From Diamond to BC8 to Simple Cubic—and Back: Kinetic Pathways of Carbon’s Post-Diamond Phases from Metadynamics

We study structural transformations between single crystal diamond and post-diamond phases of carbon, employing metadynamics and machine-learning Spectral Neighbor Analysis Potential (SNAP) for carbon. Specific focus is on exploring transformation pathways to theoretically predicted but yet experimentally unobserved BC8 phase. The initial phases of diamond - cubic and hexagonal - are used to investigate the transitions to post-diamond phases at pressures of 15–21 Mbar and temperatures of 3000–5000 K. In this presentation, we will describe the atomic-scale details of early transformation stages including homogeneous nucleation inside the parent diamond phase. We estimate the free-energy barrier associated with these transformations and identify favorable conditions for the experimental observation of the diamond-to-BC8 transition. We also explore evolution of post-diamond carbon phases upon pressure release, specifically focusing on their potential recovery at ambient conditions. Our findings offer valuable guidance for experimental exploration of exotic carbon phases.