![]() 2) match the profile of the XRD pattern and show peaks for cubic diamond together with poorly resolved maxima at the d-spacings attributed to ‘lonsdaleite.’ In order to understand the structural features that give rise to these reflections and their d-spacings, we imaged these samples using a state-of-the-art ultrahigh-resolution STEM. The circularly integrated intensity profiles of these SAED patterns ( Supplementary Fig. Such data have been interpreted as evidence for ‘lonsdaleite’ projected along and 4, 5, 6, 7, 14, 15, 20. These patterns are consistent with diamond projected along or, respectively, although the streaking and hexagonally arranged 111 reflections are incompatible with defect-free single-crystal cubic diamond. ![]() The SAED patterns of numerous grains consist of either spotty rings with streaking and smeared intensities or reflections arranged nearly hexagonally ( Figs 1b,c and 2a). The XRD patterns of Canyon Diablo diamonds and the synthetic material display the poorly resolved diffraction maxima attributed 1, 2, 3, 14, 15, 19, 20 to ‘lonsdaleite’ ( Fig. In spite of the many diffraction and spectroscopic studies, unambiguous data that prove the existence of lonsdaleite as a distinct material have not been reported. ![]() Selected-area electron diffraction (SAED), Raman, electron energy-loss spectroscopy (EELS) and high-resolution transmission electron microscopy have also been used for identification of lonsdaleite 4, 5, 6, 7, 14, 15, 18, 23, 24 however, interpretation of data is ambiguous ( Supplementary Note 1). 1a,b) or match those of graphite, but well-resolved X-ray reflections for lonsdaleite have not been reported. ![]() However, these maxima either occur on the shoulders of diamond peaks ( Fig. Published powder X-ray diffraction (XRD) patterns of lonsdaleite show peaks of cubic diamond plus extra, very broad and poorly resolved maxima at 0.218, 0.193, 0.151 and 0.116 nm that have been indexed using a hexagonal unit cell 1, 2, 3, 14, 15, 20. It has been reported to form during static compression of graphite 3, 9, 13, 15, 16, 17, 18 high-pressure–high-temperature treatment of powdered diamond, graphite and amorphous carbon 19 explosive detonation and shock compression of graphite 11, 20 and diamond 21 and chemical vapour deposition of hydrocarbon gases 22 however, in all cases the synthesis product also contained cubic diamond, graphite or both. However, these exceptional properties have not been proven experimentally because of the inability to synthesize lonsdaleite as a pure phase. Lonsdaleite has also received much attention because of its potentially superior mechanical properties, such as compressive strength, hardness and rigidity, thought to rival or exceed those of cubic diamond 16, 17. Furthermore, an area centred around 18 GPa and 1,400 K in the pressure–temperature diagram for carbon was attributed to a phase called ‘retrievable hexagonal-type diamond’ 10, which corresponds to the conditions where lonsdaleite has been reported 3, 11, 15. Observations and theoretical studies suggested a structural relationship among graphite, cubic diamond and lonsdaleite and an important role of the latter during the graphite-to-diamond transition 3, 9, 10, 11, 12, 13, 14. Lonsdaleite was proposed to have a wurtzite (ZnS)-type structure with space group P6 3/mmc ( a=0.251 and c=0.412 nm) and with all structural positions occupied by carbon 1, 2, 3, 8. It has since been reported from several meteorites as well as from terrestrial sediments and has been attributed to asteroidal impacts, both extraterrestrial and on Earth 4, 5, 6, 7. Its formation was attributed to shock-induced transformation of graphite within the meteorite upon impact with Earth, and its occurrence was used as an indicator of shock 1, 2, 3. Lonsdaleite was first described almost 50 years ago from the Canyon Diablo iron meteorite 1, 2. Diamond is reported to have a number of polytypes, of which lonsdaleite (also called hexagonal diamond) has received particularly intense attention. Within this diversity are materials with extraordinary properties, paramount being cubic diamond, which has the highest known hardness and thermal conductivity. The allotropes of carbon display a wide diversity of structures that include the three-dimensional (3D) diamond and graphite, two-dimensional (2D) graphene and curved nanotubes and fullerenes.
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