This talk presents the main results of my PhD research on what happens when two types of carbon-based molecular crystals are pushed well beyond their comfort zone in high pressure devices. Although both systems are carbon-based, diamondoids and C₆₀ fullerenes respond to extreme pressure and stress in strikingly different ways.

Diamondoids are rigid sp³-bonded carbon cages wrapped in a hydrogen shell, which prevents them from forming new bonds under pressure. Instead of polymerising, they relieve stress through plastic flow. Once pressures of about 10 GPa are exceeded, strong intermolecular repulsion causes the sample to deform non-elastically and even force its way out of the confinement volume, trying to escape its fate. Remarkably, after releasing pressure, the molecular structures fully revert back to their initial form, even after exposure to pressures up to 50 GPa.
C₆₀ fullerenes take the opposite approach. Because C₆₀ can readily form carbon–carbon bonds between individual molecules, plastic deformation instead leads to a collapse of the hollow molecular cages into sp3 bonded carbon phases. Crucially, this collapse transformation occurs at much lower pressures when non-hydrostatic stress is present.

By deliberately adding shear using a rotational diamond anvil cell, this collapse transition was triggered at pressures as low as 15 GPa, highlighting shear as a powerful and previously underutilised control parameter for sp²-to-sp³ transformation in carbon. This approach enables diamond-like material synthesis at moderate pressures at room temperature.

The experimental work combines in situ synchrotron X-ray diffraction and Raman spectroscopy with detailed transmission electron microscopy and electron energy-loss spectroscopy on recovered samples. Finally, a phenomenological model is introduced to estimate shear strain in rotational diamond anvil cells, showing that even modest rotations can generate strains well beyond typical yield points, ensuring that plastic deformation plays a central role.

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