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.

Developing a universal strategy to design piezochromic luminescent materials with desirable properties remains challenging. Here, we report that insertion of a non-emissive molecule into a donor (perylene) and acceptor (1,2,4,5-tetracyanobezene) binary cocrystal can realize fine manipulation of intermolecular interactions between perylene and 1,2,4,5-tetracyanobezene (TCNB) for desirable piezochromic luminescent properties. A continuous pressure-induced emission enhancement up to 3 GPa and a blue shift from 655 to 619 nm have been observed in perylene-TCNB cocrystals upon THF insertion, in contrast to the red-shifted and quenched emission observed when compressing perylene-TCNB cocrystals and other cocrystals reported earlier. By combining experiment with theory, it is further revealed that the inserted non-emissive THF forms blue-shifting hydrogen bonds with neighboring TCNB molecules and promote a conformation change of perylene molecules upon compression, causing the blue-shifted and enhanced emission. This strategy remains valid when inserting other molecules as non-emissive component into perylene-TCNB cocrystals for abnormal piezochromic luminescent behaviors.

Raman spectra of single-walled carbon nanotubes (SWNTs) with diameters of 0.6–1.3 nm have been studied under high pressure. A “plateau” in the pressure dependence of the $G$-band frequencies was observed in all experiments, both with and without pressure transmission medium. Near the onset of the $G$-band plateau, the corresponding radial breathing mode (RBM) lines become very weak. A strong broadening of the full width at half maximum of the RBMs just before the onset of the $G$-band plateau suggests that a structural transition starts in the SWNTs. Raman spectra from SWNTs released from different pressures also indicate that a significant structural transition occurs during the $G$-band plateau process. Simulations of the structural changes and the corresponding Raman modes of a nanotube under compression show a behavior similar to the experimental observations. Based on the experimental results and the theoretical simulation, a detailed model is suggested for the structural transition of SWNTs, corresponding to the experimentally obtained Raman results in the high-pressure domain.