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.