Amorphous diamond -- A high-pressure superhard carbon allotrope

Compressing glassy carbon above 40 GPa, we have observed a new carbon allotrope with a fully \textit{sp}$^{3}$-bonded amorphous structure and diamond-like strength. Synchrotron x-ray Raman spectroscopy revealed a continuous pressure-induced \textit{sp}$^{2}$-to-\textit{sp}$^{3}$ bonding change, while x-ray diffraction confirmed the perseverance of non-crystallinity. The transition was reversible upon releasing pressure. Used as an indenter, the glassy carbon ball demonstrated exceptional strength by reaching 130 GPa with a confining pressure of 60 GPa. Such an extremely large stress difference of $>$70 GPa has never been observed in any material besides diamond, indicating the high hardness of this high-pressure carbon allotrope. The nanoscale transmission x-ray microscopy is being utilized for accurate pressure-volume determination of glassy carbon and its high-pressure phase.