  Graphite and diamond are two well-known allotropes of carbon with distinct
physical properties due to different atomic connectivity. Graphite has a
layered structure in which the honeycomb carbon sheets can easily glide, while
atoms in diamond are strongly bonded in all three dimensions. The transition
from graphite to diamond has been a central subject in physical science. One
way to turn graphite into diamond is to apply the high pressure and high
temperature (HPHT) conditions. However, atomistic mechanism of this transition
is still under debate. From a series of large-scale molecular dynamics (MD)
simulations, we report a mechanism that the diamond nuclei originate at the
graphite grain boundaries and propagate in two preferred directions. In
addition to the widely accepted [001] direction, we found that the growth along
[120] direction of graphite is even faster. In this scenario, cubic diamond
(CD) is the kinetically favorable product, while hexagonal diamond (HD) would
appear as minor amounts of twinning structures in two main directions.
Following the crystallographic orientation relationship, the coherent interface
t-(100)gr//(11-1)cd + [010]gr//[1-10]cd was also confirmed by high-resolution
transmission electron microscopy (HR-TEM) experiment. The proposed phase
transition mechanism does not only reconcile the longstanding debate regarding
the role of HD in graphite-diamond transition, but also yields the atomistic
insight into microstructure engineering via controlled solid phase transition.
