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

Four carbon phases including amorphous, cubic, and hexagonal forms of diamond as well as graphite were found through transmission electron microscopy analysis after non-hydrostatically compressing glassy carbon to 145 GPa at room temperature. Crystallinity and preferred orientation of the recovered phases indicate that shear stress is critical in driving the phase transformations from the sp2-bonded carbon precursor. A Gibbs free energy landscape, as a function of density and crystallinity, illustrates proposed transformation pathways from glassy carbon to a common high-density parent phase that then converts to the phases observed, depending on the local stress environment in the sample. We show how it is possible for four distinct recoverable phases of carbon to form from the glassy carbon precursor following compression in a non-hydrostatic environment. Our approach creates nanodiamonds with a wide range of sizes down to ~2 nm which have potential application as a source of custom-sized nanodiamonds for applications in biomedical and quantum sensing applications.