Abstract
Using classical molecular dynamics with a more reliable reactive LCBOPII potential, we have performed a detailed study on the direct graphite-to-diamond phase transition. Our results reveal a new so-called “wave-like buckling and slipping” mechanism, which controls the transformation from hexagonal graphite to cubic diamond. Based on this mechanism, we have explained how polycrystalline cubic diamond is converted from hexagonal graphite, and demonstrated that the initial interlayer distance of compressed hexagonal graphite play a key role to determine the grain size of cubic diamond. These results can broaden our understanding of the high pressure graphite-to-diamond phase transition.
Highlights
Using classical molecular dynamics with a more reliable reactive LCBOPII potential, we have performed a detailed study on the direct graphite-to-diamond phase transition
In summary, a new ‘‘wave-like buckling and slipping’’ mechanism is proposed to probe into the atomistic mechanism for the direct graphite-to-cubic diamond phase transition
We found that the initial interlayer distance of the compressed hexagonal graphite determines the grain size of cubic diamond
Summary
We first report the results of the simulation from compressed hexagonal graphite (with 0.240 nm interlayer distance) to perfect cubic diamond (see Fig. 1). At the elastic compression stage, the stress along the x direction increases linearly with increasing strain up to Figure 1 | Transformation of hexagonal graphite with 0.240 nm interlayer distance into momocrystal cubic diamond. We can see that the hexagonal graphite layers first buckle at a critical compressive stress, and in the meantime the slipping of the graphite layers leads to the stacking order of the graphite layers transforming from ‘‘ABAB’’ to ‘‘AAAA’’. Our simulations show that when the interlayer distance of the hexagonal graphite is between 0.240–0.228 nm, the graphite will transform into a perfect crystal of cubic diamond with a critical buckling angle of 30u. When the stress along the x direction reaches 92.1 GPa, the graphite begins to transform into hexagonal diamond It can be see that the maximum energy for interlayer distance 0.208 nm (1.18 eV) is much lower than those for other interlayer distances, which indicates that it is much easier to form hexagonal diamond than to form cubic diamond from compressing hexagonal graphite, this is because the high symmetry of the hexagonal graphite to hexagonal diamond transformation pathway
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