Quantifying hexagonal stacking in diamond

Abstract

Diamond is a material of immense technological importance and an ancient signifier for wealth and societal status. In geology, diamond forms as part of the deep carbon cycle and typically displays a highly ordered cubic crystal structure. Impact diamonds, however, often exhibit structural disorder in the form of complex combinations of cubic and hexagonal stacking motifs. The structural characterization of such diamonds remains a challenge. Here, impact diamonds from the Popigai crater were characterized with a range of techniques. Using the MCDIFFaX approach for analysing X-ray diffraction data, hexagonality indices up to 40% were found. The effects of increasing amounts of hexagonal stacking on the Raman spectra of diamond were investigated computationally and found to be in excellent agreement with trends in the experimental spectra. Electron microscopy revealed nanoscale twinning within the cubic diamond structure. Our analyses lead us to propose a systematic protocol for assigning specific hexagonality attributes to the mineral designated as lonsdaleite among natural and synthetic samples.

Complex nanostructures in diamond

“…Known and predicted carbon allotropes from the SACADA database 9 are plotted as dark grey dots. The inset (rectangle) shows the range of energies for stacking disordered sp 3 -bonded polytypes (turquoise) between cubic (3C) (orange dot) and the hexagonal (2H) diamond structure (blue) 11 . Points for Type 1 and Type 2 diaphite structures described in 14 are shown as yellow and red dots, respectively.…”

“…A range of h-c stacked sequences is well known to occur among SiC, BN and other sp 3 -bonded materials that exhibit a wide range of ordered (repetitive) polytypes and disordered structures. It is proposed that h-c sp 3 -bonded layered structures are also present in natural and laboratory-shocked diamonds, and these could account for the appearance of hexagonal features in the diffraction patterns 10,11 . The method of DIFFaX analysis, which had been used to interpret the diffraction patterns of layered stacking motifs in H 2 O ice, was applied to a range of impact diamonds and laboratory shocked samples to measure their degrees of 'hexagonality'-the percentage of hexagonal stacking units present in the structure 10 .…”

“…The method of DIFFaX analysis, which had been used to interpret the diffraction patterns of layered stacking motifs in H 2 O ice, was applied to a range of impact diamonds and laboratory shocked samples to measure their degrees of 'hexagonality'-the percentage of hexagonal stacking units present in the structure 10 . This analysis approach supplemented by Monte Carlo techniques (MCDIFFaX) leads to the practical "stackogram" tool that was developed to identify and describe degrees of h-c ordered versus disordered layering patterns within the sp 3 -bonded structures, and also to differentiate among samples that had experienced different shock regimes 11 . The possibility to engineer nanoscale twinning among cubic and h-c diamond polytype structures adds additional capability to improving mechanical performance and thermal resistance 17 .…”

Characterization of carbon phases in Yamato 74123 ureilite to constrain the meteorite shock history

“…The occurrence of diamonds in ureilites poses the question of how this high-pressure mineral formed and whether diamonds in ureilites are similar or not to those formed by shock in terrestrial impact structures (e.g., Masaitis 1998;Hough et al 1995;Koeberl et al 1997;Ohfuji et al 2015;Murri et al 2019). Three main hypotheses have been proposed for the formation of diamonds in ureilites: (i) static high-pressure conditions in the deep interior of large parent bodies (Urey 1956), (ii) direct transformation from graphite due to shock (e.g., Lipschutz et al 1964;Bischoff et al 1999;Grund and Bischoff 1999;Nakamuta et al 2000Nakamuta et al , 2016Hezel et al 2008;Le Guillou et al 2010;Ross et al 2011;Lorentz 2019), also strongly supported by De Carli et al (1995;2002), and (iii) growth from a dilute gas phase, i.e., at low pressure in the solar nebula by a chemical vapor deposition (CVD) process (Fukunaga et al 1987).…”

Impact shock origin of diamonds in ureilite meteorites

“…Most ureilites, however, are shocked to various degrees and in these samples the graphite areas, though still having external blade-shaped morphologies, are internally polycrystalline (18). Diamonds and lonsdaleite [diamond with stacking faults and twinning defects (26)] occur embedded in these areas, constituting a volumetrically minor (thus disproportionately illustrious) component of ureilites.…”