Shear-Driven Transformations and Plastic Deformation in Diamondoids and C60 under High Pressure

Abstract

Carbon's flexible bonding produces numerous stable and metastable allotropes. Subjecting carbon materials to high pressure conditions is essential for synthesising technologically relevant sp3-hybridised phases such as nanodiamonds and other ultrahard materials. Conventional sp3 synthesis requires high pressures and temperatures, making it energetically costly. Recently, combining high pressure with high shear stresses has emerged as an alternative, enabling phase transformations at milder pressures, although the effect on carbon reaction pathways remains poorly understood.

This thesis examines two classes of molecular crystalline carbon-based materials under pressure: sp3-bonded diamondoid hydrocarbons (adamantane, diamantane, triamantane and tetramantane) and sp2-bonded hollow C60 fullerenes, to assess how precursor structure influences phase evolution and shear-driven transformations.

Diamond-anvil cells are used for high-pressure compression, while a rotational diamond anvil cell (rDAC) was used to apply shear. Samples were analysed in situ using X-ray diffraction and Raman spectoscropy. Recovered samples were characterised via transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS).

Cage-structured diamondoids were found to be mechanically ultra-resilient, withstanding 50 GPa and fully recovering their initial geometry upon decompression. Their main response to compression is plastic flow driven by intermolecular repulsion, while the sp3 framework remains intact. The plastic flow manifests in an anomalous lateral expansion of the sample chamber, pushing out the gasket. The onset of this effect is around 10 GPa and coincides with Raman peak splitting and increasing compressive stiffness.

In contrast, C60 molecules irreversibly transform under pressure. Initial cross-linking stiffens the structure, followed by cage collapse into sp3-rich phases. Non-hydrostatic conditions reduce the collapse threshold from 45 GPa to 30 GPa. Upon collapse, amorphous carbon with 85% sp3 bonding, interlaced by nanodiamond (ND) bands and residual C60 fragments forms.

Inducing shear through twisting the sample under pressure in the rDAC enables the molecular collapse at pressures as low as 12 GPa, when the molecular system is sufficiently cross-linked. Shearing at 15-17 GPa produces dense sp3-rich amorphous carbon with up to 3.4 g/cm3 and 95% sp3 bonding, interlaced with ND bands and a continuous ND surface layer. Shearing at higher pressures forms homogeneous high-density amorphous carbon without ND.

Lastly, a henomenological model to estimate shear strain in the rDAC is introduced, demonstrating that the setup is influenced by grain size, boundary slip, elasticity and rigidity. This model shows that even modest twist can cause shear strains over 10%, coinciding with the yield point of most materials. This indicates that the rDAC experimental setup introduces plastic deformation which considerably reduces phase transformation thresholds.

The results of this thesis demonstrate a moderate-pressure, ambient-temperature route to bulk sp3-bonded carbon phases and highlight shear as a potent driver of carbon phase transformation. These findings open various future research avenues in applying shear-driven synthesis approaches in other carbon precursor materials, utilising functionalised fullerene precursors for targeted amorphous carbon synthesis with desirable atom terminations, and tackling the challenge of quantifying shear stresses in high pressure experiments. Show more