Tunable electrical properties of C_(60)·m-xylene and the formation of semiconducting ordered amorphous carbon clusters under pressure

Ordered amorphous carbon clusters(OACC)transformed from m-xylene solvated C_(60)(C_(60)·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 C_(60)·m-xylene are systematically investigated under pressure using impedance spectroscopy,four-probe resistance measurements,and Raman spectroscopy.It is found that C_(60)·mxylene is an insulator at ambient pressure.The resistance decreases sharply starting at the pressure around 8 GPa due to the pressure-induced dimerization of C_(60)verified by the Raman study.The presence of solvent hinders further polymerization of C_(60)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-tosemiconducting 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.

Dehydro-Diels-Alder reaction and diamondization of bowl-shaped clusters C_(18)Te_(3)Br_(4)(Bu-O)_(6)

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 C_(18)Te_(3)Br_(4)(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 sp^(2) to the intermolecular spa 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.