18 research outputs found
Twinning of cubic diamond explains reported nanodiamond polymorphs
The unusual physical properties and formation conditions attributed to h-, i-, m-, and n-nanodiamond polymorphs has resulted in their receiving much attention in the materials and planetary science literature. Their identification is based on diffraction features that are absent in ordinary cubic (c-) diamond (space group: Fd-3m). We show, using ultra-high-resolution transmission electron microscope (HRTEM) images of natural and synthetic nanodiamonds, that the diffraction features attributed to the reported polymorphs are consistent with c-diamond containing abundant defects. Combinations of {113} reflection and rotation twins produce HRTEM images and d-spacings that match those attributed to h-, i-, and m-diamond. The diagnostic features of n-diamond in TEM images can arise from thickness effects of c-diamonds. Our data and interpretations strongly suggest that the reported nanodiamond polymorphs are in fact twinned c-diamond. We also report a new type of twin ( rotational), which can give rise to grains with dodecagonal symmetry. Our results show that twins are widespread in diamond nanocrystals. A high density of twins could strongly influence their applications
Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material
Lonsdaleite, also called hexagonal diamond, has been widely used as a marker of asteroidal impacts. It is thought to play a central role during the graphite-to-diamond transformation, and calculations suggest that it possesses mechanical properties superior to diamond. However, despite extensive efforts, lonsdaleite has never been produced or described as a separate, pure material. Here we show that defects in cubic diamond provide an explanation for the characteristic d-spacings and reflections reported for lonsdaleite. Ultrahigh-resolution electron microscope images demonstrate that samples displaying features attributed to lonsdaleite consist of cubic diamond dominated by extensive {113} twins and {111} stacking faults. These defects give rise to nanometre-scale structural complexity. Our findings question the existence of lonsdaleite and point to the need for re-evaluating the interpretations of many lonsdaleite-related fundamental and applied studies
Electron Energy Loss Near Edge Structure (ELNES) on the Carbon K-Edge in Transition Metal Carbides with the Rock Salt Structure
The mono-carbides of the 3d, 4d and 5d transition
metals of groups IVA (Ti, Zr, Hf) and VA (V, Nb, Ta) of the
periodic table have the rock salt (B1) structure. They are-found
in many technologically important systems. The binary compounds
exist over a range of compositions, normally with a deficit
of carbon. Since their structures are identical and their lattice
parameters similar, there is considerable mutual solubility
between so that a wide range of compounds of the form
exist where M is a mixture of transition metals. This paper
presents a comparison of the electron energy loss near edge
structure (ELNES) on the carbon K-edges from the binary carbides
with compositions close to stoichiometry. The spectra show
systematic
changes of the ELNES and the threshold energy as the metal species
is changed. The threshold energy and intensity increase on going
from group IVA to group VA. Equivalent features can be identified
in the spectrum from each compound. The energy of each feature,
relative to the threshold energy, varies systematically with
the lattice parameter of the compound. The systematic change
in the ELNES with composition has potential applications where
mixed metal carbides, nitrides and carbo-nitrides co-exist.
In such cases, it can be difficult to obtain an unambiguous
interpretation of microanalytical data and ELNES offers one
more channel of information
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Surface electronic states of meteoritic nanodiamonds
The C K edge of Orgueil nanodiamonds (C-delta diamonds) was acquired by electron energy-loss spectroscopy (EELS), with an energy resolution of 300 meV. The spectra show peaks at 282.5, 284.7, and 286.4 eV, which occur in the band gap below the main diamond edge and are absent from the bulk diamond spectrum. These peaks are attributed to transitions from degrees C 1s surface core levels to unoccupied surface states, and arise from single and pi-bonded dangling bonds and C-H bonds. A shoulder to the main absorption edge at 287.8 eV may correspond to hydrocarbon adsorbates. These results can be used to further our understanding of C-delta diamond structure and may reveal the presence of a fullerene-like surface. The unique surface electronic states of the C-delta diamond surfaces are expected to affect their optical properties, which are dependent on features such as extent of H coverage, particle size, and surface structure.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202
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Carbonaceous materials in the acid residue from the Orgueil carbonaceous chondrite meteorite
Insoluble organic matter (IOM) dominates the HF/HCl residue of the Orgueil (CI) carbonaceous chondrite meteorite. The IOM is composed primarily of two C-rich particle types. The first has a fluffy texture similar to crumpled tissue paper, and the second type occurs as solid or hollow nanospheres. High-resolution transmission electron microscope (HRTEM) images of the fluffy material show it is poorly ordered, with small, irregularly shaped regions having fringes with 0.34-0.38 nm spacings and locally 0.21 nm cross-fringes. Nanodiamonds occur in the fluffy material. The rounded C-rich particles are common in the residue and their HRTEM images show neither fringes nor nanodiamonds. Both types of carbonaceous materials have a high aromatic component, as revealed by electron energy-loss spectroscopy (EELS), with up to 10 at% substitution by S, N, and O. The average compositions of the fluffy material and nanospheres are C100S1.9N3.7O4.9 and C100S2.4N5.0O3.9, respectively. The structural and chemical heterogeneity of the carbonaceous materials may represent material from multiple sources.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202
Transformation of graphite to diamond via a topotactic mechanism
Several mechanisms and intermediate steps have been proposed to explain the transformation of graphite to diamond. However, the mechanism continues to be debated, in part because graphite that is incompletely transformed to diamond has not been reported; although such material could be used to better understand the diamond-forming process. Here we report the discovery of nano-sized grains of interstratified graphite and diamond from Gujba, an extraterrestrially shocked meteorite. We use high-resolution transmission electron microscopy (HRTEM) data from these grains to show that diamond formed via a reconstructive, topotactic rather than martensitic mechanism. Electron diffraction and HRTEM images show the following three-dimensional crystallographic relationships between the interstratified graphite and diamond: (001)g||(111) d, (100)g||(211)d, and (120) g||(011)d. These relationships yield the transition matrix linking the graphite and diamond unit cells, which become coincident for graphite compressed to 7 GPa. The specific product, whether single-crystal or twinned diamond, is dictated by the initial graphite polytype and transformation route. The derivation of a three-dimensional transition matrix is consistent with a topotactic relationship between graphite and the newly formed diamond. © 2014 by Walter de Gruyter GmbH & Co. 2014
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Scanning electron microscopical and cross sectional analysis of extraterrestrial carbonaceous nanoglobules
Carbonaceous nanoglobules are ubiquitous in carbonaceous chondrite (CC) meteorites. The Tagish Lake (C2) meteorite is particularly intriguing in containing an abundance of nanoglobules, with a wider range of forms and sizes than encountered in other CC meteorites. Previous studies by transmission electron microscopy (TEM) have provided a wealth of information on chemistry and structure. In this study, low-voltage scanning electron microscopy (SEM) was used to characterize the globule forms and external structures. The internal structure of the globules was investigated after sectioning by focused ion beam (FIB) milling. The FIB-SEM analysis shows that the globules range from solid to hollow. Some hollow globules show a central open core, with adjoining smaller cores. The FIB with an SEM is a valuable tool for the analysis of extraterrestrial materials, even of sub-micron "soft" carbonaceous particles. The rapid site-specific cross-sectioning capabilities of the FIB allow the preservation of the internal morphology of the nanoglobules, with minimal damage or alteration of the unsectioned areas.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202