Fullerenes, represented by C₆₀ and C₇₀, are typical molecular-crystal carbon allotropes. Under high-temperature and high-pressure (HTHP) conditions, they can undergo a continuous sequence of structural evolution, including orientational ordering, initial intermolecular bonding, low-dimensional polymerization, multidimensional cross-linking, cage collapse, and amorphization. Therefore, fullerenes serve as an important bridge between molecular-crystal carbon and high-density covalent carbon networks. This review focuses on the phase transitions and polymerization behavior of fullerenes under combined pressure-temperature control. The structural features and formation mechanisms of the fcc to sc orientational transition, dimerization, typically through [2+2] cycloaddition, one-dimensional chain polymerization, two-dimensional layered polymerization, including tetragonal and rhombohedral phases, and possible three-dimensional structures are systematically summarized. The effects of different loading paths and kinetic factors on phase boundaries and product ordering are also discussed. In addition, the multi-pathway competition and disordering tendency of C₇₀ driven by molecular anisotropy are comparatively reviewed. The regulatory roles of guest species in metallofullerenes and solvated fullerenes on the polymerization pathways and physical properties of fullerenes under HTHP conditions are further discussed. Finally, based on recent progress in fullerene-derived sp³-rich superhard amorphous carbon and related novel carbon structures, potential research directions for the controllable synthesis of high-pressure carbon materials through precursor engineering and multidimensional regulation strategies are proposed.

Dehydro-Diels–Alder (DDA) reaction is a textbook reaction for preparing six-membered rings in solution but is scarcely seen in solid-state synthesis. In this work, using multiple characterization techniques, we demonstrate that the bowl-shaped clusters C₁₈Te₃Br₄(Bu-O)₆ might experience a DDA reaction at room temperature and high pressure between 5.5 and 7.4 GPa. Above 17.0 GPa, it is found that the bonding conversion from the intramolecular sp² to the intermolecular sp³ occurred, in the form of pressure-induced diamondization. The recovered samples from 20.0 and 36.1 GPa showed incomplete reversibility, while the decompression-induced graphitization of glassy carbon was observed during decompression from 46.5 GPa. The electrochemical impedance spectroscopy results indicated that the transport properties changed from grain boundary dominant to grain dominant due to the DDA reaction and the grain boundary effect disappeared as the intermolecular sp³ bonding building-up and carrier transmission channel formation above 17.0 GPa. The results in this study open a new route to construct the crystalline carbon materials with different transport properties.

Ordered amorphous carbon clusters (OACC) transformed from m-xylene solvated C₆₀ (C₆₀·m-xylene) are known as the first crystalline material constructed from amorphous building blocks and have attracted a lot of attention. The formation mechanism and physical properties of this material are of great importance for the design of more materials with such structural characteristics. In this article, the transport and structural properties of C₆₀·m-xylene are systematically investigated under pressure using impedance spectroscopy, four-probe resistance measurements, and Raman spectroscopy. It is found that C₆₀·m-xylene is an insulator at ambient pressure. The resistance decreases sharply starting at the pressure around 8 GPa due to the pressure-induced dimerization of C₆₀ verified by the Raman study. The presence of solvent hinders further polymerization of C₆₀ under higher pressures. The temperature-dependence of resistance exhibits a semiconducting characteristic at > 8–26.9 GPa, and is well described by Mott's three-dimensional variable-range hopping model (3D-VRH), indicating an insulating-to-semiconducting transition accompanied with pressure-induced dimerization. The resistance and hopping energy are both found to decrease monotonically with pressure and reach the minimum near 24 GPa. Above the pressure, resistance and hopping energy values start to rise, suggesting a transition to another semiconducting state, which is attributed to the pressure-induced formation of OACC. The conductivity shows a large hysteresis during decompression from higher than 24 GPa, confirming a different transport behavior of the sample with retained fullerenes versus OACC. The findings of our study suggest that the transport property of fullerene is tunable by introducing solvates and further enhance our understanding of the OACC.