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Glassy carbon is a technologically important material with isotropic properties that is nongraphitizing up to ∼3000 °C and displays complete or “superelastic” recovery from large compression. The pressure limit of these properties is not yet known. Here we use experiments and modeling to show permanent densification, and preferred orientation occurs in glassy carbon loaded to 45 GPa and above, where 45 GPa represents the limit to the superelastic and nongraphitizing properties of the material. The changes are explained by a transformation from its sp2 rich starting structure to a sp3 rich phase that reverts to fully sp2 bonded oriented graphite during pressure release.

Diamond is an attractive material due to its extreme hardness, high thermal conductivity, quantum optical, and biomedical applications. There is still much that is not understood about how diamonds form, particularly at room temperature and without catalysts. In this work, a new route for the formation of nanocrystalline diamond and the diamond-like phase lonsdaleite is presented. Both diamond phases are found to form together within bands with a core-shell structure following the high pressure treatment of a glassy carbon precursor at room temperature. The crystallographic arrangements of the diamond phases revealed that shear is the driving force for their formation and growth. This study gives new understanding of how shear can lead to crystallization in materials and helps elucidate how diamonds can form on Earth, in meteorite impacts and on other planets. Finally, the new shear induced formation mechanism works at room temperature, a key finding that may enable diamond and other technically important nanomaterials to be synthesized more readily.

Glassy carbon (GC) is a class of disordered carbon materials that is known to be superelastic and non-graphitizing up to 3000 °C. The maximum heat treatment temperature is often used as a proxy to denote structure and physical properties. GC synthesised at low temperatures (~1000 °C) is often classified as Type I GC which has advantages of higher elastic modulus, resistance to oxidation, and lower permeability to gases. Type II GC is synthesised at higher temperatures (>2000 °C), has fewer impurities, is more electrically conductive, and is rated to a higher service temperature. Here Type I and II GC samples sourced from two suppliers are investigated using Rutherford backscattering spectrometry and elastic recoil detection analysis for composition, Raman spectroscopy, transmission electron microscope imaging, X-ray and neutron diffraction for structure determination, nanoindentation for mechanical properties, and Van der Pauw measurements for resistivity. The results show that the broad classifications of Type I or Type II do not correlate with the physical properties of the samples. We conclude that the quoted maximum heat treatment temperature alone is not sufficient to specify the properties of GC and that a careful microstructural examination of the material should be used to inform materials selection.

Room temperature compression of graphitic materials leads to interesting superhard sp3 rich phases which are sometimes transparent. In the case of graphite itself, the sp3 rich phase is proposed to be monoclinic M-carbon; however, for disordered materials such as glassy carbon the nature of the transformation is unknown. We compress glassy carbon at room temperature in a diamond anvil cell, examine the structure in situ using x-ray diffraction, and interpret the findings with molecular dynamics modeling. Experiment and modeling both predict a two-stage transformation. First, the isotropic glassy carbon undergoes a reversible transformation to an oriented compressed graphitic structure. This is followed by a phase transformation at ∼35 GPa to an unstable, disordered sp3 rich structure that reverts on decompression to an oriented graphitic structure. Analysis of the simulated sp3 rich material formed at high pressure reveals a noncrystalline structure with two different sp3 bond lengths.

Hexagonal diamond, a potentially superhard material, forms from a glassy carbon precursor at pressuresof ~100 GPa at the relatively low temperature of 400 C. The formation mechanism of the hexagonaldiamond phase was investigated by performing microstructural analysis on cross-sections of therecovered samples. Three distinct structures have been observed, a graphitic region near the centre of thesample with low density, a hexagonal diamond region at the edge of the sample with high density, and amixed region containing significant proportions of both the graphitic structure and hexagonal diamond.The hexagonal diamond was more likely to occur at greater radial distance from the centre of the samplewith some evidence for greater amounts also near the diamond anvil faces. The observed distribution ofthe hexagonal phase correlates well to regions of greatest shear strain expected from modelling studiesof strainfields in diamond anvil cells. Thefindings support the proposition that shear strain plays animportant role in the formation of hexagonal diamond, and that it may be a driving force for the naturaloccurrence of hexagonal diamond in the shear zone of meteorite impact craters.

Carbon exhibits a large number of allotropes and its phase behaviour is still subject to significant uncertainty and intensive research. The hexagonal form of diamond, also known as lonsdaleite, was discovered in the Canyon Diablo meteorite where its formation was attributed to the extreme conditions experienced during the impact. However, it has recently been claimed that lonsdaleite does not exist as a well-defined material but is instead defective cubic diamond formed under high pressure and high temperature conditions. Here we report the synthesis of almost pure lonsdaleite in a diamond anvil cell at 100 GPa and 400 °C. The nanocrystalline material was recovered at ambient and analysed using diffraction and high resolution electron microscopy. We propose that the transformation is the result of intense radial plastic flow under compression in the diamond anvil cell, which lowers the energy barrier by “locking in” favourable stackings of graphene sheets. This strain induced transformation of the graphitic planes of the precursor to hexagonal diamond is supported by first principles calculations of transformation pathways and explains why the new phase is found in an annular region. Our findings establish that high purity lonsdaleite is readily formed under strain and hence does not require meteoritic impacts.