  The direct graphite-to-diamond transformation mechanism has been a subject of
intense study and remains debated concerning the initial stages of the
conversion, the intermediate phases, and their transformation pathways. Here,
we successfully recover samples at early conversion stage by tuning
high-pressure/high-temperature conditions and reveal direct evidence supporting
the nucleation-growth mechanism. Atomistic observations show that intermediate
orthorhombic graphite phase mediates the growth of diamond nuclei. Furthermore,
we observe that quenchable orthorhombic and rhombohedra graphite are stabilized
in buckled graphite at lower temperatures. These intermediate phases are
further converted into hexagonal and cubic diamond at higher temperatures
following energetically favorable pathways in the order: graphite ->
orthorhombic graphite -> hexagonal diamond, graphite -> orthorhombic graphite
-> cubic diamond, graphite -> rhombohedra graphite -> cubic diamond. These
results significantly improve our understanding of the transformation
mechanism, enabling the synthesis of different high-quality forms of diamond
from graphite.
