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 < Tc < 1423 K, 0.06 GPa < Pc < 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 EF in LDL, which becomes more shallow in HDL.