Transparent, superhard amorphous carbon phase from compressing glassy carbon

Raman spectroscopy has been used to study the transformations of glassy carbon (GC) under high pressure. A GC sphere has been observed to transform into a transparent carbon phase above 33 GPa. The transformation is associated with a change in bonding character of carbon from sp2 to sp3 hybridization and an increase in hardness. The yield strength of the GC sphere reaches a value of 120 GPa at a confining pressure of 62 GPa, which is comparable to that of diamond at ambient conditions. The stress induced by the pressure medium is important for the observed transformations of GC under pressure.