Novel Superhard $s{p}^3$ Carbon Allotrope from Cold-Compressed ${\mathrm{C}}_{70}$ 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 ${\mathrm{C}}_{70}$ peapods, which most likely can be identified with a fully $s{p}^3$-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 $\sim 400\mathrm{GPa}$, 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 $s{p}^3$-hybridized carbon structures by using fullerene@nanotubes carbon precursor containing odd-numbered rings in the structures.

Figure 1

(a) Raman spectra of ${\mathrm{C}}_{70}$ peapods at selected pressures. The backgrounds of the Raman spectra at pressures above 60 GPa have been carefully subtracted. (b) XRD patterns of the starting material and of samples recovered from 68 GPa. The background of the diffraction pattern of the pristine sample has been carefully subtracted. The peaks marked with rhombus and asterisk represent the diffraction from a two-dimensional triangular lattice and the intertube spacing of SWNTs, respectively. (c) TEM image of the recovered samples. (d) The corresponding SAED pattern. (e) HRTEM image of the recovered samples.

Figure 2

(a) Predicted $V$ carbon structure. (b) Calculated enthalpy per atom for $V$ carbon and for previously proposed carbon allotropes versus pressure relative to graphite 2H. (c) Calculated phonon dispersion curves of $V$ carbon at 0 GPa. (d) Calculated electronic band structure of $V$ carbon at 0 GPa.

Figure 3

(a) Simulated XRD patterns of $V$ carbon and of previously proposed post-graphite phases compared with our experimental XRD pattern. The asterisk denotes the peak from a graphitelike structure. The experimental new peaks are marked by green arrows. The x-ray wavelength is 0.6199 Å in all cases. (b) Raman spectra of ${\mathrm{C}}_{70}$ peapods upon decompression at pressures as indicated. Simulated Raman spectra of $V$ carbon at ambient pressure are shown at the bottom. The computed band profiles with the Lorentzian broadening of $20{\mathrm{cm}}^{-1}$ (red line) and $2{\mathrm{cm}}^{-1}$ (blue line) are shown for a comparison with the experimental data. The gray shadow is a guideline for the eyes, corresponding to the envelope of the high-frequency Raman-active modes for $V$ carbon based on the simulation.

Figure 4

Transformation pathway of (10,10) CNT bundles with encapsulated ${\mathrm{C}}_{70}$ molecules into the $V$ carbon structure under compression.