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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.

Recent indentation experiments indicate that wurtzite BN (w-BN) exhibits surprisingly high hardness that rivals that of diamond. Here we unveil a novel two-stage shear deformation mechanism responsible for this unexpected result. We show by first-principles calculations that large normal compressive pressures under indenters can compel w-BN into a stronger structure through a volume-conserving bond-flipping structural phase transformation during indentation which produces significant enhancement in its strength, propelling it above diamond's. We further demonstrate that the same mechanism also works in lonsdaleite (hexagonal diamond) and produces superior indentation strength that is 58% higher than the corresponding value of diamond, setting a new record.

The diamond, lonsdaleite and sodalite structures are generalized to N dimensions and described. Ring counts and coordination sequences for the N + 1-connected nets are given for N ≤ 6. Simple analytic expressions are given for coordination sequences for diamonds, sodalites and the primitive hypercubic lattice. Replacing the vertices of diamonds and sodalites by regular simplices produces rare (open) stable sphere packings; general expressions for the density of these are given.

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 1200-1400 cm(-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 325 nm) with two Raman spectrometers. UV-Raman was less fluorescent. All the new data were coherent in establishing an sp(3) band centred at 1324+/-4 cm(-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 1225 cm(-1) is discussed with respect to the alternative of one continuous asymmetrical band.

13C-graphite treated in a diamond anvil high-pressure cell was studied by high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and electron energy loss spectroscopy. It was found that 13C-diamond and lonsdaleite originate after the treatment under high pressure and shear deformation.

A series of diamond polytype structures are described and their IR and Raman vibrational modes predicted. The diamond polytypes are analogous to the well-known silicon carbide polytypes. The intermediate 6H diamond polytype was recently identified by single crystal electron diffraction of vapor precipitated diamond powder. In addition, end member polytypes of 3C (cubic diamond) and 2H diamond (hexagonal lonsdaleite) have been previously established, and polytypes such as 4H, 8H, 15R, and 21R diamond are predicted, but may be difficult to isolate and identify. The various diamond polytype structures differ only in the stacking sequences of identical puckered hexagonal carbon layers. These identical carbon layers lie parallel to the cubic 3C {111} and the hexagonal 2H {001} planes. A new method for uniquely labeling the structural layers in the polytype stacking sequences is presented. Factor group analysis was used to determine the IR and Raman selection rules for five diamond polytypes with structures intermediate between those of end members diamond and lonsdaleite. Brillouin zone folding techniques were used to determine band positions, in analogy with analyses of SiC polytypes discussed in the literature. The results predict that (i) all diamond polytypes are Raman active, (ii) limiting polytypes 3C and 2H are not IR active, and (iii) polytypes 4H, 6H, 8H, 15R, and 21R have IR active modes.

Five precursors covering the whole range of carbon structural organization, i.e. a quasi-amorphous soot (QAS), a raw carbon black (CB), a carbon black heat-treated at 2600 °C (HTCB), a polycrystalline graphite (PCG) and a highly oriented pyrolytic-graphite (HOPG) were run at 15 GPa in the 1500–1900 °C range between 15 and 60 min. Full transformation into nano-diamonds was not always achieved and the corresponding run products preserved the tracks of the transformation mechanisms which led to diamond formation. These mechanisms and their kinetics were characterized combining X-ray powder diffraction, Raman micro-spectroscopy and high-resolution TEM. Globally, the disordered precursors react faster than the crystalline ones: they achieve higher transformation rates and become transparent more easily. For the spherical CB particles, nano-diamond preferentially nucleates in their centre. The graphitic layers in the QAS directly transform into diamond without any prior graphitization. The crystalline organization is even found to decrease for the graphitized precursors (HTCB and PCG) as evidenced by HRTEM images showing graphite delamination. These precursors mostly evolve according to a diffusion-limited reconstructive mechanism which initiates at the precursor structural defects. HOPG behaves differently since it mostly transforms into lonsdaleite rather than cubic diamond.

A novel Genetic Algorithm is described that is suitable for determining the global minimum energy configurations of crystal structures and which can also be used as a polymorph search technique. This algorithm requires no prior assumptions about unit cell size, shape or symmetry, nor about the ionic configuration within the unit cell. This therefore enables true ab initio crystal structure and polymorph prediction. Our new algorithm uses a real-space representation of the population members, and makes use of a novel periodic cut for the crossover operation. Results on large Lennard-Jones systems with FCC- and HCP-commensurate cells show robust convergence to the bulk structure from a random initial assignment and an ability to successfully discriminate between competing low enthalpy configurations. Results from an ab initio carbon polymorph search show the spontaneous emergence of both Lonsdaleite and graphite like structures., Comment: Accepted for publication in Phys. Rev. B. 6 pages, 8 figures, 1 table

A combination of micro-ionoluminescence (micro-IL) and micro-PIXE was used to characterize diamond grains inside a type of meteorites known as ureilites. Ureilites are a group of achondrites unique in containing relatively large amounts of carbon occurring as diamond, graphite or lonsdaleite. A shock origin for ureilitic diamonds has been widely accepted though an exact knowledge of the conditions during high-pressure graphite conversion is not yet achieved. Micro-IL is a very powerful technique for material investigation and particularly for diamond analysis. Using this technique we were able to identify the occurrence of the diamond phase inside carbon meteoritic inclusions and to perform micro-PIXE analysis on single diamond grains. In fact, IL in low nitrogen content diamonds is dominated by A-band emission (centered at about 2.9 eV) and so, considering only IL monochromatic map at such a spectral band, it was possible to identify them. By making measurements directly on the meteorites, contamination during chemical extraction processes was avoided and it was possible to study not only the diamond phase, but also its distribution inside carbon inclusions.

The most common naturally occurring crystalline forms of carbon are cubic diamond and a-graphite. Graphite is one of the softest known materials whereas diamond is the hardest. Although these are the most common they are by no means the only crystalline forms of carbon. There are at least 5 others which are rarely found in nature but can be produced in the laboratory. These are s-graphite, lonsdaleite, chaoite, carbon (IV) and most recently fullerenes. Such a wide array of available materials and contrasting properties arises from the versatility of the atomic structure of the carbon atom. It has 6 electrons in total. Two of these lie in the ls core and hence do not contribute to bonding. The other 4 are in the 2s and 2p configuration and can bond in three distinct ways to their neighboring atoms to form a sp3, sp2 or sp1 bond. See figure 1.

We studied the effect of postgrowth low energy (1.5 keV) Ar+ ion irradiation of various amorphous carbon (a-C) films with different microstructure and bonding. Detailed x-ray diffraction was used to identify the existing phases in thin (30-200 nm) a-C films, deposited on Si and subjected to Ar+ ion irradiation and to study the mechanisms of formation of diamond, lonsdaleite (diamond-2H), silicon carbide, and graphite grains. The relative content of each crystalline phase, the grain size, and distribution depend strongly on the microstructure of the as-grown films that is affected by the bias voltage applied during deposition of the films. Ion bombardment promoted the formation of graphite throughout the films and diamond and SiC grains close to the a-C/Si interface. Diamond and SIC nucleation at the a-C/Si interface area occurs beyond the penetration depth of the Ar+ ions and therefore is not attributed to a direct, ballistic ion-solid interaction but rather to a mechanism of charge transfer through conductivity channels. The latter is also considered and discussed in terms of other features of the as-grown a-C films such as density, voids content, electrical resistivity, and surface and interface morphology. (C) 2000 American Institute of Physics. [S0021-8979(00)06224-1]. Journal of Applied Physics

Virtually all the allotropic forms of carbon known naturally have been at some time or another recognised within meteorites or as a part of phenomena associated with meteorite impacts. Thus, three types of graphite the well known cubic and rhombohedral, plus a unique cubic form (cliftonite) have been encountered. There are several reported occurrences of buckminster-fullerene and its siblings. Diamonds are amongst the most interesting form of carbon in meteorites, their incidence having been noted for as long as the mineral was known to be found on Earth in kimberlites. Both the cubic and hexagonal (Lonsdaleite) forms are seen, the latter almost always denoting the advent of shock conditions. A particularly unusual type of crystal, which has been found to be ubiquitous in primitive unmetamorphosed chondrites, consists of nanometre sized grains. Originally the acid resistant residues which have transpired to be diamond were first thought to be the sp carbon allotrope generically known as carbyne. This identification proved to be highly controversial, but chaoite, one of the initial carbyne candidates, was definitely located in the impact breccia suevite found at the Ries Crater, S. Germany. Tiny crystals of chaoite seem to be elusive grains coexisting with meteorite nanodiamonds where they give EELS data which demonstrate they are different from other types of carbon. Molecules with multiple triple bonds as the main structural unit are common in the interstellar medium.

The effects of shock waves are evident in such common occurrences as the thunderclap following a lightening strike or the craters visible on the Moon [1]. Shock waves can result from or cause various physical and chemical processes. For example, shock waves can cause chemical reactions which subsequently couple with the shock wave to generate an explosive self-sustaining detonation (see, for example, Ref. 2). The ability of chemical reactions to couple with shock waves has direct implications on the strength and sensitivity of explosives and, therefore, their usefulness and safety in industrial and defense applications. Likewise, shock waves can also cause physical changes such as a transition to a high-pressure phase in a material [3]. This polymorphic phase transition can couple with the shock to produce a split shock wave profile [4]. These phase changes might place the material in a state that remains stable upon the release of pressure, such as the transition of graphite to diamond (lonsdaleite) [5]. Shock waves can therefore transform a material into a state that has more desirable properties or economic value and at the same time produce interesting phenomena such as shock wave splitting.

This chapter explains the atomic and crystal structures of diamond. An inclusive understanding of diamond nucleation and growth processes during chemical vapor deposition (CVD) necessitates knowledge of atomic and crystal structures of diamond, as well as of the competing crystallites or amorphous phases that may be produced by CVD. Three hybrid carbon orbitals available for bonding—sp, sp 2 , and sp 3 —complete the series of electronic building blocks of all carbon allotropes and compounds. Diamond has two basic crystal structures, one with a cubic symmetry (more common and stable) and the other with a hexagonal symmetry (rare but well established, found in nature as the mineral lonsdaleite). The cubic structure is the dominant crystal structure in both natural and synthetic diamond since the staggered conformation is more stable than the eclipsed due to the slightly lower energy. Diamond polytypes and carbyne phases form only during the homogeneous nucleation and growth of diamond powder.

Shock wave synthesis of diamond was an unexpected result of experiments designed to explore the effects of shock waves on a variety of materials. The initial announcement in 1959 was controversial; shock synthesis of diamond had been shown to be unlikely, on the basis of kinetic arguments. Jamieson confirmed the identification and suggested a diffusionless mechanism, c-axis compression of rhombohedral graphite. Subsequent work has provided strong evidence that shock wave synthesis of cubic diamond is a conventional thermally activated nucleation and growth process. Thermal inhomogeneities provide the requisite high temperatures; quenching via thermal equilibration is implicit in the process. Shock synthesis of adamantine BN phases appears to be quasi-martensitic; a martensitic mechanism may partially account for the Lonsdaleite (hexagonal diamond) observed in some meteorites and in some artificial shock products. Diamond is also formed as a detonation product in oxygen-deficient explosives. The polycrystalline product of shock synthesis is similar to natural carbonado. The association of carbonado with an ancient giant impact crater is noted.

Attempts were made to produce bands of hexagonal diamond (Lonsdaleite) by high-temperature indentation of cubic diamond. (011) wafers of type I diamond were indented over the temperature range 1000–1300°C and the microstructure of the indentation plastic zone investigated by transmission electron microscopy (TEM). No hexagonal diamond was produced; instead, the plastic zone consisted of arrays of dislocations lying on the {001} planes. A possible mechanism for the generation of such dislocations, and reasons for the absence of hexagonal diamond, are discussed.

The disordered tetrahedral carbon films have been grown by ion beam deposition at ∼120 eV. An absorption maximum 6.15-5.80 eV (E 0 B ) and a threshold 4.8-4.5 eV were resolved and tentatively attributed to Γ 5 + - Γ 5 - and Γ 5 + - K2 transitions in lonsdaleite. The cubic sp3-carbon manifested as maxima at 5.45-5.50 eV (Γ′25 - Δx) and 7.0 eV (Γ′25 - Γ15). Urbach tails of different steepness were resolved. A 6.508 eV maximum of 0.52 eV half-width at 90K was resolved. The spectroscopic data on absorpsion and luminescence of sp3-bonded carbon films of different allotropies are discussed.

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

A new 3D coordination polymer was solvothermally synthesized. The coordination polymer possesses a lon network built from 4-connecting Co2 clusters and tetracarboxylate ligands, and exhibits selective gas sorption behavior as well as antiferromagnetic interactions.

A rare 4-connected neb framework has been constructed using a multi-carboxylic flexible ligand. It represents a unique 66 topology, which is evidently distinct from diamond and lonsdaleite topologies. In addition, the complex displays intriguing structural features of perpendicular right- and left-handed helical chains, as well as ferromagnetic interactions. Furthermore, the ferroelectric property is preliminarily discussed.

In a previous paper, Zallen [Phys. Rev. 173, 824 (1968)] reported a group-theoretical analysis of the competition between unit-cell complexity and crystal symmetry in determining the presence or absence of infrared-active phonons in an elemental crystal. Here we correct an error in that paper's treatment of certain hexagonal space groups. Our results modify the minimum-complexity condition for infrared activity: For 228 of the 230 space groups, a necessary and sufficient condition for the existence of symmetry-allowed infrared-active modes in an elemental crystal is the presence of three or more atoms in the primitive unit cell. The two exceptional space groups are P6/mmm (${\mathit{D}}_{6\mathit{h}}^{1}$) and P${6}_{3}$/mmc (${\mathit{D}}_{6\mathit{h}}^{4}$); for each of these symmetries, there exists one structure with four atoms per cell and no infrared modes. The P${6}_{3}$/mmc structure includes, as special cases, Lonsdaleite (or ``wurtzite silicon'') as well as a c-axis-aligned hcp arrangement of diatomic molecules which is relevant to models of solid molecular hydrogen at high pressure.

Information on orbital hybridization is very important to understand the structural, physical, and chemical properties of a material. Results of a comparative first-principles study on the behaviours of orbital hybridization in the two-dimensional single-element phases by carbon, silicon, and germanium are presented. From the well-known three-dimensional hexagonal lonsdaleite structure, in which the atoms are in ideal sp(3)-bonding, the layer spacing along c-axis is gradually stretched to simulate the evolutions of structural and electronic properties from three-dimensional to two-dimensional lattice configurations in the three materials. A turning point of the total system energy due to the sp(3) to sp(2) transition is observed during this process in carbon. In contrast, no such phenomenon is found in silicon and germanium. The differences in electronic structure and bonding behaviour are further examined through comparative investigation of atomic angular-momentum projected density of states and electronic energy band spectrums of these materials. We demonstrate that the valence electronic orbital in the two-dimensional hexagonal crystals of Si and Ge shows sp(3)-like behaviour for the partial hybridization of s and p(z), which leads to their different lattice configurations to graphene. The role of π-bonds in stabilizing the flat configuration of graphene is also discussed.

Single-crystal X-ray analysis revealed that isomorphous metal formates [M6(HCOO)12]·DEF (M = Mg2+, Fe2+, Co2+, Zn2+) have a framework with the lonsdaleite net, which is one of the rare topologies in metal–organic frameworks; iron and cobalt formates with metal sites bridged by single atoms showed dominant ferromagnetic and antiferromagnetic interactions, respectively.

Electrodeposition of diamondlike carbon films through oxidation of acetylides prepared both in situ and ex situ dissolved in dimethylsulfoxide DMSO was carried out at room temperature by potentiodynamic, potentiostatic, and galvanostatic methods onto gold substrates. Diamondlike carbon films and nanoinclusions of diamond, graphite, and lonsdaleite phases were prepared from acetylene-saturated liquid ammonia at a temperature of 33°C by potentiodynamic, potentiostatic, galvanostatic, and pulsed deposition methods. The films were characterized in situ by electrochemical methods and ex situ by Raman spectroscopy, optical microscopy, transmission electron microscopy, profilometry, and electron diffraction. Films were successfully deposited by both methods. The fastest growth rate was achieved with acetylides prepared ex situ and oxidized in the presence of NaBH4 in DMSO solution. Film thickness was found to increase either by processing at higher values of potential or for longer deposition times. Films of the highest quality and greatest uniformity were prepared by pulsed deposition from acetylene-saturated liquid ammonia. Nanodiamond and lonsdaleite inclusions were obtained by potentiostatic deposition from the same medium. Diamond has many properties that make it attractive as an engineering material. The physical properties of diamond include extreme mechanical hardness ca. 90 GPa, high wear resistance, high bulk modulus 1.2 10 12 Nm 2 , low compressibility 8.3 10 13 m 2 N 1 , high room-temperature thermal conductivity 2 10 3 Wm 1 K 1 , very low thermal expansion coefficient at room temperature 1 10 6 K 1 , large range of optical transparency deep ultraviolet UV to far infrared IR, and high sound propagation velocity 17.5 km s 1 . Very resistant to chemical corrosion and biologically compatible, diamond surfaces can possess very low or even “negative” electron affinity. With a bandgap energy of 5.4 eV, diamond is a very good electrical insulator. 1

Using the local-density-theory and the linear-muffin-tin-orbitals method, we calculate the electron band structures of hexagonal (lonsdaleite) and cubic diamond. Even though the arrangement of atoms is very similar between the two crystal structures, we find significant differences in the electron bands, especially in the conduction bands. In particular, including estimated corrections on top of the local-density results, we find the lowest theoretical gap of hexagonal diamond to be 4.5 eV, i.e., a remarkable 1.1-eV drop as compared to that of cubic diamond. The lowest gap in the hexagonal structure is still indirect as in the cubic structure, but the gap is now from \ensuremath{\Gamma} to K. The reduction of the band gap should be observable in optical-absorption or reflectivity experiments.

Current issues and problems in the chemical vapor deposition (CVD) of diamond are those which relate to its characterization, its nucleation on foreign surfaces, the question of its formation in preference to the other phases of solid carbon (for example, graphite, chaoite, or lonsdaleite), why different morphologies and crystallographic orientations (textures) are seen in different experiments or with different parameters in the same experiment, and finally whether well-crystallized metastable phases can be obtained by CVD in other material systems or are only a peculiarity of carbon chemistry. Whether a given carbon coating is justly described as diamond has been such an issue, and coatings should clearly show evidence for diamond by x-ray diffraction and Raman spectroscopy before the claim of diamond is made. Experimental results have not been consistent in many cases, and much work remains to be done before an accurate assessment can be made of the technological impact of the development.

Thin layers of tetrahedral carbon phases (diamond, lonsdaleite etc.) were crystallized under reduced pressure (10 -3 / Pa) using a coaxial plasma generator. On the basis of comparison of known diamond crystallization processes under reduced pressure it was assumed that the electric charge plays an important part in the nucleation process.

As the technology for diamond film preparation by plasma-assisted CVD and related procedures has advanced, Raman spectroscopy has emerged as one of the principal characterization tools for diamond materials. Cubic diamond has a single Raman-active first order phonon mode at the center of the Brillouin zone. The presence of sharp Raman lines allows cubic diamond to be recognized against a background of graphitic carbon and also to characterize the graphitic carbon. Small shifts in the band wavenumber have been related to the stress state of deposited films. The effect is most noticeable in diamond films deposited on hard substrates such as alumina or carbides. The Raman line width varies with mode of preparation of the diamond and has been related to degree of structural order. The Raman spectrum of hexagonal diamond (lonsdaleite) is distinct from that of the cubic diamond and allows it to be recognized.

Two earlier papers by Teo and the author [J. Chem. Phys. 83, 6520 (1985); Inorg. Chem. 25, 2315 (1986)] studied circular and spherical clusters in the simplest close‐packed structures in two and three dimensions. The present work considers clusters in other fundamental structures (the hexagonal net, diamond), and applies the results to study clusters in related structures (Lonsdaleite, graphite) and in binary arrays with the structure of the idealized ionic crystals NaCl, CsCl, ZnS, CaF2, TiO2, O3Bi2.

1. Experiments were conducted on shock compression (with conservation of material) of the carboniferous chondrite Migei at p=500 kbar and of the graphite from the Yardymlinskii iron meteorite at p=1000 kbar with the goal of explaining the transformations of meteoritic material due to strong shock waves. 2. Diamonds were synthesized from the carbonaceous matter in the carboniferous chondrite Migei (p=500 kbar) and from the graphite in the Yardymlinskii iron meteorite (p-1000 kbar) as microcrystals (

Many previous studies of vitreous carbon by x-ray diffraction show broad diffraction maxima typical of vitreous materials. However, electron diffraction studies reveal many regions that give single crystal patterns. Although some of the patterns are of graphite, as expected, many patterns were obtained that correspond to other carbon crystal forms such as chaoite and carbon VI. In addition, ring patterns of lonsdaleite and diamond were found. Finally, patterns were obtained that do not correspond to any of the known carbon allotropes. (Author)

Ureilites are a rare group of achondrites. They are composed mainly of olivine and pigeonite in a matrix of carbonaceous material, including graphite, lonsdaleite, diamond, and metal. In most respects Kenna is a typical ureilite with the requisite mineralogical and chemical properties of the group. Differences of the Kenna ureilite from previously studied ureilites are related to a greater density, the occurrence of exceedingly minute quantities of feldspar, and a very strong elongation lineation of the silicate minerals. A description is presented of a study which indicates a complex history for Kenna, including igneous, mild metamorphic, and shock processes.

Abundances of carbon and sulfur in the Kenna ureilite are 2.219 plus or minus 0.060 wt. % C and 0.179 plus or minus 0.008 wt. % S. Secondary carbonates resulting from terrestrial weathering account for 0.25 plus or minus 0.02 wt. % C. No hydrocarbons were detected during gas release measurements. Most of the carbon is in graphite, diamond, or lonsdaleite. The sample of Kenna contained 0.95 plus or minus 0.05 wt.% H2O. Total carbon and sulfur measurements were made on three additional ureilites: Havero, Dingo Pup Donga, and North Haig. Ureilite carbon abundances are similar to those of C-2 chondrites, whereas sulfur abundances are a factor of 10 less than C-2 chondrites and ordinary chondrites. The elemental abundances, ratios, and phases present in the ureilites rule out a direct genetic relationship between the ureilites and the carbonaceous chondrites.

A new crystalline form of carbon—hexagonal diamond—has been synthesized in the laboratory under conditions of static pressure exceeding about 130 kbar and temperature greater than about 1000°C. It is necessary to start with well‐crystallized graphite in which the c axes of the crystallites are parallel to each other and to the direction of compression. There is electrical evidence that the transformation starts at room temperature but hexagonal diamond is not retrieved unless a setting temperature exceeding about 1000°C is applied. The electrical and crystal characteristics have been studied. The crystal structure is hexagonal with a=2.52 A and c=4.12 A. The theoretical density is 3.51+g/cm3, same as cubic diamond. It has also been prepared recently in another laboratory from crystalline graphite by a method involving intense shock compression and strong thermal quenching. More recently it has been discovered to be present to the extent of over 30% in the Canyon Diablo meteorite diamonds.

Based on its mineral composition and structure the Havero meteorite, which fell on August 2, 1971, in Finland, is a ureilite, the sixth meteorite in this rare group. Elementary carbon in this meteorite is represented by diamond, lonsdaleite, graphite and chaoite microcrystals in the intergrowths. The presence of lonsdaleite and chaoite indicates that diamonds were formed in the Havero meteorite due to the action of strong shock waves

Review of the mineralogy and texture of the Haveroe and of some of the six other known ureilites, and assessment of the implications of their observed characteristics. In common with other ureilites, Haveroe shows mineralogical and textural evidence of preterrestrial shock. The olivines and metal grains have been recrystallized, the fabric includes angular voids, and graphite-rich areas are present that have been partially converted to diamonds with some indication of lonsdaleite. On the basis of such features, a subdivision of ureilites into particular types is proposed.

Of the many meteorites recovered so far from the Allan Hills, Antarctica, only nine have been irons. One of these, ALHA77283, contains troilite(FeS)–graphite(C)–schreibersite((Fe,Ni)3P)–cohenite(Fe3C) inclusions rich in the carbonado-type diamond–lonsdaleite ‘nodules’ previously described from the Canyon Diablo meteorite1,2. The Canyon Diablo, Arizona, meteorite, the excavator of Meteor Crater, is the only other iron meteorite known to contain these high-pressure minerals, and their occurrence in that meteorite has been explained as the result of shock-induced transformation of graphite, most probably at the moment of terrestrial impact and disintegration of the projectile during crater formation3–6. The suggestion7 that formation was by high gravitational pressure has not been accepted. Virtually identical diamond–lonsdaleite-containing material in ALHA77283 occurs in a meteorite specimen with a well developed heat-altered zone produced by atmospheric ablation. It seems, therefore, that the diamond and lonsdaleite were present in the meteoroid before its final ablative passage through the atmosphere and soft landing on the ground. The shock event that produced these high pressure phases, therefore, must have taken place on its parent body or have been associated with the disruption of that body. Here we present the metallographic and X-ray diffraction data on which this conclusion is based.

A new polymorph of carbon, hexagonal diamond, has been discovered in the Canyon Diablo and Goalpara meteorites. This phase had been synthesized recently under specific high-pressure conditions in the laboratory. Our results: provide strong evidence that diamonds found in these meteorites were produced by intense shock pressures acting on crystalline graphite inclusions present within the meteorite before impact, rather than by disintegration of larger, statically grown diamonds, as some theories propose.

TWO well known atomic arrangements found among tetrahedrally co-ordinated AX compounds such as silicon carbide (SiC) and zinc sulphide (ZnS) are those based on a cubic close-packed array or on a hexagonal close-packed array of tetrahedra. The two polymorphs are generally designated as the sphalerite and wurtzite types, respectively, from their occurrence in zinc sulphide. The crystal structures of the Group IV elements, carbon, silicon, germanium and α-tin, are based on the cubic close-packed sphalerite arrangement (aside from graphite, tetragonal tin and tetragonal germanium) with all A and X sites occupied by the same kind of atom. The existence of polymorphs analogous to wurtzite among these elements is not surprising, because the two types of structure are so similar geometrically that the energy differences between them must be small. A wurtzite-like polymorph of silicon has already been synthesized1. For carbon a rhombohedral polymorph called beta-diamond, which corresponds to the 3R polytype of wurtzite, has been reported as birefringent lamellae in terrestrial diamonds2,3. The synthesis of a hexagonal wurtzite polymorph of diamond by shock conversion of graphite has been disclosed in a patent application4. A substance earlier called delta-carbon, synthesized at the General Electric Company, has been characterized by Bundy5 as the wurtzite-like polymorph of carbon under the name hexagonal diamond. While the present work was in progress Hanneman et al.6 put forward X-ray evidence for the occurrence of this polymorph in meteorites.

The recent interest in the growth of thin diamond films has led us to consider the differences between the hexagonal (lonsdaleite) and cubic structures. Both phases have very similar properties, and empirical and theoretical considerations indicate that their structural energies are nearly identical. When thin films are grown the hexagonal phase may compete with the cubic phase, making characterization of the film difficult. Cubic diamond has one Raman active mode, while hexagonal diamond has three. The opportunity thus exists for Raman spectroscopy to differentiate between the two tetrahedrally bonded phases (as well as "amorphous" or graphitic phases). Electronic structure calculations can be used to obtain theoretical Q=0 frequencies of the Raman active modes in both structures. We have used the first principles Linearized Augmented Plane Wave method within the local density approximation to calculate the zone center phonon frequencies. The calculated frequency of the cubic diamond Raman mode is 1336 cm-1, very close to the experimental value of 1333 cm-1. Our calculations indicate that the hexagonal structure A t has a zone-center frequency of 1269 cm-1, the Elg mode is at 1215 cm-1, and the E1g mode is at 430 cm-1. Anharmonic corrections are rather large (2-3%) in the cubic diamond Raman mode and in the hexagonal Al mode, but are not important in the E2g and Elg modes. We will compare our results with the available experimental information.

Many previous studies of vitreous carbon by x-ray diffraction show broad diffraction maxima typical of vitreous materials. However, electron diffraction studies reveal many regions that give single crystal patterns. Although some of the patterns are of graphite, as expected, many patterns were obtained that correspond to other carbon crystal forms such as chaoite and carbon VI. In addition, ring patterns of lonsdaleite and diamond were found. Finally, patterns were obtained that do not correspond to any of the known carbon allotropes. (Author)

This study was based on diamonds showing absorption peaks in the range from 7 to 10 pm (1010, 1100, 1175, and 1331 cm−1 bands). In ultraviolet light (λ = 365 nm) these show yellow-green luminescence, wavelength λ = 503 nm [1–6]. No evidence has previously been available on the defects responsible for this type of luminescence.

There is an old puzzle associated with the series of compounds formed by first row elements, C, BN, and BeO: similar to the other members of the family of octet semiconductors, these compounds exist in dense phases like diamond and zincblende, or hexagonal diamond (lonsdaleite) and wurtzite; however, whereas C and BN also exist in low density graphitic phases, BeO does not. Until now it has not been understood if the absence of BeO in this latter series is caused by simple technical difficulties in preparing the compound or by an intrinsic structural instability of the graphitic phase with respect to the dense wurtzite phase. There is some interest in layered BeO because of its expected exotic intercalation chemistry, as well as some possible unusual properties in this highly polar 2-D oxide. In this report, the stability of graphitic BeO is discussed based on all-electron full-potential linearized augmented plane wave method. 11 refs., 3 figs.

The giant diamond-containing Canyon Diablo meteorite is in composition a typical representative of the widespread group of iron meteorites — the coarse octahedrites. But it is unique in a number of scientifically important aspects. When it fell, it formed the Arizona (Barringer meteorite) crater (1220 m in diam), which is of the explosive type. The investigations of the shock recrystallization of the crater rocks and the meteoritic material are of importance for planetology, and in particular for the eludication of matter recrystallization conditions during the collisions of large cosmic bodies. The study of the diamonds contained in the Canyon Diablo meteorite is of importance to various branches of carbon cosmochemistry. The Canyon Diablo meteorite fell in Arizona, U.S.A., some tens of thousands years ago. However, the Arizona meteorite crater is well preserved owing to the semi-arid climatic conditions. Signs of shock recrystallization of the rocks — shatter cones, impactites, dense and super-dense silica modifications were revealed in the Arizona meteorite crater. Around the crater many samples of the Canyon Diablo iron meteorite have been found (at distances of up to 9 km from the crater), together with a great amount of finely dispersed meteorite matter. The total weight of the material attributed to the meteorite is put at 30 tons. A number of meteorite fragments have been oxidized to different degrees during terrestrial weathering. Typical samples of the Canyon Diablo meteorite are represented by coarse octahedrite matter with kamacite band widths of 1.8–2.2 mm. In many meteorite fragments, especially the fragments found on the crater rim, the Widmanstatten pattern has been destroyed as a result of the explosion which occurred when the meteorite hit the Earth. The meteorite is rich in accessory minerals (cohenite, schreibersite, troilite.). The Ni content is, in typical samples of the meteorite, about 7.16%, in rare, atypical, medium octahedrite samples the Ni content reached 8.2 %. In the content of trace elements the meteorite may be classified with the I Ga-Ge group. In the content of stable isotopes of elements there is no substantial difference between the Canyon Diablo meteorite and other octahedrites. Radioactive cosmogenic isotopes are represented by isotopes with a large half-life. The diamonds in the Canyon Diablo meteorite are unevenly distributed and are found inside the highly recrystallized meteorite fragments at the rim of the crater. Diamonds are present in the form of intergrowths of microcrystals, crystallite sizes are < 1μ, the sizes of the intergrowths reach 2–5 mm. Admixtures of graphite and the hexagonal diamond lonsdaleite are present in the intergrowths. From the evidence of shock recrystallization of the meteorite matter it would seem that the diamond containing fragments of the Canyon Diablo meteorite have undergone shock pressures of from 400 kbar to 1 mgbar at impact, at these pressures the diamonds would crystallise. The diamond-containing sample of the Canyon Diablo meteorite investigated by the author has experienced a shock pressure of up to 1 mgbar during the explosion.

We announce a new class of carbon allotropes. The basis of this new classification resides on the concept of combining hexagonal diamond (sp3 bonded carbon − lonsdaleite) and ring carbon (sp2 bonded carbon − graphene). Since hexagonal diamond acts as an insulator and sp2 bonded rings act as conductors, these predicted materials have potential applications for transistors and other electronic components. We describe the structure of a proposed series of carbon allotropes, novamene, and carry out a detailed computational analysis of the structural and electronic properties of the simplest compound in this class: the single-ring novamene. In addition, we suggest how hundreds of different allotropes of carbon could be constructed within this class.

We present an ab initio study of boron nitride as well as carbon allotropes. Their relative thermodynamic stabilities and structural phase transitions from low- to high-density phases are investigated. Pressure-temperature phase diagrams are calculated and compared to experimental findings. The calculations are performed using quantum chemical wave-function-based as well as density functional theories. Our findings reveal that predicted energy differences often depend significantly on the choice of the employed method. Comparison between calculated and experimental results allows for benchmarking the accuracy of various levels of theory. The produced results show that quantum chemical wave-function-based theories allow for achieving systematically improvable estimates. We find that on the level of coupled cluster theories the low- and high-density phases of boron nitride become thermodynamically degenerate at 0 K. This is in agreement with recent experimental findings, indicating that cubic boron nitride is not the thermodynamically stable allotrope at ambient conditions. Furthermore, we employ the calculated results to assess transition probabilities from graphitic low-density to diamondlike high-density phases in an approximate manner. We conclude that the stacking order of the parent graphitic material is crucial for the possible formation of metastable wurtzite boron nitride and hexagonal carbon diamond also known as lonsdaleite.

Diamonds, up to 30 µm in size, were found in the iridium-rich layer from the K-T boundary site at Arroyo El Mimbral and the spherule bed from Arroyo El Penon, northeastern Mexico. Stepped heating experiments indicate two or more isotopically distinct diamond components with carbon isotopic compositions characteristic of a mixture of carbon sources. The diamonds' crystal form is cubic—not the hexagonal polymorph of diamond, lonsdaleite, which has been used previously to infer formation due to shock transformation of graphite. The size, crystallography, and mineralogic associations of K-T diamonds are similar to those of impact-produced diamonds from the Ries crater in Germany where both shock transformation of graphite and a mode of formation by condensation from a vapor plume have been inferred. The discovery of impact-produced diamonds in association with high Ir contents for these sediments supports their impact origin, K-T age, and the inference that their source was from the buried impact crater of Chicxulub on the Yucatan peninsula, Mexico.

High-quality defect-free lonsdaleite Si and Ge can now be grown on hexagonal nanowire substrates. These hexagonal phases of group-IV semiconductors have been predicted to exhibit improved electronic and optical properties for optoelectronic applications. While lonsdaleite Si is a well-characterized indirect semiconductor, experimental data and reliable calculations on lonsdaleite Ge are scarce and not consistent regarding the nature of its gap. Using ab initio density-functional theory, we calculate accurate structural, electronic, and optical properties for hexagonal Ge. Given the well-known sensitivity of electronic-structure calculations for Ge to the underlying approximations, we systematically test the performance of several exchange-correlation functionals, including meta-GGA and hybrid functionals. We first validate our approach for cubic Ge, obtaining atomic geometries and band structures in excellent agreement with available experimental data. Then, the same approach is applied to predict electronic and optical properties of lonsdaleite Ge. We portray lonsdaleite Ge as a direct semiconductor with only weakly dipole-active lowest optical transitions, small band gap, huge crystal-field splitting, and strongly anisotropic effective masses. The unexpectedly small direct gap and the oscillator strengths of the lowest optical transitions are explained in terms of symmetry and back-folding of energy bands of the diamond structure.

Carbonados are irregular polycrystalline aggregates of diamond that occur in placers and low-grade metamorphic rocks. They are characterized by fine grain size, light carbon isotope ratio, a wide variety of inclusions of crustal materials (silicates, phosphates, and oxides), and an absence of minerals characteristically associated with diamonds from the upper mantle. We propose that all these properties, and also the presence of lonsdaleite in one carbonado, can be explained by impact metamorphism of crustal rocks containing organic carbon or a graphitic derivative, including those of the Archean and Proterozoic eras.