Tunable electrical properties of C₆₀∙m-xylene and the formation of semiconducting ordered amorphous carbon clusters under pressure

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.