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Based on tetrahedral lonsdaleite‐like C4 inserted with C(sp2), C(sp1) and both C(sp2)−C(sp1) ultrahard hybrid allotropes ene‐C5, yne‐C6, and ene‐yne‐C7 were generated and identified from quantum density functional theory DFT calculations of ground states. The crystal density and the cohesive energy were found to decrease along the series C4−C5−C6−C7 from the larger openness of the structures due to C insertions; such feature led to decrease the hardness throughout the series. The novel hybrid allotropes were found stable mechanically (elastic constants and their combinations) and dynamically (phonons band structures). From the electronic band structures, the inserted carbons were found to add up rigidly to the diamond‐like bands of C4 while being characterized by small band gap semi‐conductor to metallic‐like behavior. Modified diamonds such as the simple models herein considered exist (ex. nano‐diamond) and their investigations are of growing interest in materials sciences communities with applications as in electrochemistry. [ABSTRACT FROM AUTHOR]

In 1967, a diamond polymorph was reported from hard, diamond-like grains of the Canyon Diablo iron meteorite and named lonsdaleite. This mineral was defined and identified by powder X-ray diffraction (XRD) features that were indexed with a hexagonal unit cell. Since 1967, several natural and synthetic diamond-like materials with XRD data matching lonsdaleite have been reported and the name lonsdaleite was used interchangeably with hexagonal diamond. Its hexagonal structure was speculated to lead to physical properties superior to cubic diamond, and as such has stimulated attempts to synthesize lonsdaleite. Despite numerous reports, several recent studies have provided alternative explanations for the XRD, transmission electron microscopy and Raman data used to identify lonsdaleite. Here, we show that lonsdaleite from the Canyon Diablo diamond-like grains are a nanocomposite material dominated by subnanometre-scale cubic/hexagonal stacking disordered diamond and diaphite domains. These nanostructured elements are intimately intergrown, giving rise to structural features erroneously associated with h diamond. Our data suggest that the diffuse scattering in XRD and the hexagonal features in transmission electron microscopy images reported from various natural and laboratory-prepared samples that were previously used for lonsdaleite identification, in fact arise from cubic/hexagonal stacking disordered diamond and diaphite domains. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Phase change and graphitization of diamonds from the Popigai impact crater (Krasnoyarsk Territory, Siberian platform, Russia) exposed to high-pressure high-temperature (HPHT) conditions of 5.5 GPa and 2000–2200 °C are studied by Raman spectroscopy and X-ray diffractometry (XRD). Light-color diamonds of type 1, free from inclusions, with 0 to 10 % lonsdaleite, are more resistant to HPHT effects than dark diamonds of type 2 rich in lonsdaleite and graphite. The lonsdaleite/diamond ratios in lonsdaleite-bearing impact diamonds become smaller upon annealing, possibly because lonsdaleite transforms to cubic diamond simultaneously with graphitization. Therefore, lonsdaleite is more likely a structure defect in diamond than a separate hexagonal phase. [ABSTRACT FROM AUTHOR]

The paper presents the characteristics of impact diamonds from the Popigai meteorite crater, formed by martensite transformation of graphite-bearing gneisses due to meteorite impacts. These diamonds have a nanopolycrystalline structure and consist of two phases, i.e., cubic (diamond) and hexagonal (lonsdaleite). They are distinguished by phenomenal abrasion resistance, on average twice as high as the abrasion resistance of single-crystalline diamonds. The work considers the possibility of producing similar diamonds by explosion. It is noted that diamonds, which are most similar in structure, are produced through exploding a composition of graphite and hexogen; lonsdaleite is present in the material obtained in this way. However, in any case, diamonds produced by explosion are very small in size, incomparable with natural ones. [ABSTRACT FROM AUTHOR]

Ureilite meteorites are arguably our only large suite of samples from the mantle of a dwarf planet and typically contain greater abundances of diamond than any known rock. Some also contain lonsdaleite, which may be harder than diamond. Here, we use electron microscopy to map the relative distribution of coexisting lonsdaleite, diamond, and graphite in ureilites. These maps show that lonsdaleite tends to occur as polycrystalline grains, sometimes with distinctive fold morphologies, partially replaced by diamond + graphite in rims and cross-cutting veins. These observations provide strong evidence for how the carbon phases formed in ureilites, which, despite much conjecture and seemingly conflicting observations, has not been resolved. We suggest that lonsdaleite formed by pseudomorphic replacement of primary graphite shapes, facilitated by a supercritical C-H-O-S fluid during rapid decompression and cooling. Diamond + graphite formed after lonsdaleite via ongoing reaction with C-H-O-S gas. This graphite > lonsdaleite > diamond + graphite formation process is akin to industrial chemical vapor deposition but operates at higher pressure (~1-100 bar) and provides a pathway toward manufacture of shaped lonsdaleite for industrial application. It also provides a unique model for ureilites that can reconcile all conflicting observations relating to diamond formation. [ABSTRACT FROM AUTHOR]

The novel quaternary thiogermanate Li4CdGe2S7 (tetralithium cadmium digermanium heptasulfide) was discovered from a solid‐state reaction at 750 °C. Single‐crystal X‐ray diffraction data were collected and used to solve and refine the structure. Li4CdGe2S7 is a member of the small, but growing, class of I4–II–IV2–VI7 diamond‐like materials. The compound adopts the Cu5Si2S7 structure type, which is a derivative of lonsdaleite. Crystallizing in the polar space group Cc, Li4CdGe2S7 contains 14 crystallographically unique ions, all residing on general positions. Like all diamond‐like structures, the compound is built of corner‐sharing tetrahedral units that create a relatively dense three‐dimensional assembly. The title compound is the major phase of the reaction product, as evidenced by powder X‐ray diffraction and optical diffuse reflectance spectroscopy. While the compound exhibits a second‐harmonic generation (SHG) response comparable to that of the AgGaS2 (AGS) reference material in the IR region, its laser‐induced damage threshold (LIDT) is over an order of magnitude greater than AGS for λ = 1.064 µm and τ = 30 ps. Bond valence sums, global instability index, minimum bounding ellipsoid (MBE) analysis, and electronic structure calculations using density functional theory (DFT) were used to further evaluate the crystal structure and electronic structure of the compound and provide a comparison with the analogous I2–II–IV–VI4 diamond‐like compound Li2CdGeS4. Li4CdGe2S7 appears to be a better IR nonlinear optical (NLO) candidate than Li2CdGeS4 and one of the most promising contenders to date. The exceptional LIDT is likely due, at least in part, to the wider optical bandgap of ∼3.6 eV. [ABSTRACT FROM AUTHOR]

In this paper, the aspects of studying micron-sized carbon material substances using Raman spectroscopy with laser excitation were discussed. The relevance of the study is determined by the fact that the positions of diagnostic lines in the Raman spectra of carbon materials are significantly affected as the analyzed sample region is heated during the process of spectra recording, thereby resulting in a shift of the diagnostic lines and bands, up to the burnout of the analyzed particle region or to the complete combustion of the sample. To assess the influence of the laser radiation power on the position of diagnostic lines in the Raman spectra of carbon materials, we studied the position of the lines depending on the laser power and sample size of both natural and man-made carbon phases of various structures: highly crystalline graphite, glassy carbon, cubic monocrystalline diamond, hexagonal monocrystalline diamond (lonsdaleite), and ultrananocrystalline diamond. The study was performed by Raman spectroscopy with the use of a high-resolution LabRam HR800 microspectrometer (Horiba, Jobin Yvon). For mono-, nano-, and ultrananocrystalline diamonds, a number of examples were provided to demonstrate that the exciting laser power during Raman spectroscopy measurements of carbon materials must be especially carefully monitored in particles of 10 μm or less in size. For highly crystalline graphite particles, the laser power must be controlled in samples smaller than 4 μm in size. When the Raman spectra were registered during the controlled laser heating, it was found that the samples of a black carbon variety between coal and diamond (described as togorite by V.A. Yezersky V.A. (1986)) had intergrowths of diamond and glassy carbon, a diamond core with a glass-like carbon shell. The results obtained show that the controlled use of the thermal effect of laser radiation can be helpful in identification of the detailed spectroscopic characteristics that occur during the intensive heating of samples, as well as in recovering mineral individuals from aggregates.

By means of ab initio calculations we demonstrate that perturbed hexagonal germanium is a superior material for active optoelectronic devices in the infrared spectral region. Perfect lonsdaleite germanium is a pseudodirect semiconductor, with a direct fundamental band gap but almost vanishing optical transitions. Perturbing the system by replacing a germanium atom with a silicon atom in the primitive cell increases the oscillator strength at the onset gap by orders of magnitude, with a concurrent blue shift of the transition energies. This effect is mainly due to the increased s character of the lowest conduction band because of the perturbation‐induced wave function mixing. A structural distortion of pure lonsdaleite Ge can enhance as well optical oscillator strengths, but their magnitude significantly depends on the specific details of the resulting atomic geometry. In particular, moderate tensile uniaxial strain can induce an order inversion of the two lowest conduction bands, leading to an extremely efficient enhancement of the dipole matrix elements at the minimum gap. In general, chemical and/or structural perturbations are shown to make lonsdaleite germanium a suitable material for light emitting devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In the present work, we report on structure formation of ultradispersed diamond through detonation synthesis. In particular, based on the present data, we can state that ultradispersed diamond of detonation (UDD) synthesis is characterized by a rail substructure formation, which is typical for diamonds of dynamic and static synthesis formed from graphite by the martensitic mechanism. The formation of UDD is characterized by a two-stage process: the first stage is the formation of graphite from detonation products, and the second stage is the transformation of graphite into diamond by the martensitic mechanism. A microlamellar substructure in the rails of diamonds formed from the original graphite is due to the presence of basic packing defects that is characteristic for covalent crystals with a wurtzite lattice deformed at high pressures and temperatures typical for lonsdaleite. The presence of lonsdaleite in UDD also indicates its formation in the process of synthesis of diamond from graphite through the martensitic transformation. [Display omitted] • Structure formation of ultradispersed diamond by detonation (UDD) synthesis is studied. • For UDD, a rail substructure is characteristic. • The formation of UDD is characterized by a two-stage process. • The first stage is the formation of graphite from detonation products. • The second stage is the transformation of graphite into diamond by the martensitic mechanism. [ABSTRACT FROM AUTHOR]

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 Å; the second is compressed graphite with d002 = 3.21 Å. 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. [ABSTRACT FROM AUTHOR]

ABSTRACT During the end of the last glacial period in the Northern Hemisphere near 12.9k cal a BP, deglacial warming of the Bølling-Ållerod interstadial ceased abruptly and the climate returned to glacial conditions for an interval of about 1300 years known as the Younger Dryas stadial. The Younger Dryas Impact Hypothesis proposes that the onset of the Younger Dryas climate reversal, Pleistocene megafaunal extinctions and disappearance of the Clovis paleoindian lithic technology were coeval and caused by continent-wide catastrophic effects of impact/bolide events in North America. While there are no known impact structures dated to the Younger Dryas onset, physical evidence of the impact/bolide events is argued to be present in sediments spanning several continents at stratigraphic levels inferred to date to the Bølling-Ållerod/Younger Dryas boundary (YDB). Reports of nanometer to submicron-sized diamonds in YDB sediments, in particular the rare 2H hexagonal polytype of diamond, lonsdaleite, have been presented as strong evidence for shock processing of crustal materials. We review the available data on diamonds in sediments and provide new data. We find no evidence for lonsdaleite in YDB sediments and find no evidence of a spike in nanodiamond concentration at the YDB layer to support the impact hypothesis. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

External and internal morphologies of natural impact apographitic diamonds (paramorphoses) have been studied. The (0001) surface morphology of the paramorphoses reflects their phase composition and the structural relationship of its constituting phases. Growth and etch figures together with the elements of crystal symmetry of lonsdaleite and diamond are developed on these surfaces. The crystal size of lonsdaleite is up to 100 nm, and that of diamond is up to 300 nm. Two types of structural relations between graphite, lonsdaleite, and diamond in the paramorphoses are observed: the first type (black, black-gray, colorless and yellowish paramorphoses): the (0001) graphite face is parallel to the (100) lonsdaleite face and parallel to (111) diamond; the second type (milky-white paramorphoses): the (0001) graphite is parallel to the (100) lonsdaleite and parallel to the (112) diamond. The first type of the paramorphoses contains lonsdaleite, diamond, graphite or diamond, lonsdaleite, the second type of the paramorphoses contains predominantly diamond. The direct phase transition of graphite → lonsdaleite and/or graphite →diamond occurred in the paramorphoses of the first type. A successive phase transition graphite → lonsdaleite → diamond was observed in the paramorphoses of the second type. The structure of the paramorphoses of this type shows characteristic features of recrystallization. [ABSTRACT FROM AUTHOR]

Hexagonal diamond has been predicted computationally to display extraordinary physical properties including a hardness that exceeds cubic diamond. However, a recent electron microscopy study has shown that so-called hexagonal diamond samples are in fact not discrete materials but faulted and twinned cubic diamond. We now provide a quantitative analysis of cubic and hexagonal stacking in diamond samples by analysing X-ray diffraction data with the DIFFaX software package. The highest fractions of hexagonal stacking in materials previously referred to as hexagonal diamond are below 60%. The remainder of the stacking sequences is cubic. We show that the cubic and hexagonal sequences are interlaced in a complex way and that naturally occurring Lonsdaleite is not a simple physical mixture of cubic and hexagonal diamond. Instead, it is structurally best described as stacking disordered diamond. The future experimental challenge will be to prepare diamond samples beyond 60% hexagonality and towards the so far elusive ‘perfect’ hexagonal diamond. [ABSTRACT FROM AUTHOR]

Micro-Raman spectroscopy and X-ray diffraction method (XRD) were used to characterize impact carbonaceous rocks excavated from the Popigai crater (Siberia). The deconvolution of the first-order Raman spectra of the rocks containing different amounts of carbon phases (diamond, lonsdaleite and graphite) allowed the identification of lonsdaleite spectrum. The most intensive band at 1292-1303 cm−1 was ascribed to A1g vibration mode of lonsdaleite, whereas the less intense band at 1219-1244 cm−1 was attributed, in agreement with previously reported ab initio calculations, to E2g vibration mode. The established correlation between the intensities of Raman and XRD peaks permits a rough estimation of lonsdaleite/diamond phase ratio in the impact rocks using micro-Raman measurements. The second-order Raman spectra of lonsdaleite-diamond rocks were recorded. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Raman spectroscopy has developed into a leading tool for characterizing organic materials based on their rotational and vibrational spectral signatures. This article summarises progress to date in the Raman spectroscopy of important new carbon allotropes and nanomaterials, many of which have been synthesised only in the last two decades. This includes allotropes such as carbynes, graphynes, graphdiynes, graphene, graphite, highly-oriented pyrolytic graphite (HOPG), common fullerenes, carbon nanotubes, carbon onions, diamond, lonsdaleite, nanodiamond, ultra-nanocyrstalline diamond (UNCD), amorphous carbon (a-C), tetrahedral amorphous carbon (ta-C), amorphous diamond (a-D), and diamond-like carbon (DLC). Key spectral features are summarised for each carbon allotrope, where experimental data is available. This work aims to provide a useful "field guide" to assist other researchers in identifying new or unusual pure carbon samples. [Display omitted] • Reviews Progress in the Raman spectroscopy of new sp-, sp2-, sp3- hybridised carbon allotropes. • Raman spectra are related to their carbon lattice vibrational modes. • Noteworthy 1st- and 2nd-order spectral peaks are graphed for the most common allotropes. • Space groups and spectral peaks [nm] are tabulated for frequently-employed excitation sources. • Nanomaterials, amorphous carbons, and Mixed-hybridisation Allotropes are included. [ABSTRACT FROM AUTHOR]

Abstract: Diamond–lonsdaleite–graphite micro-samples collected from peat after the 1908 catastrophic blast in the Tunguska area were studied with scanning (SEM) and transmission electron (TEM) microscopy, NanoSecondary Ion Mass Spectrometry (NanoSIMS) and Х-ray synchrotron technique. The high-pressure carbon allotropes in the Tunguska samples are being described for the first time and contain inclusions of FeS (troilite), Fe–Ni (taenite), γ-Fe and (FeNi)3P (schreibersite). The samples are nodule-like in shape and consist of 99.5% carbon minerals, e.g. diamond>lonsdaleite>graphite. Micro- and nanoinclusions of troilite (up to 0.5vol%), taenite, γ-iron and schreibersite fill cracks, cleavages and pores in the carbon matrix. Carbon isotope studies from the two analyses of the Tunguska foil showed δ 13C=−16.0±1.9‰ and δ 13C=−15.2±2.1‰, suggesting δ 13C=−15.6±2‰ as an average characteristic of the carbon reservoir. That value is close to δ 13C of some extraterrestrial samples. A negligible concentration of Ir and Os in the carbonaceous matrix promotes some controversial interpretation of the origin of the studied materials. Attributing this fact to the primary inhomogeneity, and considering the micro-structural features such as cracks, deformation of the crystal lattices, etc. coupled with high-pressure carbon allotropes association with metals, sulfides and phosphides, and the high ratio of Fe:Ni=22:1 suggest that the studied samples are meteorite micro-remnants. [Copyright &y& Elsevier]

This study reports on the structural relationship between graphite, lonsdaleite, and diamond extracted from the Goalpara ureilite and propose a model for the formation of lonsdaleite and diamond in these stony meteorites. The study is based on data from reflected-light microscopy and laser Raman spectroscopy of a polished thin section (PTS) of the Goalpara ureilite and X-ray powder diffraction (XRPD) analyses of the grains taken out of it. Selected-area electron diffraction (SAED) analyses and high-resolution TEM (HRTEM) observations were carried out in the three unique directions of pristine graphite with two thin slices prepared from a carbon grain directly taken out of a PTS. SAED patterns reveal the relative crystal-axes orientations between graphite (Gr), lonsdaleite (Lo), and diamond (Di) as (001)Gr//(100)Lo//(111)Di, [210]Gr//[001]Lo//[...]Di, and (...)Gr//(...)Lo//(...)Di. The shapes of diffraction spots in the SAED patterns reveal that the transformation of graphite to lonsdaleite and diamond is initiated by sliding of hexagonal carbon planes of graphite along the [210] of the graphite structure. These results suggest that lonsdaleite and diamond in ureilites formed directly from graphite through boat-type buckling and chair-type puckering of hexagonal carbon planes of graphite, respectively. The results of this study confirm the shock origin of diamond in ureilites. [ABSTRACT FROM AUTHOR]

The effect of structural disordering of graphitic materials on their phase transformations into dense modifications of carbon under shock compression conditions ( P = 30 GPa, T = 3000 K) is studied. It is shown that a lower degree of three-dimensional ordering of initial structure (P) first decreases and then increases the overall yield of dense phases, reaching the maximum at P = 0. The content of lonsdaleite simultaneously decreases and that of the dense amorphous phase ( C) increases. The results obtained are attributed to gradual change of the predominant martensite transformation mechanism to predominant diffusion-controlled mechanism, and also to the fact that the metastable phases (lonsdaleite and C) that form at the initial transformation stage partly transform into a stable diamond phase. [ABSTRACT FROM AUTHOR]

Abstract: Micromorphology, internal structure and phase composition of perfect apographitic crystals of impact diamond — paramorphoses were investigated using goniometry, SEM and TEM. X-ray and electron diffraction analyses of the paramorphoses evidence that they are all crystalline and textured. They consist of up to 50vol.% lonsdaleite and diamond each with minor graphite. The paramorphoses mimic the morphology of the original graphite crystals. On the (0001) planes of the paramorphoses striations caused by twinning as well as bulges on (110) planes together with various micro- and nano-features of growth and etching are observed. Twin striations on (0001) planes of the paramorphoses create triangular-shaped ledges. The walls of the ledges correspond to {111} and {1.1.15} planes. The growth and etch figures are strictly oriented along (110). Etch pits with trigonal or hexagonal symmetry are characteristic for diamond whereas rectangular shaped etch pits and growth plates are typical for lonsdaleite. Different types of etch and growth figures allow – at least qualitatively – discrimination between diamond and lonsdaleite. The presence of lonsdaleite and diamond already can be expected from the morphological characteristics observed in SEM images. The orientation of the growth figures and etch pits on the (0001) planes (surfaces) with respect to the hexagonal contours of the paramorphoses suggests that the (0001) planes of the precursor graphite were oriented parallel to (100) planes of lonsdaleite and to (111) planes of diamond. The orientation relationship of the three phases establishing the paramorphoses are confirmed by electron diffraction data from TEM foils cut oriented from the paramorphoses. The internal structure of the paramorphoses indicates that shock compression initiated polysynthetic twinning of the precursor graphite followed by the phase transformation to lonsdaleite and/or diamond. The twinning plane in graphite is (111), and twinning occurs by a rotation of the graphite layers at 20°48′ relative to the basal plane. These twin-related bands in the paramorphoses are predominantly composed of lonsdaleite (0.3–1.0μm wide) and those composed of diamond are 1.5–2.5μm wide. The observed bulges on the (0001) planes of the paramorphoses and the orientation relationship of graphite, lonsdaleite and diamond structures suggest a martensitic transformation of graphite to lonsdaleite and/or diamond. [Copyright &y& Elsevier]

Abstract: Low cost synthesis of the diamond is a dream for the scientists. In this study, different forms of carbon were obtained due to utilizing of different cobalt-based catalytic materials in the graphitization of poly(vinyl alcohol) (PVA) electrospun nanofibers. The utilized cobalt-based catalysts were cobalt acetate tetrahydrate (CoAc), cobalt NPs, and CoAc/palladium NPs. Calcination in argon atmosphere of the polymeric nanofibers containing the aforementioned catalytic materials led to obtain different products. High yield and good nanofibrous morphology were obtained in case of CoAc-containing nanofibers, however cobalt nanoparticles destroyed the nanofibrous morphology and revealed low yield. Interestingly, diamond-like lonsdaleite carbon nanofibers were produced when CoAc–Pd NPs catalytic system was utilized; however normal graphite powder and nanofibers were obtained when Co NPs and CoAc have been used, respectively. The obtained Co-graphite and Co–Pd–lonsdaleite nanofibers could be also synthesized in the form of thin films supported on graphite disks when polyacrylonitrile disks were used as collectors. Calcination of the obtained disks with the attached electrospun nanofibers produced well affixed Co-graphite and Co–Pd–lonsdaleite nanofibers on graphite disks. Electrical properties study of the obtained films indicated that the Co–Pd–lonsdaleite nanofibers behave as semiconductor while Co-graphite nanofibers show ohmic conductivity. Magnetically, Pd has a negative impact on the magnetic properties of the cobalt. [Copyright &y& Elsevier]

Shock compression-induced phase transformations of graphite into carbon dense modifications in cylindrical recovery containers ( p = 20-36 GPa, T = 1800-3500 K) have been studied. The dependences of the lonsdaleite and diamond yields on the compression conditions have been established. The results obtained have been analyzed taking into account the formation mechanisms of the dense phases and special features of their shock-wave synthesis. [ABSTRACT FROM AUTHOR]

Based on first-principles calculations, we demonstrate that lonsdaleite exhibits many excellent static mechanical properties. For instance, the compressive strength, coefficient of stiffness and Young’s modulus of lonsdaleite all exceed the corresponding values for diamond. Moreover, the bulk modulus of lonsdaleite is as good as that of diamond, and its tensile strength is similar to that of diamond. Therefore, our results suggest that lonsdaleite is a stronger and stiffer naturally occurring substance than diamond. [ABSTRACT FROM AUTHOR]

Abstract: Soft X-ray emission and absorption spectroscopy methods were used in the present work to study experimentally the electronic structure of the dense amorphous carbon phase synthesized in a mixture with diamond as a result of shock compression of carbon black. The X-ray emission C Kα bands, representing the energy distribution of the C 2p-like states, have been recorded for the amorphous carbon phase and, for comparison, for diamond, lonsdaleite, graphite, and carbon black. For the amorphous carbon phase, diamond and lonsdaleite the spectra of quantum yields of the X-ray photoeffect in the area of the C K absorption edge have been recorded as well. Total and partial densities of states of the carbon H-6 and bct-4 phases, diamond and lonsdaleite have been calculated using the first-principles self-consistent full potential linearized augmented plane wave (FP-LAPW) method. Comparison on a common energy scale of the X-ray emission C Kα bands and C K quantum yields of the dense amorphous carbon phase as well as the theoretical curves of partial C 2p-like states of the carbon H-6 and bct-4 phases reveals that, the electronic structure of the amorphous carbon phase under consideration is well described by that of the carbon bct-4 phase. The experimental X-ray emission and absorption spectroscopy data indicate that the amorphous carbon phase studied is metallic, being in agreement with the results of the theoretical FP-LAPW calculations. [Copyright &y& Elsevier]

Abstract: The energy-loss near-edge structure (ELNES) spectra of several carbon allotropes (non-hydrogenated and hydrogenated face-centered cubic (FCC) carbon, rhombohedral carbon, glitter, hexagonite and lonsdaleite) are calculated within the supercell-core-excited density functional theory approach. In particular an experimental ELNES spectrum of ‘new diamond’ (‘n-diamond’) [Konyashin et al., Diamond Relat. Mater. 10, (2001) 99–102] is compared with the ELNES spectra of FCC carbon, rhombohedral carbon and the so-called glitter structure. Our calculations show that the ELNES spectrum considered in that publication cannot be that of FCC carbon. [Copyright &y& Elsevier]

Abstract: A UV laser has now been used to measure the Raman spectrum of lonsdaleite. This mineral species is a little-known hexagonal form of carbon having no known P–T field of stability. Lonsdaleite is known to coexist with diamond and/or graphite in certain impact structures and meteorites. Its presence in microinclusions in some ultrahigh-pressure eclogites is under discussion as there is a considerable wavenumber overlap of the sp 3 Raman band of lonsdaleite in the 1200–1400cm−1 region with certain bands of haematite, graphite and diamond, and also with “disordered-diamond” having a downshifted wavenumber. Various incoherent previously published values of the Raman bands are briefly reviewed and an attempt is made to establish a reference spectrum. Four samples of lonsdaleite from the Zapadnaya and Popigai impact structures (Ukraine) were measured with three different laser sources (488, 514.5 and 325nm) with two Raman spectrometers. UV-Raman was less fluorescent. All the new data were coherent in establishing an sp 3 band centred at 1324±4cm−1 with a FWHM about five times wider than that of diamond and an intensity about 500 times weaker. The presence of a second band giving a weak shoulder around 1225cm−1 is discussed with respect to the alternative of one continuous asymmetrical band. [Copyright &y& Elsevier]

Abstract: The observation of a 10-fold increase of tool wear during the high-speed cutting of ductile Cu–Pb alloys for journal bearings was the motivation of an in-depth analysis of the alloy and PCD-tools used in the corresponding industrial process. The high wear rate was found in the softer alloy batches and could be traced to the amount of skinpass reduction, which, while increasing hardness, diminishes toughness and the corresponding cutting energy, all other parameters being equal. At low wear rates, wear occurred by removal of small diamond particles by a brittle process. At high wear rates, clear evidence of plastic deformation was found. Parallel, needle-like inclusions were observed inside the diamond single grains, which may either represent deformation twins or martensitically formed lonsdaleite (hexagonal diamond). A review of recent literature and a qualitative analysis of the thermodynamics and mechanics of polymorphism in diamond confirm the possibility of such transformation and provide arguments against the occurrence of graphitisation as a wear mechanism under the present conditions. [Copyright &y& Elsevier]

Abstract: Characterization tools such as confocal Raman micro-spectroscopy, Laser Ablation (LA-ICP-TOF-MS) and SEM-EDS were used to characterize meteorites: primitive achondrite — not classified NWA XXX ureilite found in 2006 in Morocco and the graphite nodula from the Canyon Diablo iron meteorite. The presence of diamond was confirmed in both samples. There are two kinds of meteoritic diamonds: diamonds of the sizes of microns up to millimeters are most probably of impact origin, nanodiamonds of the sizes of 1–3 nm, called presolar diamonds because of the isotopic anomalies, are believed to be formed before our Solar System was formed. There are many theories concerning presolar diamonds formation, among them: impact shock metamorphism driven by supernovae or chemical vapor deposition (CVD) from stellar outflows. We examined the properties of diamond nanopowders obtained by the PA CVD and detonation methods. Nanodiamonds obtained by the detonation method, called ultradispersed detonation diamonds (UDD), are of the same range of sizes as presolar diamonds. The results show both differences and similarities among meteoritic, terrestrial and laboratory diamonds. The comparison will help to understand the processes during presolar nanodiamonds formation. [Copyright &y& Elsevier]

In this paper, the growth of Lonsdaleite diamond using hot-filament chemical vapor deposition (HFCVD) on flashed and reconstructed Si (100) is reported. Surface morphology studies using scanning electron microscopy (SEM) show that the film is composed of decahedron and icosahedron diamond particles. The X-ray diffraction (XRD) pattern has a strongest peak at 47° and a peak at 41°, which is indicative of Lonsdaleite nature of the grown diamond film. The Raman spectrum of the film shows a broadened diamond peak at wave number of 1,329 cm−1, which has shifted towards the peak position corresponding to Lonsdaleite nature of the diamond (1,326 cm−1). [ABSTRACT FROM AUTHOR]

A natural shockwave event led to the formation of a new crystalline polymorph of carbon in gneisses from the Popigai crater, Russia. The new species occupies the interior of a multiphase assemblage and is entirely enveloped by lonsdaleite and graphite. Polishing hardness of this new phase is greater than that of lonsdaleite. Micro-beam synchrotron X-ray diffraction, imaging and fluorescence studies revealed a pure transparent carbon phase. The diffraction pattern is indexed in terms of a cubic cell (a=14.697 A˚, space group Pm3m.). This species was neither encountered in static or dynamic high-pressure experiment nor predicted by theoretical calculations. To cite this article: A. El Goresy et al., C. R. Geoscience 335 (2003). [Copyright &y& Elsevier]