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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.

The relative stability of diamond and graphite is readdressed from the new perspective of deductive molecular mechanics. Unlike most theoretical studies that are conducted numerically, this article uses an analytical model to gain insight into the fundamental reasons behind the quasi-degeneracy of these allotropes with very different bonding patterns. The relative energies of the allotropes are derived and several general statements about the structure of these materials are proven. This analysis yields a quasi-degenerate electronic ground state for graphite and diamond at 0 K. Numerical estimates based on this analysis are in astonishingly good agreement with experimental data and recent results of numeric modeling, despite the fact that they were obtained with a drastically smaller numerical effort. An extension of the proposed interpretation to silicon allotropes proves to be very successful as well. The proposed approach is also expanded to four-coordinated carbon allotropes, and the software package Adamas is developed, which is able to calculate allotrope energies and elastic properties (elastic moduli). In the case of diamond and graphene, some general statements could be proven from deductive molecular mechanics parameters. Specifically, it is shown that among the four-coordinated allotropes the cubic diamond structure represents the true minimum. In the cases of allotropes with some C—C bonds that are stronger than those in diamond, the energy gain is compensated by the mandatory presence of weaker bonds in the same allotrope, which leads to the overall increase of the energy relative to the diamond.

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.

The formation of structure of hexagonal diamond from graphite and cubic diamond is simulated with the density functional theory. Orthorhombic AB graphite transforms into hexagonal diamond under uniaxial compression at pressures above 61 GPa. The pressure should be higher than 57 GPa to form hexagonal diamond from AA graphite. Another possible way to obtain hexagonal diamond is uniaxial compression of cubic diamond under pressures from 300 to 380 GPa. The theoretical calculations were used to interpret the diffraction data obtained after X-ray diffraction and electron microscopic analysis of natural and synthetic diamond-like materials. We found that there are no crystals with ideal cubic and hexagonal structures in diamond-like carbon materials, and their structure is characterized by random packing of molecular layers.

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.

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.

The graphitization process of impact diamonds in the helium atmosphere at 1900–1960 °C is investigated. It is shown that when an impact diamond particle with a large number of lonsdaleite defects is kept at 1900 °C for 1 min, ~5 wt.% of graphite are formed (in the paralle experiment with a particle without lonsdaleite defects the graphite formation is not observed). With an increase in the temperature to 1930 °С and 1940 °С the graphite amount increases to 70 wt.% and 80 wt.% respectively. In both cases, a mixture of two graphite phases forms: the first is turbostratic graphite with d002 = 3.38 A; the second is compressed graphite with d002 = 3.21 A. On heating a particle to 1960 °C graphite is formed in which the layer packing contains a large amount of defects and is a mixture of 2Н and 3R polytypes (the ratio 40:60) with a small degree of turbostratic disordering.

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.

International audience; A new ultra-hard rhombohedral carbon rh-C4 (or hexagonal h-C12) is reported as derived from 3R graphite through crystal chemistry construction and ground state energy within the density functional theory. A resulting cohesive extended hexagonal three-dimensional network of h-C12 is identified as formed of C4 tetrahedra like in cubic and hexagonal diamond. The electronic band structure of rh-C4 is characteristic of insulator with Egap ​= ​4 ​eV similarly to diamond. From the energy-volume equations of state and the set of elastic constants a larger value of bulk modulus versus lonsdaleite, and the largest Vickers hardness (Hv) versus both forms of diamond were revealed. The phonon band structure showing only real wave numbers further confirms the stability of the new carbon phase.

Two zeolite-like cluster organic frameworks based on Cd-Cu/Mn-Cu heterometallic supertetrahedral secondary building units have been successfully constructed under solvothermal conditions, namely, Cu[Cd4Cu6(L)4(H2O)18](Ac)9⋅DMA⋅3 H2O (1), and Cu[Mn4Cu6(L)4(Ac)3(H2O)12](Ac)6⋅CH3CN⋅13 H2O (2), where H3L=2-(hydroxymethyl)-2-(pyridin-4-yl)-1,3-propanediol, Ac=CH3COO−, DMA=N,N′-dimethylacetamide. Single-crystal X-ray structural analysis reveals that both 1 and 2 exhibit 3-dimensional zeolite-like architectures with similar 4-connected components, but possess definitely different topologies of diamondoid (dia) and uncommon lonsdaleite (lon), respectively. 1 and 2 represent the first cases of zeolite-like cluster organic frameworks containing Cd-Cu/Mn-Cu heterometallic supertetrahedral secondary building units. Furthermore, the magnetic properties and porous nature of 1 and 2 were also studied.

The structural, elastic, elastic anisotropic, thermodynamic and electronic properties of Lonsdaleite C, Si and Ge and Lonsdaleite C–Si and Si–Ge alloys are investigated using density functional theory. The elastic anisotropy calculations show that the Lonsdaleite C0.25Si0.75 alloy has the greatest anisotropy in Poisson’s ratio, shear modulus, bulk modulus and Young’s modulus. Through the mixing of carbon and silicon and silicon and germanium at certain proportions, Lonsdaleite C0.25Si0.75 with metallic properties and Lonsdaleite Si0.25Ge0.75 with a direct band gap are obtained, where Lonsdaleite Si0.25Ge0.75 is a narrow direct band gap semiconductor with a band gap of 0.76 eV at the HSE06 hybrid functional level. The minimum thermal conductivity calculations on Lonsdaleite C–Si and Si–Ge alloys show that the minimum thermal conductivities of Lonsdaleite C0.75Si0.25 and Lonsdaleite C0.5Si0.5 are greater than that of diamond C, and the minimum thermal conductivities of Lonsdaleite C–Si and Si–Ge alloys in different directions are also investigated.

Coal is considered to be an abundantly available cheap feedstock for the fabrication of carbon nanomaterials. In this Letter, a report on the formation of nanodiamond from low-grade coals during low-power ultrasonic-assisted stimulation in hydrogen peroxide (H2O2) followed by dialysis in 1 kDa is given. High resolution-transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), Raman spectroscopy, ultraviolet–visible spectroscopy (UV–vis), fluorescence (FL), and Fourier transform infrared (FT-IR) spectroscopy analyses revealed the formation of carbon nanocrystals of monocrystalline and polycrystalline form with multiple plans. The nominal size of the carbon nanocrystals are found to be in the range of 4–15 nm. The planar spacing of the crystal lattice fingers is in the range of 2.0–2.3 A and are in good agreement with the lattice planes of various diamond phases including cubic diamond and lonsdaleite. The present work significantly contributes an additional synthetic methodology of nanodiamond pr...

This work demonstrates that diamond-like nano-carbons can be rapidly grown at atmospheric pressure and near ambient temperature in Ar gas bubble discharge in liquid ethanol. The method uses a discharge between point-to-plate electrodes immersed in ethanol, with plasma being generated inside Ar gas bubbles introduced through the needle electrode. The ethanol was dissociated at the liquid/gas interface into reactive species such as C2 and CH, which are the primary species responsible for diamond formation. A mixture of lonsdaleite nano-diamonds, amorphous carbon nano-spheres, and a graphitic carbon network, was formed. The rapid bubble movement distributes the reaction products almost immediately into the liquid phase, ensuring that nucleation of new material continues throughout the process. This simple, inexpensive and fast process avoids the elevated temperatures and extreme pressures of current methods. Refereed/Peer-reviewed

This paper describes the problem of the behavior of a mixture of small graphite particles with water in the conditions of shock-wave action at a pressure of 32 GPa and a temperature of up to 1200–1600 K. Graphite particles at these pressures and temperatures are capable of transforming into cubic diamonds or at least into their hexagonal form that is lonsdaleite. It is shown that, for sufficiently small graphite particles of the order of 1 μm, their mixture with water for about 10 μs can heat up to the above-mentioned temperatures and undergo phase transformation, remain in those conditions for about 50 μs, and then efficiently cool down during the next 50 μs to the temperatures below 300 K, while remaining in the diamond phase.

We introduce a computational approach to estimate the hardness and stiffness of diamond surfaces and nanoparticles by studying their elastic response to atomic nanoindentation. Results of our ab initio density functional calculations explain the observed hardness differences between different diamond surfaces and suggest bond stiffening in bare and hydrogenated fragments of cubic diamond and lonsdaleite. The increase in hardness and stiffness can be traced back to bond length reduction especially in bare nanoscale diamond clusters, a result of compression that is driven by the dominant role of the surface tension., Comment: 7 pages, 3 figures

Polycrystalline powder of a diamond–lonsdaleite system from the Popigai impact crater (Siberia) is studied via UV micro-Raman scattering (RS) and X-ray diffraction. A spectrum of pure lonsdaleite is obtained by subtracting two experimental Raman spectra of diamond–lonsdaleite samples with close ratios of diamond and lonsdaleite. The maximum contribution to the difference spectrum is due to the difference between the contribution from lonsdaleite, and not to the change in the bandwidth of the diamond. Deconvolution of the lonsdaleite spectrum allows isolation of all three vibrational modes (E2g, A1g, and E1g) active in the first-order RS, the positions of which agree well with the results from first-principles calculations.

The electronic structure and dielectric properties of the diamond, body centered cubic diamond (bc8), and hexagonal diamond (lonsdaleite) phases of carbon are computed using density functional theory and many-body perturbation theory with the emphasis on the excitonic picture of the solid phases relevant in the regimes of high-pressure physics and warm dense matter. We also discuss the capabilities of reproducing the inelastic x-ray scattering spectra in comparison with the existing models in light of recent x-ray scattering experiments on carbon and carbon bearing materials in the Megabar range., 25 pages, 25 figures

We propose a new class of carbon allotropes obtained by combining linear sp carbon chains with sp3 bulk carbon. The structure of these crystalline carbon allotropes consists of sp chains inserted in cylindrical cavities periodically arranged in hexagonal diamond (lonsdaleite). We carry out a detailed computational analysis of the structural, electronic, and vibrational properties of a simple form in this class: a single C6 strand embedded in a nm-sized cavity. We obtain a metallic allotrope exhibiting characteristic high-frequency vibrations associated with the sp chain stretching modes, and characterized by long-time room-temperature stability. In addition, we suggest how numerous similar allotropes could be constructed within this class, that we call zayedenes, all characterized by comparable metallicity and high-frequency vibrations.

We propose a new class of carbon allotropes obtained by combining linear sp carbon chains with sp3 bulk carbon.The structure of these crystalline carbon allotropes consists of sp chains inserted in cylindrical cavities periodically arranged in hexagonal diamond (lonsdaleite).We carry out a detailed computational analysis of the structural, electronic, and vibrational properties of a simple form in this class: a single C6 strand embedded in a nm-sized cavity.We obtain a metallic allotrope exhibiting characteristic high-frequency vibrations associated with the sp chains stretching modes, and characterized by long-time room-temperature stability.In addition, we suggest how numerous similar allotropes could be constructed within this class, that we call zayedene, all characterized by comparable metallicity and high-frequency vibrations.

The concentration of diamond-bearing tagamite from the Popigai impact crater produces large amounts of graphite in addition to impact diamonds (1:100, respectively). The question arises of whether this is residual graphite not converted to diamond at the time of the Popigai impact or is a retrograde form resulting from back-conversion of impact diamond to graphite in a high-temperature tagamite melt. Experiments show that graphite from tagamite is a residual phase. Coexistence of lonsdaleite, cubic diamond, and single-crystal graphite within a limited volume may be due to different orientations of the graphite base plane relative to the impact stress direction. Thus, the diamond-bearing rocks may contain significant amounts of residual graphite, which is consistent with published evidence.

Structural stability and the possible pathways to experimental formation of lonsdaleite—a hexagonal 2H polytype of diamond—have been studied in the framework of the density functional theory (DFT). It is established that the structural transformation of orthorhombic Cmmm graphite to 2H polytype of diamond must take place at a pressure of 61 GPa, while the formation of lonsdaleite from hexagonal P6/mmm graphite must take place at 56 GPa. The minimum potential barrier height separating the 2H polytype state from graphite is only 0.003 eV/atom smaller than that for the cubic diamond. The high potential barrier is indicative of the possibility of stable existence of the hexagonal diamond under normal conditions. In this work, we have also analyzed the X-ray diffraction and electron-microscopic data available for nanodiamonds found in meteorite impact craters in search for the presence of hexagonal diamond. Results of this analysis showed that pure 3C and 2H polytypes are not contained in the carbon materials of impact origin, the structure of nanocrystals found representing diamonds with randomly packed layers. The term “lonsdaleite,” used to denote carbon materials found in meteorite impact craters and diamond crystals with 2H polytype structure, is rather ambiguous, since no pure hexagonal diamond has been identified in carbon phases found at meteorite fall sites.

Based on detailed models of interatomic cohesion and the well-developed structural-thermodynamic approach, the interatomic bond energy has been determined for some covalent and metallic crystals and it has been found to agree with experimental data. Using the proposed analytical relationship for the degree of covalency of crystals the author has elaborated a generalized model that describes experimental hardness of supersclerometer materials (diamond, cBN, SiC, and others), minerals, refractory compounds, semiconductors, ionic crystals and metals. On the basis of the mechanical-chemical approach to (nano)flow a dislocation model of crystal-mechanical anisotropy of yield strength has been put forward for monocrystalline diamond and lonsdaleite; the model makes it possible to draw a conclusion on a slight difference in their sclerometric characteristics. The experimental relationships that govern high-temperature high-pressure plasticity in some covalent crystals are discussed.

We successfully synthesized a novel anionic luminescent metal–organic framework (MOF) (NENU-524) with a lonsdaleite topology. NENU-524 contains a trigonal prismatic unit {Zn8(btca)6(2-NH2-bdc)3} that can be regarded as a double secondary building unit with an unusual triply bound triangular frustum geometry. The prepared NENU-524 had a permanent porosity and excellent stability in air. NENU-524 was used as a platform to encapsulate yellow-emitting [Ir(ppy)2(bpy)]+ cations in the nanotube channels of the blue-emitting MOF via an ion-exchange process. The [Ir(ppy)2(bpy)]+@NENU-524 MOF ([Ir(ppy)2(bpy)]+ concentration 3.86 wt%) emitted a pure white light with CIE coordinates of (0.300, 0.336) and a high quantum yield of up to 15.2%. The white light-emitting diodes assembled using [Ir(ppy)2(bpy)]+@NENU-524 as a white phosphor emitted a bright white light, suggesting that the composite is a promising material for use in lighting. The assembled white light-emitting diodes continued to emit a bright white light for up to one month. This simple and feasible approach could be used to develop luminescent luminophor@MOFs composites for practical applications.

Although diamond is the hardest material, it would still be deformed by indenting. Therefore, investigating its incipient plasticity during the indenting process should be of great interest. Through molecular dynamics simulations, we have investigated the nanoindentations of diamond and probed its incipient plasticity according to the obtained load–displacement (P–h) curve. We found that the incipient plasticity of diamond should result from the propagation of the dislocations and the structural phase transitions from cubic diamond to lonsdaleite. The probable physical processes of incipient plasticity of diamond during nanoindentation were also suggested. The results presented in this work would not only provide clues for detecting the pop-in event of diamond in the future experimental work, but also offer new insights into the plasticity of diamond, which could be beneficial to the design of novel nano-structured superhard materials and elucidating the origins of wear and friction.

Zeolitic cluster organic frameworks constructed with two different tetrahedral units, hexamethylenetetramine (hmt) and hmt-type [Cu4Cl6]2– clusters, are realized in this work for the first time. By using a tetrahedral hmt ligand to assemble in situ hmt-type Cu4Cl6 clusters, a three-dimensional cluster organic framework with lonsdaleite (lon) topology is successfully built to host the two-dimensional honeycomb-like metal–organic complex. Furthermore, a series of cluster organic frameworks based on hmt and other well-defined copper-halide clusters, such as Cu2Cl3, Cu3Cl2, Cu4Cl4, Cu5Cl6, and Cu6Cl6, are also presented, which exhibit rich topological nets. These compounds are further investigated by infrared, powder X-ray diffraction, thermogravimetric analysis, infrared and photoluminescent spectroscopy. Photoluminescent studies reveal that these compounds display yellow green emissions based on halide to metal charge transfer. This work reveals a versatile synthetic method to generate a large family of cop...

The possible presence of the high-density carbon polymorph with hexagonal symmetry known as "lonsdaleite" provides an important marker for shock impact events. It is typically considered to form as a metastable phase produced from graphite or other carbonaceous precursors. However, its existence has recently been called into question. Here we collected high-resolution synchrotron X-ray diffraction data for laboratory-shocked and natural impact diamonds that both show evidence for deviations from cubic symmetry, that would be consistent with the appearance of hexagonal stacking sequences. These results show that hexagonality can be achieved by shocking diamond as well as from graphite precursors. The diffraction results are analyzed in terms of a general model that describes intermediate stacking sequences between pure diamond (fully cubic) and "lonsdaleite" (fully hexagonal) phases, with provision made for ordered . vs disordered stacking arrangements. This approach provides a "hexagonality index" that can be used to characterize and distinguish among samples that have experienced different degrees of shock or static high pressure-high temperature treatments. We have also examined the relative energetics of diamond and "lonsdaleite" structures using density functional theoretical (DFT) methods. The results set limits on the conditions under which a transformation between diamond and "lonsdaleite" structures can be achieved. Calculated Raman spectra provide an indicator for the presence of extended hexagonal stacking sequences within natural and laboratory-prepared samples. Our results show that comparable crystallographic structures may be developed by impact-generated shockwaves starting from ambient conditions using either of the two different allotropes of carbon (diamond, graphite). This broadens the scope for its occurrence in terrestrial and planetary systems.

High-resolution transmission electron microscopy (HRTEM) studies of silicon after treatment in a planetary mill have been performed. It is shown that along with the initial phase of silicon, Si-I, the sample also contains some high-pressure phases: Si-III (Kasper phase) and Si-IV (lonsdaleite). We studied the orientation relationship between the particles of different phases, finding that there are, in general, two mechanisms of formation of Si-IV: (1) through the stacking faults formation; (2) through the transformation first to the Kasper phase (Si-III), and then from the Kasper phase to Si-IV. Estimations of temperature and pressure conditions in the planetary ball mill made previously are in accordance with the conditions of formation of the above-mentioned phases.

A new family of three-dimensional hydrocarbon polymers which are more energetically favorable than benzene is proposed. Although structurally these polymers are closely related to well known diamond and lonsdaleite carbon structures, using topological arguments we demonstrate that they have no known structural analogs. Topological considerations also give some indication of possible methods of synthesis. Taking into account their exceptional optical, structural and mechanical properties these polymers might have interesting applications.

The structures of solid phases of water and IV(b) group elements of the periodic table (carbon, silicon, germanium, and tin) are compared. Both water and mentioned elements are characterized by high polymorphism. All mentioned elements and ice have a modification with a diamond structure. For all substances considered, except tin, modifications with the structure of a hexagonal analogue of diamond (lonsdaleite) have been known. The modification with the β-Sn structure has not been known only for ice and carbon. One of the germanium modifications is isostructural to ice III. Tin has a modification with the α-Fe structure. The same structure is observed for ices VII, VIII, and X. Both water and IVb group elements can form clathrate compounds with cavities in the form of Allen’s polyhedra. In water clathrates these cavities contain noble gas atoms (Ar, Kr, Xe) and various molecules. Clathrates of IVb group elements contain metal atoms as well as noble gas atoms.