Mechanism of thermally assisted stabilization of pressure-induced sp3 bonds in amorphous carbon

Various sp³-bond-rich noncrystalline carbon materials with extraordinary properties have been synthesized through structural transitions of sp²-bonded amorphous carbon under high pressure and high temperature (HPHT). However, the specific role of pressure and temperature in the process of the structural transition remains elusive due to the lack of in situ investigation. Here, combining in situ HPHT synchrotron x-ray diffraction and first-principles simulations, we investigated the structural evolution of glassy carbon (GC) (an archetype amorphous carbon) under pressure and thermal annealing. We found GC transformed to a recoverable superstrong amorphous carbon with a considerable number of sp³ bonds when compressed to {~}58 GPa and annealed at {~}728 K. Our results indicate that thermal annealing (even far below 1000 K) plays a key and unique role in stabilizing the sp³ carbon bonds by lowering the free energy through local structural rearrangements. Otherwise, most pressure-induced sp³ carbon bonds could not be preserved upon pressure release. Therefore, temperature is an essential tuning parameter for synthesizing sp³-bond-rich amorphous carbon with tailorable properties. These results improve our fundamental understanding of the structural transition of amorphous carbon under HPHT and provide crucial guidance for synthesizing novel amorphous carbon materials.