Three-step formation of diamonds in shock-compressed hydrocarbons: Dissociation, species separation, and nucleation

The accumulation and circulation of carbon and hydrogen contribute to the chemical evolution of ice giant planets. Species separation and diamond precipitation have been reported in carbon–hydrogen systems and have been verified by static and shock compression experiments. Nevertheless, the dynamic formation processes underlying these phenomena remain insufficiently understood. In combination with a deep learning model, we demonstrate that diamonds form through a three-step process involving dissociation, species separation, and nucleation processes. Under shock conditions of 125 GPa and 4590 K, hydrocarbons decompose to give hydrogen and low-molecular-weight alkanes (CH4 and C2H6), which escape from the carbon chains, resulting in C/H species separation. The remaining carbon atoms without C–H bonds accumulate and nucleate to form diamond crystals. The process of diamond growth is associated with a critical nucleus size at which the dynamic energy barrier plays a key role. These dynamic processes of diamond formation provide insight into the establishment of a model for the evolution of ice giant planets.