Liquid–liquid phase transition in simulations of ultrafast heating and decompression of amorphous ice

Abstract

• Ultrafast heating experiments on amorphous ice clarified by simulations. • High density amorphous ice heated to liquid state by ultrafast laser pulse. • Decompression of high density liquid reveals liquid–liquid transition. • Simulations detect structural transition along out-of-equilibrium paths. A recent experiment [K. H. Kim, et al. , Science 370 , 978 (2020)] showed that it may be possible to detect a liquid–liquid phase transition (LLPT) in supercooled water by subjecting high density amorphous ice (HDA) to ultrafast heating, after which the sample reportedly undergoes spontaneous decompression from a high density liquid (HDL) to a low density liquid (LDL) via a first-order phase transition. Here we conduct computer simulations of the ST2 water model, in which a LLPT is known to occur. We subject various HDA samples of this model to a heating and decompression protocol that follows a thermodynamic pathway similar to that of the recent experiments. Our results show that a signature of the underlying equilibrium LLPT can be observed in a strongly out-of-equilibrium process that follows this pathway despite the very high heating and decompression rates employed here. Our results are also consistent with the phase diagram of glassy ST2 water reported in previous studies.