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.