Novel Superhard sp3 Carbon Allotrope from Cold-Compressed C70 Peapods

Design and synthesis of new carbon allotropes have always been important topics in condensed matter physics and materials science. Here we report a new carbon allotrope, formed from cold-compressed C-70 peapods, which most likely can be identified with a fully sp(3)-bonded monoclinic structure, here named V carbon, predicted from our simulation. The simulated x-ray diffraction pattern, near K-edge spectroscopy, and phonon spectrum agree well with our experimental data. Theoretical calculations reveal that V carbon has a Vickers hardness of 90 GPa and a bulk modulus similar to 400 GPa, which well explains the ring crack left on the diamond anvils by the transformed phase in our experiments. The V carbon is thermodynamically stable over a wide pressure range up to 100 GPa, suggesting that once V carbon forms, it is stable and can be recovered to ambient conditions. A transition pathway from peapod to V carbon has also been suggested. These findings suggest a new strategy for creating new sp(3)-hybridized carbon structures by using fullerene@nanotubes carbon precursor containing odd-numbered rings in the structures.