First-order liquid–liquid phase transition in AsS melt caused by change of intermediate-range order

Using ab initio molecular dynamics simulations, we provide direct evidence for the existence of a first-order liquid–liquid phase transition (LLPT) in AsS melt, with a negative Clapeyron slope of the transition line and a critical point around (1323 K < T _c < 1423 K, 0.06 GPa < P _c < 0.16 GPa). Our predicted transition line is in good agreement with the boundary between high density liquid (HDL) and low density liquid (LDL) suggested in experiment, especially the location of the HDL–LDL–Crystal triple point. Our results show that this first-order LLPT primarily originates from the change of intermediate-range order, i.e., the fourth coordination shell at 4.7 Å and the fifth at 6.7 Å around a S atom in LDL merge into one at 5.4 Å in HDL. Based on our results, a negative thermal expansion coefficient in LDL is predicted. With increasing pressure, the number of 2-fold S-coordinated As atoms decreases in HDL, while that of 3-fold S-coordinated As atoms increases. The total electronic density of states has a deep dip at E_F in LDL, which becomes more shallow in HDL.