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Spherical crystals in minerals from prismatine-bearing rock from Waldheim, including ultrahigh-pressure (UHP) minerals such as stishovite and coesite, were previously described in uncommon crustal environments. To determine if this was an outlier phenomenon, we searched for equivalent inclusions in other rocks, which we indeed discovered in a Variscan tin-bearing granite sensu stricto from the Erzgebirge/Germany. The identification of more examples of this phenomenon implies a novel, very rapid transcrustal transport mechanism, which, however, is not unique. We demonstrate the unusual occurrence of UHP minerals (moissanite, diamond, lonsdaleite, stishovite, coesite, kumdykolite, and cristobalite-II) in topaz the investigated granitic samples, which reflects the direct interaction of mantle and crust via supercritical fluids or extremely volatile-rich melts. Mostly, the UHP minerals we recognized occur as tiny inclusions in moissanite. The trapping by this mineral prevents a fast reaction in an exogenous environment.

Silicon nanocrystals (NCs) have great potential for applications in optoelectronics, photovoltaics and biomedicine. The photo-physical characteristics of these particles strongly depend on whether they are crystalline or amorphous. This structural order is sensitive to the synthesis details. To understand the morphology of hydrogen-passivated silicon clusters and find how it depends on the passivation degree, we calculated the optimal structures of SinH2m clusters with n ≤ 21 and 2m ≤ 30. We found that as the hydrogen amount increases, clusters run through three structural types: (i) amorphous clusters with dangling bonds (DBs), (ii) amorphous clusters without DBs at intermediate passivation, and (iii) crystalline clusters. We describe a mechanism which removes dangling bonds in the amorphous clusters of the second type and shows its key importance for cluster structure formation. The crystalline lattice (diamond or lonsdaleite) is found to emerge when all broken bonds at the NC surface are passivated. We constructed the phase P-T diagram of Si-H clusters, compared it with the available experimental data and discussed the transfer of our results to large Si nanoparticles.

The plumbides CaTPb2 (T = Rh, Pd), EuTPb2 (T = Rh, Pd, Pt), SrTPb2 (T = Rh, Pd, Pt) and BaTPb2 (T = Pd, Pt) were obtained by direct reactions of the elements in sealed tantalum tubes in an induction furnace. The moisture sensitive polycrystalline samples were characterized by X-ray powder diffraction. They crystallize with the orthorhombic MgCuAl2-type structure, space group Cmcm. The structures of CaRhPb2 (a = 433.78(3), b = 1102.06(8), c = 798.43(6) pm, wR = 0.0285, 432 F2 values and 16 variables) and EuPdPb2 (a = 457.24(5), b = 1158.27(13), c = 775.73(8), wR = 0.0464, 464 F2 values and 16 variables) were refined from single crystal X-ray diffractometer data. The characteristic structural motif is the distorted tetrahedral substructure built up by the lead atoms with Pb–Pb distances of 326–327 pm in CaRhPb2 and of 315–345 pm in EuPdPb2. With increasing size of the alkaline earth (Eu) cation, the lead substructure becomes more anisotropic with a shift of the [TPb2] polyanions from three- to two-dimensional, leading to significantly increased moisture sensitivity. Temperature dependent magnetic susceptibility studies reveal Pauli paramagnetism for SrRhPb2, SrPtPb2, BaPdPb2 and BaPtPb2. EuRhPb2 and EuPdPb2 are Curie–Weiss paramagnets with stable divalent europium as is also evident from 151Eu Mössbauer spectra. EuRhPb2 is a ferromagnet with T C = 17.7(2) K, while EuPdPb2 orders antiferromagnetically at T N = 15.9 K. This is in agreement with the full magnetic hyperfine field splitting of the 151Eu Mössbauer spectra at T = 6 K.

It has been found recently in experiments that diamond/lonsdaleite biphase could possess excellent thermal-mechanical properties, implying that the properties of carbon materials can be improved by reasonably designing their internal structures. The mechanism of the excellent performance arising from biphasic structure is still unknown and needs to be revealed. In this paper, we established a series of possible diamond/lonsdaleite biphasic structures and revealed the optimization mechanism of the biphasic structure using first principles calculations. It shows in our ab-initio molecular dynamics simulations that the lonsdaleite cannot exist stably at room temperature, which could explain why pure lonsdaleite can hardly be found or synthesized. Detailed analysis shows that partial slip would occur in the lonsdaleite region if the applied strain is sufficiently large, leading to the transition from biphasic phase to cubic phase. Then, further shear strain would be applied along the hard shear direction of the cubic structure, resulting in an ascent of stress. The results presented could offer an insight into the structural transformation at high temperature and large strain.

We present ab initio calculations of the electronic and optical properties of hexagonal SiGe alloys in the lonsdaleite structure. Lattice constants and electronic band structures in excellent agreement with experiment are obtained using density-functional theory. Hexagonal Si has an indirect band gap, while hexagonal Ge has a pseudo-direct gap, i.e. the optical transitions at the minimum direct band gap are very weak. The pseudo-direct character of pure hexagonal Ge is efficiently lifted by alloying. Already for a small admixture of Si, symmetry reduction enhances the oscillator strength of the lowest direct optical transitions. The band gap is direct for a Si content below 45 %. We validate lonsdaleite group-IV alloys to be efficient optical emitters, suitable for integrated optoelectronic applications.

In this report, a new carbon allotrope named nanoleite is proposed. Its crystal structure is constructed by embedding carbon nanotubes into the matrix of lonsdaleite periodically, leading to a hexagonal primitive unit cell. The equilibrium structure of nanoleite is fully relaxed by density functional theory calculation, and we demonstrate that nanoleite is a semiconductor with an indirect energy bandgap of 2.06 eV. Furthermore, it has a high absorption coefficient in the visible spectrum range, which is comparable to that of the gallium arsenide and indium phosphide. X-ray diffraction patterns and phonon modes are also studied.

Diamond is a material of immense technological importance and an ancient signifier for wealth and societal status. In geology, diamond forms as part of the deep carbon cycle and typically displays a highly ordered cubic crystal structure. Impact diamonds, however, often exhibit structural disorder in the form of complex combinations of cubic and hexagonal stacking motifs. The structural characterization of such diamonds remains a challenge. Here, impact diamonds from the Popigai crater were characterized with a range of techniques. Using the MCDIFFaX approach for analysing X-ray diffraction data, hexagonality indices up to 40% were found. The effects of increasing amounts of hexagonal stacking on the Raman spectra of diamond were investigated computationally and found to be in excellent agreement with trends in the experimental spectra. Electron microscopy revealed nanoscale twinning within the cubic diamond structure. Our analyses lead us to propose a systematic protocol for assigning specific hexagonality attributes to the mineral designated as lonsdaleite among natural and synthetic samples.

Complementary nano- and atomic-scale data from SEM, FIB, HRTEM, and EELS observations of after-coal impact diamonds from the giant Kara astrobleme are described, presenting their particular nano-sized porous polycrystalline structure, which consists of well-shaped single 20-30 nm nanocrystals that are free of deformation defects and do not contain lonsdaleite. The porous micro- and nanostructure is a special typomorphic feature of after-coal diamonds that suggests a crystallisation mechanism through short distance diffusion. The data for the after-coal impact diamonds presented here demonstrate their distinguishing characteristics from after-graphite impact diamonds, and have some similarity with the enigmatic carbonado, providing new insights to the origin of the latter.

The phase stabilities as well as some intrinsic properties of hydrogenated diamondized bilayer graphenes, 2-dimensional materials adopting the crystal structure of diamond and of lonsdaleite, are investigated using a first-principles approach. Our simulations demonstrate that hydrogenated diamondized bilayer graphenes are thermodynamically stable with respect to bilayer graphene and hydrogen molecule even at 0 GPa, and additionally they are found to withstand the physical change in structure up to at least 1000 K, ensuring their dynamical and thermal stabilities. The studied hydrogenated diamondized bilayer graphenes are predicted not only to behave as direct and wide band gap semiconductors, but also to have a remarkably high resistance to in-plane plastic deformation induced by indentation as implied by their high in-plane elastic constants comparable to those of diamond and of lonsdaleite. The mechanical stability of the materials is confirmed though the fulfilment of the Born stability criteria. Detailed analysis of phonon vibrational frequencies of hydrogenated diamondized bilayer graphenes reveals possible Raman active and IR active modes, which are found to be distinctly different from those of hydrogenated diamond-like amorphous carbon and defective graphene and thus could be used as a fingerprint for future experimental characterization of the materials.

The importance of hexagonal Lonsdaleite silicon-germanium has been growing lately due to its possible uses in optoelectronic devices. However, very little is known about defects in the hexagonal phases of group-IV semiconductors. We extend here an efficient constrained structure prediction algorithm designed for interface reconstructions to the study of point defect geometries. With this method we perform an exhaustive structure prediction study of the most energetically favorable intrinsic defects in Lonsdaleite silicon. We obtain among the lowest-energy structures the hexagonal counterparts of all known defects of cubic silicon, together with other often more complex geometries. Neutral vacancies, fourfold-coordinated, and Frenkel defects have comparable formation energies in both hexagonal and cubic phases, while some interstitial defects become considerably more stable in the hexagonal lattice. Furthermore, due to the reduced symmetry, formation energies can depend on the orientation of the defect with respect to the $c$ axis. Finally, we calculate the density of states of the defective supercells to determine which defects lead to electronic states in the band gap, potentially affecting the performance of optoelectronic devices based on hexagonal group-IV crystals.

Germanium is crystalized in the cubic diamond structure, but its high energy hexagonal Ge (lonsdaleite) phase has many novel properties such as direct band gap. Using first-principles calculations, we show that the hexagonal lonsdaleite phase of Ge can be stabilized by introducing carriers, either electrons or holes, because Ge in the cubic and hexagonal phases form a type-I band alignment with both electrons and holes localized at the hexagonal site. This result is distinct from that in zinc-blende compounds such as ZnSe, because due to the lack of inversion symmetry, the crystal-field splitting, zone folding, and symmetry-controlled level repulsion between valence and conduction band states lead to a type-II band alignment between its cubic and hexagonal phases, so the hexagonal (wurtzite) phase of ZnSe can only be stabilized, in principle, by holes. This distinction reveals that, due to the symmetry differences, the well-investigated understanding of band structure differences between zinc-blende and wurtzite phases should not be simply extended to that of diamond and lonsdaleite phases despite the remarkable structure resemblance between the two cases.

Bicontinuous membranes in cell organelles epitomise nature’s ability to create complex functional nanostructures. Like their synthetic counterparts, these membranes are characterised by continuous membrane sheets draped onto topologically complex saddle-shaped surfaces with a periodic network-like structure. In cell organelles, their structure sizes around 50–500 nm and fluid nature make Transmission Electron Microscopy (TEM) the analysis method of choice to decipher nanostructural features. Here we present a tool to identify bicontinuous structures from TEM sections by comparison to mathematical “nodal surface” models, including the hexagonal lonsdaleite geometry. Our approach, following pioneering work by Deng and Mieczkowski (1998), creates synthetic TEM images of known bicontinuous geometries for interactive structure identification. We apply the method to the inner membrane network in plant cell chloroplast precursors and achieve a robust identification of the bicontinuous diamond surface as the dominant geometry in several plant species. This represents an important step in understanding their as yet elusive structure-function relationship.

International audience; Here, we report the first occurrence so far of cubic diamond and lonsdaleite in a siliceous breccia hosted in nickel laterite from the Tiébaghi mine, New Caledonia. Diamond and lonsdaleite polytype occurred as minerals in surficial formed siliceous rocks. The vibrational properties and the calculated elastic properties and electronic band structure of 3C-diamond and lonsdaleite were studied by micro-Raman spectroscopy and ab initio calculation. Our measured Raman spectra demonstrate the coexistence of cubic diamond and lonsdaleite based on the correlation between ab initio calculation and experimental results. We showed that the position of the single Raman vibration mode for cubic diamond corresponds to the first order scattering of F2g symmetry at 1332 cm−1. For lonsdaleite, the ab initio calculations predict three fundamental vibrational modes: 1207 cm−1 (E2g), 1307 cm−1 (A1g), and 1330 cm−1 (E1g). The calculated indirect bandgaps of 3C-diamond and lonsdaleite at room temperature are 5.7 eV and 5.2 eV, respectively. The elastic anisotropy calculations show that lonsdaleite has the greatest shear modulus, bulk modulus and young’s modulus compared to cubic diamond, which indicates a high degree of hardness of this hexagonal structure of diamond.

The discovery of the carbon atom, an element forming the basis of life, had a great impact on technological development that continues to benefit from its valency that gives this element the ability to form many allotropes. Natural carbon allotropes, like diamond and graphite, have been under intense research, and many other synthetic allotropes are continuously being produced with extraordinary structural and chemical properties. When researching carbon-based nanostructures and their applications, numerous experiments are usually necessary to optimize laboratory protocols and achieve confidence in the overall approach. Furthermore, nanoscale laboratory approaches are time-consuming and expensive, and despite following consistent protocols, the results may be difficult to interpret and without clarity. In such cases, computational methods may be particularly helpful for understanding the interactions between carbon atoms and different biomolecules. This chapter focuses on updated carbonaceous materials that exist in various forms across all dimensions: 0D (e.g., fullerene—C60, C540, C70, etc.), 1D (e.g., carbon nanotubes—single-walled, multi-walled), 2D (e.g., graphene, penta-graphene), and 3D (e.g., graphite, diamond, lonsdaleite). For each form of carbon allotrope, a brief description is provided along with the chemical structure and the most important properties of the materials. Also, interesting applications are discussed in order to emphasize the key novel properties of these carbonaceous materials. Moreover, state of the art and future prospects of carbon allotropes are presented, highlighting the novel carbon-based nanostructures. The reader will be introduced to the field of computational chemistry , followed by a brief introduction into ab initio methods and density functional theory. The chapter ends with a short presentation on carbonaceous materials that were researched using computational methods, either before or after being synthesized in the laboratory.

Conventional diamond-structured silicon (Si) and germanium (Ge) possess indirect fundamental band gaps, limiting their potential for applications in light-emitting devices. However, $\mathrm{Si}_{x}\mathrm{Ge}_{1-x}$ alloys grown in the lonsdaleite (“hexagonal diamond”) phase have recently emerged as a promising direct-gap, Si-compatible material system, with experimental measurements demonstrating strong room temperature photoluminescence. When grown in the lonsdaleite phase, Ge possesses a narrow ( $\sim 0.3 \mathrm{eV}$ ) “pseudo-direct” fundamental band gap. Lonsdaleite Si is indirect-gap ( $\sim 0.8 \mathrm{eV}$ ), creating the possibility to achieve direct-gap lonsdaleite $\mathrm{Si}_{x}\mathrm{Ge}_{1-x}$ alloys across a Ge-rich composition range. We present a first principles analysis of the electronic and optical properties of lonsdaleite $\mathrm{Si}_{x}\mathrm{Ge}_{1-x}$ alloys, elucidate the electronic structure evolution and direct- to indirect-gap transition, and describe the impact of alloy band mixing effects on inter-band optical transition strengths.

We explore the structural evolution of highly oriented pyrolytic graphite (HOPG) under detonation-induced shock conditions using in situ synchrotron x-ray diffraction in the ns timescale. We observe the formation of hexagonal diamond (lonsdaleite) at pressures above 50 GPa, in qualitative agreement with recent gas gun experiments. First-principles density functional calculations reveal that under uniaxial compression, the energy barrier for the transition toward hexagonal diamond is lower than that for cubic diamond. Finally, no indication of cubic diamond formation was observed up to $g70$ GPa.

For the first time, lonsdaleite-rich impact diamonds from one of the largest Popigai impact crater (Northern Siberia) with a high concentration of structural defects are investigated under hydrostatic compression up to 25 GPa. It is found that, depending on the nature of a sample, the bulk modulus for lonsdaleite experimentally obtained by X-ray diffraction in diamond-anvil cells is systematically lower and equal to 93.3-100.5% of the average values of the bulk moduli of a diamond matrix. Density functional theory calculations reveal possible coexistence of a number of diamond/lonsdaleite and twin diamond biphases. Among the different mutual configurations, separate inclusions of one lonsdaleite (001) plane per four diamond (111) demonstrate the lowest energy per carbon atom, suggesting a favorable formation of single-layer lonsdaleite (001) fragments inserted in the diamond matrix. Calculated formation energies and experimental diamond (311) and lonsdaleite (331) powder X-ray diffraction patterns indicate that all biphases could be formed under high-temperature, high-pressure conditions. Following the equation of states, the bulk modulus of the diamond (111)/lonsdaleite (001) biphase is the largest one among all bulk moduli, including pristine diamond and lonsdaleite.

Electrical insulators with ultrahigh thermal conductivity (κL) are highly desirable for thermal management to facilitate heat extraction in many electronic devices. In this work, we select three typical carbon allotropes (lonsdaleite, Bct-C4, and Z-carbon) with ultrahigh κL from the 522 carbon allotropes in the Samara Database by using Boltzmann transport theory combined with first-principles calculations. We find that the thermal conductivity (κL) of the three carbon allotropes is 1686.67, 1411.02 and 1262.05 W m−1 K−1, respectively, at room temperature. A further analysis of both harmonic and anharmonic properties reveals that such high κL is attributed to their exceptional atomic structures. They all are composed of pure sp3 hybridized carbon with a short bond length, small unit cell and strong chemical bonding. Together with diamond, they are the top four κL materials (beyond 1000 W m−1 K−1) reported so far among the 3D carbon allotropes. Equally important, we propose a simple descriptor, namely the scaled Pugh ratio (G/K/n), to characterize the strength of chemical bonding for high-throughput thermal material screening. The three identified carbon allotropes in this study would be alternatives to diamond in future thermal energy devices.

We report the results of a study of the polycrystalline powder of the diamond-lonsdaleite from the Popigai crater (Siberia) using UV micro-Raman spectroscopy and high-resolution synchrotron X-ray diffraction. By subtracting two experimental Raman spectra of diamond-lonsdaleite samples with close amounts of diamond and lonsdaleite, we were able to identify the polytypic composition of impact diamonds in contrast to the method of X-ray diffraction. We have managed to get for the first time the spectrum of “pure” lonsdaleite. Its deconvolution has allowed us to identify all the three Raman - active vibrational modes E2g, A1g, and E1g whose positions agree well with the results of ab initio calculations.

The search for new nanostructural topologies composed of elemental carbon is driven by technological opportunities as well as the need to understand the structure and evolution of carbon materials formed by planetary shock impact events and in laboratory syntheses. We describe two new families of diamond-graphene (diaphite) phases constructed from layered and bonded sp3 and sp2 nanostructural units and provide a framework for classifying the members of this new class of materials. The nanocomposite structures are identified within both natural impact diamonds and laboratory-shocked samples and possess diffraction features that have previously been assigned to lonsdaleite and postgraphite phases. The diaphite nanocomposites represent a new class of high-performance carbon materials that are predicted to combine the superhard qualities of diamond with high fracture toughness and ductility enabled by the graphitic units and the atomically defined interfaces between the sp3- and sp2-bonded nanodomains.

Strong, directional, N bonding led surface stabilization is induced to obtain free-standing two-dimensional (2D) layered assemblies of Lonsdaleite and diamond-phase carbon by surface termination at three coordinated C sites. These assemblies have strong bonding and surface N lone pair induced self-protection and achieve respective bulk-like electronic, mechanical, and crystallographic properties at as low as seven atomic layer thickness. Lonsdaleite phase subnano thickness free-standing as small as three layered assembly shows bulk-like thermal stability up to 2900 K. Size confinement effect dominates up to four layered assembly, and then electronic and crystallographic properties are dominated by bulk stabilization which saturates toward respective bulk-like features from seven layered assembly. Similarly, free-standing hybrid assemblies of Lonsdaleite phase carbon with BeF (C5NBeF) and BS (C5NBS) layer are also proposed along with low symmetric Cm-C3N4 layered assembly. The combination of N and O is sho...

For the first time, 60 large diamond aggregates were found inside the Popigai meteorite crater during washing of alluvial deposits along the Dogoi river crossing the crater. These aggregates are similar in appearance to yakutites from the placers of Northern Yakutia (YPY), and we regard them as yakutites from the Popigai crater (YPC). The structure and optical properties of Popigai impact diamonds from the impact melt rocks (tagamites) in the crater (PIDT) and yakutites YPC/YPY were compared in detail. In all these cases, a polycrystalline structure consisting of nanoscale grains of cubic and twinned cubic diamond (lonsdaleite) was found. This is the result of a solid-phase graphite-diamond transition due to an impact event 35 million years ago. The diamond aggregates show the following features: a red shift of the short-wave edge of the transmission, broadening of the diamond Raman peaks, signals from other diamond polytypes and numerous inclusions of other minerals in the Raman spectra, and a dominant broadband photoluminescence (PL). PL in the N3 system associated with N3V centers in PIDT diamonds indicates a high-temperature annealing of these aggregates with resulting aggregation of impurities during the prolonged cooling of large impact melt pockets and pools. It is assumed that some of the impact diamonds were ejected from the crater during the impact event and experienced rapid cooling. Some of these diamonds fell back into the crater (YPC yakutites), others have been deposited outside the crater and displaced during erosion (YPY yakutites). Difference in size and shape between the PIDTs and yakutites YPC/YPY is due to the difference in size of original graphite flakes or aggregates and/or due to the fundamentally different technologies of diamond extraction.

We present the results of our study of a polycrystalline diamond–lonsdaleite powder from the Popigai crater (Siberia) using UV micro-Raman scattering and synchrotron X-ray diffraction. By subtracting two experimental Raman spectra of diamond–lonsdaleite samples with a close ratio of the diamond and lonsdaleite fractions, when the maximum contribution to the difference spectrum is related to the difference of the lonsdaleite contributions and not to the change in the diamond band width, we have managed to obtain the spectrum of “pure” lonsdaleite. Its deconvolution has allowed us to identify all three Raman-active vibrational modes E2g, A1g, and E1g whose positions agree well with the results of ab initio calculations.

Recent experiments have reported that nanotwinned and nanopolycrystalline diamond-like materials exhibit unprecedented hardness, enhanced fracture toughness, and improved thermal stability. This extraordinary finding has raised fundamental issues on atomistic hardening mechanisms governing the combined resistance to indentation of the chemical bonds in these nanostructured hardest covalent solids. Indentation-induced transition from diamond, nanopolycrystalline diamond and nanotwinned diamond into graphite at the area of indentation has been experimentally confirmed. Here, using molecular dynamics simulation, we unveil a new graphitization resistance dominated hardening mechanism that hinders graphitization by introducing a large number of twin boundaries (TBs) and grain boundaries (GBs) commonly known in nanostructured diamond, leading to greatly enhanced hardness of an indented diamond lattice. This remarkable nanostructure-generated additional graphitization resistance mechanism offers fundamental insights for understanding and improving the stress response of nanotwinned and nanopolycrystalline diamond under indentation and is verified by comparing the results with those from the experiment.

Described are unusual nano-microcrystals of natural diamond found in a meteorite crater of Ukraine and advised about the earlier unknown mechanism of diamond polyhedra growth—the formation by globules. It was revealed that diamond nano-microcrystals in a meteorite crater are very similar to globular crystals, and at the same time have octahedral faceting. The morphology and composition of diamond nano-microcrystals are studied by scanning electron microscopy and with an energy dispersive X-ray analyser. These tiny crystals are grown on the grains of impact apographitic diamond from the Bilylivka meteorite crater (Zapadnaya impact crater) on the Ukrainian Shield. Their surface morphology indicates that the nano-microdiamonds are grown, most probably, by a vapor deposition process immediately after the formation of the impact diamond–transformation of the graphite into diamond and lonsdaleite.

International audience; The purpose of this paper is to predict the existence of novel 3D covalent ultra-hard carbon allotropes in view of synthesizing them eventually. Stability and spectroscopic characteristics of the rarely occurring tricarbon linear molecule from quantum chemistry calculations are extended to the solid state based on crystal chemistry considerations and quantum density functional theory ground state calculations letting devise rhombohedral rh-C3 (h-C9) and hexagonal h-C6 proposed as ultra-hard carbon allotropes like lonsdaleite. Two crystallographic different carbons forming linear C2-C1-C2 unit are characterized as sp3-like C2 with an angle of 106.17° smaller than ideal 109.4°, and sp1 C1. The calculated elastic constants are positive and their combinations obey the required stability criteria; furthermore they point to a strong anisotropy of the mechanical properties of the two allotropes rh-C3 (h-C9) and h-C6 with exceptionally large C33 values (1636 GPa and 1610 GPa, respectively), exceeding that of lonsdaleite (1380 GPa). The phonon dispersion curves stress furthermore the dynamical stability of the two carbon allotropes by showing positive frequencies throughout the hexagonal Brillouin zone. A significant separation between the highest frequencies of the optic modes illustrates the rigidity of the bonding within linear C3 building block and the whole structure. Both allotropes are characterized by large bulk moduli and high hardness values that are only slightly less than those of lonsdaleite and diamond. Weak metallic behavior of both new carbon allotropes were identified from electronic band structure calculations and explained by the mixed sp1-sp3 hybridizations, likewise other carbon allotropes from literature: m-C12 (sp1-sp2) and h-C18 (sp2-sp3).

The stacking sequence significantly affects the thermal conductivity of diamond. However, the inaccuracy of some widely used empirical potentials to calculate the anharmonic phonon transport in diamond, hinders the use of molecular dynamics simulations from revealing the detailed mechanism of phonon transport in diamond and other related materials with different stacking sequences. In this paper, a comparative study of four common carbon empirical potentials - Tersoff, Airebo, 2nd-rebo, and COMPASS - is performed to simulate the phonon thermal transport of diamond and lonsdaleite. The results show that the Tersoff potential, together with Airebo and 2nd-rebo potentials, is not suitable for describing the phonon transport mechanism in different diamond polytypes, both for harmonic phonon dispersion and anharmonic phonon scattering caused by stacking disorders. COMPASS potential gives a good fit for the acoustic phonon branches in all directions of diamond and accurately describes the band gaps between acoustic and optical branches, and is capable of representing the lattice dynamics and phonon thermal transport in diamond and lonsdaleite. The COMPASS potential is suitable for simulating the phonon thermal transport properties in diamond and lonsdaleite, and thus it is suitable for simulating the stacking disorder effect on the phonon thermal transport in diamond.

The structural, mechanical and electronic properties of carbon allotropes were systematically investigated by the first-principles calculations in this study, and the origin of high hardness of carbon allotropes was analyzed. We studied the mechanical properties, including the elastic moduli, Poisson’s ratio and Vickers hardness, of six novel carbon allotropes and compared with two known superhard carbon phases (diamond and lonsdaleite). It was found that six novel carbon allotropes exhibit the potential to be superhard materials. Besides, the band structures and electronic density of states of these carbon allotropes were calculated as well, and the superhard properties of the six novel carbon allotypes were preliminarily revealed. Finally, the tiling approach was adopted to analyze the origin of high hardness of six novel carbon allotropes from the perspective of structure.