Mechanisms of nanodiamond and amorphous diamond-like carbon formation following room temperature compression of C60

The transformation of carbon into sp 3 -rich phases is of broad interest for developing superhard materials, yet the role of non-hydrostatic pressures during such transformations remains poorly understood. While buckminsterfullerene (C 60 ) is known to collapse under high pressures and temperatures, the structural evolution pathways under non-hydrostatic conditions have not been fully established. Here, we report the formation of transparent, amorphous diamond-like carbon ( a-D ) from C 60 compressed to 49 GPa at room temperature under non-hydrostatic stress. Transmission electron microscopy of the recovered material reveals a predominantly sp 3 -bonded network with (85 ± 5)% sp 3 bonds and a density of (3.05 ± 0.10) g cm −3 , interlaced with residual C 60 molecules and narrow bands of nanocrystalline diamond. To understand the transformation mechanism, we performed molecular dynamics simulations using an atomic cluster expansion potential. The simulations show that under uniaxial stress, fullerene cages collapse at lower pressures (29–32 GPa) than under hydrostatic conditions (42–45 GPa), forming a structure consistent with amorphous diamond-like carbon. Annealing simulations on the amorphous structure at 50 GPa result in nanocrystalline diamond formation, suggesting that localised heat spikes generated by an adiabatic shear band process drive the formation of observed nanodiamond regions. • Formation of recoverable sp 3 -bonded carbon at room temperature. • Formation of nanodiamonds in shear bands. • Non-hydrostatic stresses promote C 60 cage collapse. • Machine learning potentials are powerful tools to model high pressure responses in carbon.