Modeling the Liquid-Liquid Phase Transition in Polymeric Sulfur

We suggest a simple model to describe the recently observed phase separation in liquid sulfur (Nature, 584, 382, 2020), which polymerizes at high temperatures and pressures. The model contains three types of interactions: i) atoms attract each other by van der Waals forces which produce the liquid-gas phase transition at low pressures, ii) atoms may form covalent bonds which induce polymerization, and iii) polymerized atoms attract each other by van der Waals forces stronger than nonpolymerized atoms. We show that the latter interaction couples polymerization with phase segregation and produces the liquid-liquid phase transition that was recently observed in sulfur. We implement these three conditions using a discrete molecular dynamics approach. The phase diagram and structure factor demonstrate a qualitative agreement with the behavior of real sulfur.