure rate, oC12-, hP6-, and cI24-AlAs may be acquired through quenching NiAs-AlAs. The elastic constants and phonon dispersion spectra are calculated to certify the mechanical and dynamic stabilities of three newly discovered phases. On the basis of first-principle study, we explore phase transformations under pressure for several AlAs polymorphs. The calculation of mechanical properties illustrates that oC12-, and hP6-AlAs possess similar hardness levels, which are higher than that of cI24-AlAs. Meanwhile, oC12-, and hP6-AlAs hold similar shear anisotropic factors, which are smaller than that of cI24-AlAs. Electronic band structure calculation reveals that at zero pressure, oC12-, and hP6-AlAs possess indirect band gaps of 0.468 eV and 1.356 eV, respectively. cI24-AlAs is a direct semiconductor with a gap value 1.761 eV.

As the stable compound in boron nitrides, stoichiometric BN is a well-known insulator, irrespective of its structure and dimensionality. The exploration of novel B–N compounds with various stoichiometric ratios can lead to the discovery of unexpected electrical and mechanical properties. To the best of our knowledge, previously reported graphite-like or diamond-like B–N compounds obtained from experimental synthesis and theoretical prediction are mostly insulators or semiconductors. In this paper, a sp2–sp3 hybridised tetragonal phase of B3N4 (t-B3N4) possessing unique two-dimensional (2D) metallicity in a 3D ultra-strong framework has been predicted through an unbiased swarm structure search. The structure of t-B3N4 can be considered as sp3-hybridised cubic BN blocks interlinked by sp2 N–N bonds. Noticeably, t-B3N4 is metastable at ambient pressure, but becomes stable under high pressure. The transition pressure from layered B3N4 to t-B3N4 is 14.7 GPa, and the calculated formation enthalpies of t-B3N4 with respect to h-BN and N2 become negative at pressures above 20 GPa, indicating its viability under pressure. Its structure stability has been confirmed by the criteria of both elastic constants and phonon frequency dispersions. The analyses of the band structure, density of states, and electron orbitals show that the metallic behaviour of t-B3N4 mainly originates from the N 2p electrons, and that the conduction is interrupted by the insulated boron sheets stacked along the c axis, giving rise to the 2D metallicity of the material. The theoretical Vickers hardness of t-B3N4 is estimated to reach 42.5 GPa, which is the highest among all proposed B3N4 polymorphs. Furthermore, t-B3N4 exhibits ultra-high axial incompressibility even beyond that of diamond, due to the existence of strong short N–N bonds.

In this paper, a layer-like sp2-/sp3-hybridized orthorhombic phase of B3N5, named as oA-B3N5, is theoretically predicted to be stable at ambient pressure by first-principles calculations. The mechanical and dynamical stability has been confirmed by the elastic constants and phonon frequency curves calculations. The analyses of electronic properties show the metallicity of oA-B3N5 originates from the delocalized 2p electrons of sp2-hybridized N atoms, forming several independent 1D conductive channels, giving the 1D metallicity in 3D framework. Furthermore, the incompressibility of oA-B3N5 exhibits highly anisotropic, and higher axial incompressibility along the N-N bonds direction comparing to the predicted semiconductor C2221-B3N5.

Diamond-like B C compounds exhibit the favorable properties of high hardness and superconductivity. The explorations for novel B C phases are kept on going and it was desirable to explore whether the diamond-like structure can still exist stably or not when the content of boron increases. Herein we predicted a nondiamond-like structure t2-BC took place the lowest energy position of the diamond-like structure, which is completely different from the BCx (x ≥ 2) system. Based on elastic constants and phonon dispersion calculations, the mechanical and dynamic stability of t2-BC were confirmed. Furthermore, hardness and electron–phonon calculations reveal that t2-BC is superhard (Hv = 55.4 GPa) and superconductive with superconducting critical temperature reaching 6.4 K. These results have practical significance for the experimental synthesis of diamond and nondiamond like B C compounds.

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.

A monoclinic BC3 phase (denoted M-BC3) has been predicted using first principles calculations. The M-BC3 structure is formed by alternately stacking sequences of metallic BC-layers and insulating C atom layers, thus, the structure exhibits two-dimensional conductivity. Its stability has been confirmed by our calculations of the total energy, elastic constants, and phonon frequencies. The pressure of phase transition from graphite-like BC3 to M-BC3 is calculated to be 9.3 GPa, and the theoretical Vickers hardness of M-BC3 is 43.8 GPa, this value indicates that the compound is a potentially superhard material. By comparing Raman spectral calculations of M-BC3 and previously proposed structures with the experimental data, we speculate that the experimentally synthesized BC3 crystal may simultaneously contain M-BC3 and Pmma-b phases.

An oP10–FeB4 phase [H. Gou, et al., Phys. Rev. Lett., 2013, 111, 157002] was recently synthesized based on previous theoretical predictions. In this study, a high-pressure phase of FeB4 (tP10–FeB4) was proposed through first-principles calculations. The tP10–FeB4 structure, which contains two formula units per unit cell, belongs to tetragonal symmetry with the space group P42/nmc. The boron atoms in tP10–FeB4 are present as tetrahedron configurations. Enthalpies as a function of pressure indicate that this new phase is probable to achieve through a phase transition from the oP10–FeB4 phase above ∼65.9 GPa. The softening of acoustic phonon at T points in the Brillouin zone may be the driving force behind the phase transition. Further analyses reveal that the tP10–FeB4 phase is a potential superhard semiconductor.