Pressure-induced change in the local Peierls distortion structure of molten selenium

It was recently reported that molten sulfur exhibits a first-order liquid-liquid transition (LLT) with a critical point. The phase diagrams of selenium (Se) and sulfur are very similar, and experiments have suggested that LLT may also happen in molten Se. Here, complete ab initio molecular dynamics simulations (AIMD) and a combination of AIMD and machine learning are used to study the pressure-induced structural change of molten Se. Here the simulation density range is expanded, the number of atoms and simulation time are significantly increased, and the van der Waals correction (VDW) is considered. According to our findings, the liquid structure undergoes two changes along the 1000 K isotherm. Nevertheless, neither change is first order. The significance of the VDW interaction is demonstrated by the perfect agreement of the transition pressure with the experiments after considering the VDW correction. We contend that the increase in conductivity seen in the experiments is connected to the first structural change, which results from the effect of temperature on the weakly Peierls distorted structures. The maximum of the melting curve of crystalline Se-I is associated with the second structural change, which is the pressure-induced Peierls transition.