44 / 2023-04-08 10:56:47

The Graphite-Hexagonal Diamond Phase Transition Mechanism resolved by Molecular Dynamics

Hexagonal Diamond,,Phase Transition Mechanism

高压物理与材料科学 > 相变与机制-海报

In the past several years, most high-temperature and high-pressure experiments have found that graphite mainly transforms into cubic diamonds, with a small number of hexagonal diamonds existing as twins. It was once believed that hexagonal diamond could not exist as a discrete phase. However, recently, researchers from the HPSTAR obtained the pure phase of hexagonal diamond using laser heating method, and measured its hardness for the first time in experiments, discovering that its hardness exceeded that of cubic diamond. However, so far, it is still unclear why graphite transforms into hexagonal diamond under this condition. In this work, we used molecular dynamics simulations to study the phase transition mechanism of "graphite hexagonal diamond" at such local high temperatures, and resolved the phase transition selective effect. Under local heating conditions, stress is generated in the central heating zone of graphite, which changing the microstructure, controlling the subsequent phase selectivity. Specifically, the main product of graphite phase transformation under quasi-hydrostatic pressure conditions is hexagonal diamond, while cubic diamond is the main product in hydrostatic pressure and large uniaxial pressure simulations (Figure 1). The reason is that under local heating conditions, different pressure environments can lead to different microstructural changes, such as varying the degree of buckling of the graphite, resulting in different phase transition. This study reveals the microscopic mechanism of laser heating synthesis of diamond, providing new ideas for large-scale synthesis of superhard hexagonal diamond.