Cold-compressed Graphite Research Articles

High-pressure phase of cold-compressed bulk graphite and graphene nanoplatelets

We have studied the high-pressure vibrational and structural behavior of bulk graphite and graphene nanoplatelets at room temperature by means of high-pressure Raman spectroscopic and x-ray diffraction probes. We have detected a clear pressure-induced structural transition in both materials, evidenced by the appearance of new Bragg peaks and Raman features, deviating from the starting hexagonal graphitic structure. The high-pressure phase is identified as a partially disordered orthorhombic structure, consisting of mixed $s{p}^{2}$- and $s{p}^{3}$-type bonding. Our experimental findings serve as direct evidence for the existence of a metastable transient modification in cold compressed carbon, lying between the $s{p}^{2}$-type graphite and ${sp}^{3}$-type diamond allotropes.

Three tetragonal superhard sp3 carbon allotropes

Four tetragonal anisotropic superhard carbon phases, T-C8, T-C12, T-C20 and T-C28 were generated by RG2 program, T-C8 has been reported and named bct-Carbon. The calculated relative enthalpy, elastic constants and phonon dispersion spectra show that these structures are stable. Their lattice failure modes are mainly tensile. Thermodynamic stability, thermal performance and hardness are all in the order T-C20 >T-C8>T-C12>T-C28, which is contrary to the proportion of the four membered ring in the corresponding structure, suggesting that strained four-membered ring is detrimental to the stability and mechanical properties of carbon phases. Among them, T-C20 with hardness of 90.8 GPa is more stable than bct-Carbon and graphite under 20 GPa, suggesting that it is likely to be transformed by cold compressed graphite.

The stability, electronic, mechanical and thermal properties of three novel superhard carbon crystals

Three novel orthorhombic carbon crystals were designed and their stability, sound velocities, thermal conductivity and dynamically and mechanical properties were investigated. Density functional theory calculations show that the three carbon crystals are energetically stable with cohesive energies less than −8.94 eV and their elastic constants satisfy the mechanical stability criterion of orthorhombic system. They are anisotropic superhard materials with Vickers hardness of over 79 GPa based on four different models. Their Debye temperature are over 2100 K, only a little lower than that of diamond. The calculations show that they may be synthesized from cold-compressed graphite under pressure of less than 20 GPa. Their similar structural features to diamond and excellent mechanical performance suggest that diamond can be a useful structural template of superhard materials.

Stability, electronic and mechanical properties of superhard materials formed by 4+6+8 membered rings of carbon

In order to study the relationship between structures and properties, a series of 4 ​+ ​6+8 carbon allotropes including oC24, oC32, oC40 and oC48 are designed by inserting eight-atom fragments between the four- and eight-membered rings of bct-Carbon and oC16-Ⅱ carbon. The calculations prove that these carbon phases are thermodynamically, mechanically and dynamically stable and the new designed oC40 and oC48 possess a higher bulk module, shear module, Young’s modulus and Vickers hardness than the theoretically proposed o-C32 and M-carbon. The calculated relative enthalpies against pressure suggest that oC40 and oC48 can be transformed from graphite under a lower pressure than bct-Carbon and o-C16-Ⅱ. Thus oC40 and oC48 are expected to be experimentally synthesized from cold-compressed graphite. More importantly, the calculated results show that the stability, bandgaps, bulk modules, shear modules, Young’s modulus, incompressibility, ideal tensile strengths and Vickers hardness of these structurally similar allotropes increase with increasing proportion of the six-membered rings of carbon, suggesting that their stability and properties can be achieved by engineering their microstructures.

A review on the structure of cold-compressed graphite phase

The room-temperature structural phase transition of graphite at elevated hydrostatic pressure has drawn considerable scientific interest for its fundamental importance in condensed matter physics and materials science over the past few decades. A pressure-induced phase transition has been demonstrated in previous experiments by the measurement of electrical resistance, optical spectrum, X-ray diffraction spectrum, inelastic X-ray scattering and Raman spectroscopy. However, the nature of the cold-compressed graphite phase has been puzzling the experts and pioneers in the field of high pressure research due to some inherent factors, until recently a monoclinic structure, i.e. [Formula: see text]-carbon, stands out from the other structure candidates and successfully accounts for the crystal structure of the cold-compressed graphite phase both in theory and experiment that eventually putting an end to this long-lasting controversial issue. This paper reviews the recent progress on the pressure-induced phase transitions of graphite at room temperature especially for the theoretical investigations. The review will focus on the recent proposed novel carbon allotropes as candidate structures of the cold-compressed graphite phase by using different crystal structure prediction methods. The history of structure determination of cold-compressed graphite phase is discussed.

Ab initio models for polycrystalline diamond constructed from cold-compressed disordered graphite

A general scheme is proposed to construct systematically a family of superhard sp3 carbon phases of cold-compressed graphite by combining hexagonal to cubic diamond (named as X-Carbon). Based on calculations employing density functional theory (DFT), we find that our currently proposed X-carbon can occur by compressing disordered graphite, and the X-carbon is more stable in energy than the previously proposed M, Z, W, bct-C4, P allotropes. Thus, the X-carbon is predicted to be the transition of cold-compressed graphite. The results show that the simulated x-ray diffraction pattern, Vickers hardness and bulk modulus of X-Carbon match well with the experimental data (Mao et al Science 302, 425 (2003)). These new phases are transparent superhard materials with a large hardness and wide electronic band gaps comparable to cubic diamond.

Pressure-induced transformations of onion-like carbon nanospheres up to 48 GPa

Raman spectra of onion-like carbon nanospheres (OCNSs) have been studied under pressure up to 48 GPa. A transformation related to a change from sp(2) to sp(3) bonding of carbons in OCNSs was observed at pressures above 20 GPa. The Raman spectra exhibit some vibrational features similar to those of the theoretically proposed Z-carbon phase of cold-compressed graphite, while the transition pressure is obviously higher than that for graphite. In contrast to the transformations in compressed graphite, interlayer bonds are formed on the nanoscale between buckled layers in OCNSs under pressure due to the concentric configuration, and sp(2)-sp(3) conversion is incomplete even up to 48 GPa. This is confirmed by TEM observations on the decompressed samples. Moreover, the onion-like carbon structure is extremely stable and can be recovered even after a compression cycle to 48 GPa. This high stability, beyond that of other sp(2) carbon materials, is related to the unique onion-like configuration and to the interlayer bonding. The transformed material should have excellent mechanical properties so that it can sustain very high pressure.

First-principles investigation in the Raman and infrared spectra of sp3 carbon allotropes

The Raman and infrared intensities of Bct-C4, C-carbon, F-carbon, M-carbon, O-carbon, P-carbon, T12, W-carbon, X-carbon and Z-carbon are calculated based on the density-functional theory and two discoveries are found. Firstly, their Raman peaks are distributed into two regions: from 600 to 1150 cm(-1) and from 1150 to 1450 cm(-1). As the Raman peaks of diamond and graphite are above 1300 cm(-1), great attention should be paid between 600 and 1150 cm(-1) in experiments. Secondly, differently from diamond, all of these newly proposed structures have infrared active modes. This suggests that infrared spectrum may give valuable information about the crystal structure of cold-compressed graphite. (C) 2014 Elsevier Ltd. All rights reserved.

Prediction of a novel monoclinic carbon allotrope

A novel allotrope of carbon with P2/m symmetry was identified during an ab initio minima-hopping structural search which we call M10-carbon. This structure is predicted to be more stable than graphite at pressures above 14.4 GPa and consists purely of s p 3 bonds. It has a high bulk modulus and is almost as hard as diamond. A comparison of the simulated X-ray diffraction pattern shows a good agreement with experimental results from cold compressed graphite.

An ab initio study on the transition paths from graphite to diamond under pressure

We calculate and compare the transition paths from graphite to two types of diamond using the variable cell nudged elastic band method. For the phase transition from graphite to cubic diamond, we analyze in detail how the π bonds transit to the σ bonds in an electronic structure. Meanwhile, a new transition path with a lower energy barrier for the transformation from graphite to hexagonal diamond is discovered. The path has its own peculiar sp2–sp3 bonding configurations, serving as a transition state. Further calculation suggests that the sp2–sp3 transition state represents an expected general phenomenon for cold-compressed graphite.

From soft to superhard: Fifty years of experiments on cold-compressed graphite

In recent years there have been numerous computational studies predicting the nature of cold-compressed graphite yielding a proverbial alphabet soup of carbon structures (e.g., bct-C4, K4-, M-, H-, R-, S-, T-, W- and Z-carbon). Although theoretical methods have improved, the inherent nature of graphite (i.e., low-Z) and the subsequent room-temperature, high-pressure phase transition (i.e., low symmetry, nanocrystalline and sluggish), make experimental measurements difficult to execute and interpret even with the current technology of 3rd generation synchrotron sources. The room-temperature, high-pressure phase transition of graphite has been detected by numerous kinds of experiments over the past fifty years, such as electrical resistance measurements, optical microscopy, X-ray diffraction, inelastic X-ray scattering, and Raman spectroscopy. However, the identification and characterization of high-pressure graphite is replete with controversy since its discovery more than fifty years ago. Recent experiments confirm that this phase has a monoclinic structure, consistent with the M-carbon phase predicted by theoretical computations. Meanwhile, experiments demonstrate that the phase transition is sluggish and kinetics is important in discerning the phase boundary. Additionally, the post-graphite phase appears to be superhard with hardness comparable to that of diamond.

Communication: From graphite to diamond: Reaction pathways of the phase transition

Phase transitions between carbon allotropes are calculated using the generalized solid-state nudged elastic band method. We find a new reaction mechanism between graphite and diamond with nucleation characteristics that has a lower activation energy than the concerted mechanism. The calculated barrier from graphite to hexagonal diamond is lower than to cubic diamond, resolving a conflict between theory and experiment. Transitions are calculated to three structures of cold compressed graphite: bct C4, M, and Z-carbon, which are accessible at the experimentally relevant pressures near 17 GPa.

Polymorphic Phases of sp3-Hybridized Carbon under Cold Compression

It is well established that graphite can be transformed into superhard carbons under cold compression (Mao et al. Science 2003, 302, 425). However, structure of the superhard carbon is yet to be determined experimentally. We have performed an extensive structural search for the high-pressure crystalline phases of carbon using the evolutionary algorithm. Nine low-energy polymorphic structures of sp(3)-hybridized carbon result from the unbiased search. These new polymorphic carbon structures together with previously reported low-energy sp(3)-hybridized carbon structures (e.g., M-carbon, W-carbon, and Cco-C(8) or Z-carbon) can be classified into three groups on the basis of different ways of stacking two (or more) out of five (A-E) types of buckled graphene layers. Such a classification scheme points out a simple way to construct a variety of sp(3)-hybridized carbon allotropes via stacking buckled graphene layers in different combinations of the A-E types by design. Density-functional theory calculations indicate that, among the nine low-energy crystalline structures, seven are energetically more favorable than the previously reported most stable crystalline structure (i.e., Cco-C(8) or Z-carbon) in the pressure range 0-25 GPa. Moreover, several newly predicted polymorphic sp(3)-hybridized carbon structures possess elastic moduli and hardness close to those of the cubic diamond. In particular, Z-carbon-4 possesses the highest hardness (93.4) among all the low-energy sp(3)-hybridized carbon structures predicted today. The calculated electronic structures suggest that most polymorphic carbon structures are optically transparent. The simulated X-ray diffraction (XRD) spectra of a few polymorphic structures are in good agreement with the experimental spectrum, suggesting that samples from the cold-compressed graphite experiments may consist of multiple polymorphic phases of sp(3)-hybridized carbon.

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

A simple (5 + 6 + 7)–sp3 carbon (denoted as F-carbon) with eight atoms per unit cell predicted by a newly developed ab initio particle-swarm optimization methodology on crystal structure prediction is proposed. F-carbon can be seen as the reconstruction of AA-stacked or 3R-graphite, and is energetically more stable than 2H-graphite beyond 13.9 GPa. Band structure and hardness calculations indicate that F-carbon is a transparent superhard carbon with a gap of 4.55 eV at 15 GPa and a hardness of 93.9 GPa at zero pressure. Compared with the previously proposed Bct-, M- and W-carbons, the simulative x-ray diffraction pattern of F-carbon also well matches the superhard intermediate phase of the experimentally cold-compressed graphite. The possible transition route and energy barrier were observed using the variable cell nudged elastic band method. Our simulations show that the cold compression of graphite can produce some reversible metastable carbons (e.g. M- and F-carbons) with energy barriers close to diamond or lonsdaleite.

Families of Superhard Crystalline Carbon Allotropes Constructed via Cold Compression of Graphite and Nanotubes

We report a general scheme to systematically construct two classes of structural families of superhard sp(3) carbon allotropes of cold-compressed graphite through the topological analysis of odd 5+7 or even 4+8 membered carbon rings stemmed from the stacking of zigzag and armchair chains. Our results show that the previously proposed M, bct-C(4), W and Z allotropes belong to our currently proposed families and that depending on the topological arrangement of the native carbon rings numerous other members are found that can help us understand the structural phase transformation of cold-compressed graphite and carbon nanotubes (CNTs). In particular, we predict the existence of two simple allotropes, R and P carbon, which match well the experimental x-ray diffraction patterns of cold-compressed graphite and CNTs, respectively, display a transparent wide-gap insulator ground state and possess a large Vickers hardness comparable to diamond.

Lowest enthalpy polymorph of cold-compressed graphite phase

Based on an ab initio evolutionary algorithm, a novel carbon polymorph with an orthorhombic Cmcm symmetry is predicted, named as C carbon, which has the lowest enthalpy among the previously proposed cold-compressed graphite phases.

Superhard Monoclinic Polymorph of Carbon

We report a novel phase of carbon possessing a monoclinic C2/m structure (8 atoms/cell) identified using an ab initio evolutionary structural search. This polymorph, which we call M-carbon, is related to the (2x1) reconstruction of the (111) surface of diamond and can also be viewed as a distorted (through sliding and buckling of the sheets) form of graphite. It is stable over cold-compressed graphite above 13.4 GPa. The simulated x-ray diffraction pattern and near K-edge spectroscopy are in satisfactory agreement with the experimental data [W. L. Mao, Science 302, 425 (2003)10.1126/science.1089713] on overcompressed graphite. The hardness and bulk modulus of this new carbon polymorph are calculated to be 83.1 and 431.2 GPa, respectively, which are comparable to those of diamond.