Temperature-pressure phase diagram of confined monolayer water/ice at first-principles accuracy with a machine-learning force field

B Lin and J Jiang and XC Zeng and L Li, NATURE COMMUNICATIONS, 14 (2023).

DOI: 10.1038/s41467-023-39829-z

Understanding the phase behaviour of nanoconfined water films is of fundamental importance in broad fields of science and engineering. However, the phase behaviour of the thinnest water film - monolayer water - is still incompletely known. Here, we developed a machine- learning force field (MLFF) at first-principles accuracy to determine the phase diagram of mono layer water/ice in nanoconfinement with hydrophobic walls. We observed the spontaneous formation of two previously unreported high-density ices, namely, zigzag quasi-bilayer ice (ZZ-qBI) and branched-zigzag quasi-bilayer ice (bZZ-qBI). Unlike conventional bilayer ices, few inter-layer hydrogen bonds were observed in both quasi-bilayer ices. Notably, the bZZ-qBI entails a unique hydrogen-bonding network that consists of two distinctive types of hydrogen bonds. Moreover, we identified, for the first time, the stable region for the lowest-density 4.8(2) monolayer ice (LD-48MI) at negative pressures (< -0.3 GPa). Overall, the MLFF enables large-scale first- principle-level molecular dynamics (MD) simulations of the spontaneous transition from the liquid water to a plethora of monolayer ices, including hexagonal, pentagonal, square, zigzag (ZZMI), and hexatic monolayer ices. These findings will enrich our understanding of the phase behaviour of the nanoconfined water/ices and provide a guide for future experimental realization of the 2D ices.

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