A Revisited Mechanism of the Graphite-to-Diamond Transition at High Temperature

• Published in: Matter Vol. 3; no. 3; pp. 864 - 878
• Main Authors: Zhu, Sheng-cai, Yan, Xiao-zhi, Liu, Jin, Oganov, Artem R., Zhu, Qiang
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 02.09.2020; Cell Press/Elsevier
• Subjects: crystal growth; interfaces; MATERIALS SCIENCE; molecular dynamics simulation; phase transition; superhard materials; MAP 3: Understanding; interfaces; superhard materials; phase transition; crystal growth; molecular dynamics simulation
• ISSN: 2590-2385; 2590-2385
• DOI: 10.1016/j.matt.2020.05.013

The graphite-diamond transition, under high-pressure and high-temperature conditions, has been a central subject in physical science. However, its atomistic mechanism remains under debate. Employing large-scale molecular dynamics (MD) simulations, we report a mechanism whereby the diamond nuclei in the graphite matrix propagate in two preferred directions, among which the graphite [120] is about 2.5 times faster than [001]. Consequently, cubic diamond (CD) is the kinetically favorable product, while only a few hexagonal diamonds (HDs) can exist as the twins of CDs. The coherent interface of t-(100)gr//(11-1)cd + [010]gr//[1-10]cd observed in MD simulation was confirmed by our high-resolution transmission electron microscopy experiment. The proposed mechanism not only clarifies the role of HD in graphite-diamond transition but also yields atomistic insight into strengthening synthetic diamond via microstructure engineering. [Display omitted] •Both experiment and simulation were employed to study the graphite-diamond transition•The simulation suggests that diamond grows faster in graphite [120] than in [001]•This graphite-diamond interface model from simulation is consistent with experiments•The uncovered mechanism can be used to improve the synthetic diamond The graphite-diamond phase transition is a central subject in physical science. Among the debates after many years of studies, one outstanding issue is the role of hexagonal diamond (HD), which was argued to be the preferable product according to the simulation but never reported in the compression experiments on graphite under high-pressure and high-temperature (HPHT) conditions. From a synergy between experiment and simulation, we investigated the atomistic mechanism of the graphite-diamond transition in HPHT conditions. Our study suggests that the growth of diamond has a preferred direction, which notably favors the formation of cubic diamond (CD). On the other hand, HD only appears as the twin structures of CD. We further investigated the possibility of harvesting the twin structures via microstructure engineering, which may have the potential to advance the fabrication process in the synthetic diamond industry. We performed large-scale molecular dynamics simulations on the graphite-diamond transition under high-temperature, high-pressure conditions. The simulations suggested that diamond nuclei would emerge due to the corrugation and thermal fluctuation of graphite layers and then grow in a preferred direction along the graphite [120] direction, resulting in the cubic diamond phase being the kinetically favorable product while the hexagonal phase would appear as minor amounts of twin structures. The simulated coherent interface is confirmed by subsequent high-resolution transmission electron microscopy experiments.