Phase Diagram and Dynamic Transformation Pathways of Carbon under Extreme Compression

Carbon exhibits complex structural transformations, melting behavior, and electronic-property evolution under extreme compression. This behavior is relevant to carbon-rich planetary interiors, the dynamic response of high-density carbon ablators in inertial confinement fusion, and the formation of post-diamond phases. This review focuses on the dynamic phase diagram of carbon. We summarize the equilibrium reference states of graphite, diamond, liquid carbon, and BC8 carbon, and discuss typical dynamic loading paths under shock, ramp, and multiple-shock compression together with their corresponding in situ diagnostics. We focus on the graphite-to-diamond/lonsdaleite-like transformation, diamond melting and liquid-carbon structure, and the kinetic accessibility of BC8 carbon. Existing studies show that the phases observed under dynamic compression are governed not only by pressure and temperature, but also by loading time, stress state, initial structure, and diagnostic window. Therefore, a dynamic phase diagram of carbon should extend the conventional pressure-temperature (p-T) description by incorporating loading path, leading to a p-T-path representation. This perspective may provide guidance for experimental design, in situ structural diagnostics, and equation-of-state model constraints for carbon under extreme conditions.