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Complex diamond nanostructures created by asteroid impacts

X-ray diffraction and other techniques were used to probe the complex structure of diamonds formed at extreme pressures and temperatures during an asteroid impact. The results may open opportunities for engineering carbon materials with unique mechanical and electronic properties.

Around 50,000 years ago, a ~60-m asteroid travelling at 11 km/s impacted in northern Arizona, creating the 200-m-deep and 1.2-km-diameter Meteor Crater. The impact, estimated to correspond to 10 megatons of TNT, generated an intense shockwave, affecting the impacted rocks as well as surviving pieces of the asteroid, called the Canyon Diablo iron meteorite. As early as 1891, diamonds were reported embedded in the metal of the Canyon Diablo meteorites [1]. In 1967, scientists investigating fragments of the meteorite announced the discovery of a new form of diamond [2], which they speculated to have been formed in the extreme impact conditions. Terrestrial diamonds have cubic symmetry, but these ones had hexagonal symmetry. The new material was named lonsdaleite after the pioneering British crystallographer, Professor Dame Kathleen Lonsdale. Since then, lonsdaleite has been reported in several other meteorites associated with asteroid impacts. Scientists also speculated that the hexagonal diamond could possess properties superior to that of cubic diamond, stimulating attempts to synthesise pure lonsdaleite, though without success.

In this study, the structure of Canyon Diablo diamonds was probed using microfocus synchrotron X-ray diffraction (XRD) at beamline ID27 combined with DiFFaX modelling, state-of-the-art transmission electron microscopy (TEM) and multiwavelength Raman spectroscopy supported with density functional theory (DFT) calculations. The results challenged the prevailing view of the mineral as consisting of single-phase hexagonal diamond. The study revealed a remarkable structural diversity of cubic/hexagonally stacked diamond and their association with diamond-graphite nanocomposites containing sp3-/sp2-bonding patterns, called diaphites (Figure 22). The XRD data was pivotal for identifying the intergrowth between diamond and graphene, revealing

defects in the repeating patterns of the diamond atomic layers and mapping the structural complexity of the diaphite material at the submicron scale (Figure 23).

TEM investigations revealed the continuum of sp2- and sp3-bonded structures intimately associated at the nanometre scale (Figure 22). The XRD data showed evidence for two diaphite types and unusual, short graphene spacings (Figure 23), which is explained by the unique environments of carbon atoms occurring at the interface between diamond and graphene. In addition, Raman data combined with DFT calculations suggested that previously neglected or misinterpreted Raman bands can be associated with diaphite structures. This finding is significant as diaphite structures can be detected using a widely available spectroscopic technique.

This multi-analytical investigation shows that hard carbon grains found in natural and synthetic samples, many of which have been claimed to contain lonsdaleite, correspond to a mixture of micro-to-nanoscale structures containing

Fig. 22: Diaphite (diamond-graphite)

from the Canyon Diablo meteorite.

The central ~1.5 nm region outlined

in red represents nanocrystalline

diamond surrounded by graphite in green. The transition colour

between red and green refers to the

transition bond type of diamond and

graphite.

Fig. 23: Selected 2D XRD maps obtained from two different 2 µm3 areas of a Canyon Diablo diamond grain and their corresponding 1D intensity profiles (black curves). Red circles mark hexagonally arranged features and the 0.313 nm d-spacing arising from

compressed graphene layers of type 2 diaphite, respectively. The asymmetric and broad peaks indicate stacking disordered diamond and intimately intergrown diaphite structures. The simulated patterns using type 1 (Fdg=Fgd=0.2, Fg+=Fg-=0.4) and type 2 diaphite

(Fgg = 0.9, Fdg = 0.0053, Fg = 0.1) are shown in blue and pink, respectively.