From soft to superhard: Fifty years of experiments on cold-compressed graphite

In recent years there have been numerous computational studies predicting the nature of cold-compressed graphite yielding a proverbial alphabet soup of carbon structures (e.g., bct-C-4, K-4-, M-, H-, R-, S-, T-, W- and Z-carbon). Although theoretical methods have improved, the inherent nature of graphite (i.e., low-Z) and the subsequent room-temperature, high-pressure phase transition (i.e., low symmetry, nanocrystalline and sluggish), make experimental measurements difficult to execute and interpret even with the current technology of 3rd generation synchrotron sources. The room-temperature, high-pressure phase transition of graphite has been detected by numerous kinds of experiments over the past fifty years, such as electrical resistance measurements, optical microscopy, X-ray diffraction, inelastic X-ray scattering, and Raman spectroscopy. However, the identification and characterization of high-pressure graphite is replete with controversy since its discovery more than fifty years ago. Recent experiments confirm that this phase has a monoclinic structure, consistent with the M-carbon phase predicted by theoretical computations. Meanwhile, experiments demonstrate that the phase transition is sluggish and kinetics is important in discerning the phase boundary. Additionally, the post-graphite phase appears to be superhard with hardness comparable to that of diamond.