High-pressure pathways for the formation of amorphous diamond and other tetrahedrally-bonded phases from glassy carbon

We show that non-hydrostatic compression of glassy carbon to 145 GPa at room temperature produces cubic, hexagonal and an amorphous form of diamond in the same recovered sample. The amorphous diamond phase exhibits a microstructure comparable to tetrahedral amorphous carbon thin films and to that formed from fullerene samples subjected to high pressure treatment. The crystallinity and preferred orientation of the phases indicate that shear stress is critical in driving the phase transformations from the randomly oriented graphitic glassy carbon precursor. A Gibbs free energy landscape, mapped as a function of density and crystallinity, illustrates proposed transformation pathways from glassy carbon to a common high-density parent phase that forms under compression that then converts to the phases observed, depending on the local stress environment. Modelling demonstrates that the common high-density parent phase has a preferred orientation in its atomic structure which explains the microstructure in many of the recovered daughter phases. Our results demonstrate that non-hydrostatic stress can be used to synthesise desirable diamond-like carbon phases without heating, including amorphous diamond and ultra-fine nanodiamonds with sizes down to 2 nm in diameter.