Abstract
Phase transitions between carbon allotropes are calculated using the generalized solid-state nudged elastic band method. We find a new reaction mechanism between graphite and diamond with nucleation characteristics that has a lower activation energy than the concerted mechanism. The calculated barrier from graphite to hexagonal diamond is lower than to cubic diamond, resolving a conflict between theory and experiment. Transitions are calculated to three structures of cold compressed graphite: bct C4, M, and Z-carbon, which are accessible at the experimentally relevant pressures near 17 GPa.
Highlights
Carbon has many crystalline phases, including graphite, cubic diamond (CD), hexagonal diamond (HD), and cold compressed graphite (CCG)
It has been found that both transitions can occur at 15 GPa static pressure and the temperature required to form HD is lower than CD,4,5 indicating that the barrier to form HD is lower at this pressure
Previous saddle point calculations have focused on the concerted mechanism, where the barrier to HD is higher than to CD, in disagreement with experiments
Summary
Carbon has many crystalline phases, including graphite, cubic diamond (CD), hexagonal diamond (HD), and cold compressed graphite (CCG).
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