In situ analysis of the structural transformation of glassy carbon under compression at room temperature

Room temperature compression of graphitic materials leads to interesting superhard sp3 rich phases which are sometimes transparent. In the case of graphite itself, the sp3 rich phase is proposed to be monoclinic M-carbon; however, for disordered materials such as glassy carbon the nature of the transformation is unknown. We compress glassy carbon at room temperature in a diamond anvil cell, examine the structure in situ using x-ray diffraction, and interpret the findings with molecular dynamics modeling. Experiment and modeling both predict a two-stage transformation. First, the isotropic glassy carbon undergoes a reversible transformation to an oriented compressed graphitic structure. This is followed by a phase transformation at ∼35 GPa to an unstable, disordered sp3 rich structure that reverts on decompression to an oriented graphitic structure. Analysis of the simulated sp3 rich material formed at high pressure reveals a noncrystalline structure with two different sp3 bond lengths.J.E.B. would like to acknowledge the Australian Research Council (ARC) for financial support through a Future Fellowship (Grant No. FT130101355). J.E.B. and D.G.M. acknowledge funding under the ARC Discovery Project scheme (Grant No. DP140102331). B.H. acknowledges funding through the ORNL Neutron Scattering Facilities, DOE Office of Science User Facilities operated by the Oak Ridge National Laboratory. N.A.M. acknowledges financial support through a fellowship, Grant No. FT120100924. I.S.-M. acknowledges financial support through a fellowship, Grant No. FT140100191. Work by R.B. was supported by the Energy Frontier Research in Extreme Environments (EFree) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under Award No. DE-SC0001057. Computational resources are provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia. The XRD measurements presented here were performed at HP CAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974, with partial instrumentation funding by NSF. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DEAC02-06CH11357. This work has been partially supported by the U.S. Department of Energy. ORNL is managed by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 for the U.S. Department of Energ