**Square ice formation in CrOCl and graphene confinement**

YJ Wang and JG Chen, JOURNAL OF STATISTICAL MECHANICS-THEORY AND EXPERIMENT, 2023, 063203 (2023).

DOI: 10.1088/1742-5468/acd693

The structure and thermodynamic properties of water in nanoscale confinement environment are greatly involved in the research field of material science and nanotechnology. However, a complete picture of the ordered structure formation and thermodynamics behavior of the confined water inside two parallel nanosheets with different surface atomic arrangement is still lacking. In this paper, by using molecular dynamics (MD) simulations, we study the structural variation and thermodynamics behavior for water molecules confined between two parallel CrOCl with a square surface atomic arrangement and two parallel graphene nanosheets with a hexagonal surface atomic arrangement. Square ice, with a lattice constant 2.1 and 2.0 angstrom, is observed inside the two parallel CrOCl and graphene nanosheets, respectively. By calculating the configuration entropy of the molecular dipoles S-t , it is found that, in the CrOCl confinement, S-t reaches a peak value and then is greatly reduced due to the square ice formation. On the other hand, in the graphene confinement, S-t continues to grow after the square ice formation and is then reduced after reaching its peak value. Interestingly, it is found that the square ice could be stable at a higher entropy state under the external pressure than the bulk water at ambient condition. By calculating the orientational order parameters M, it shows that the conventional tetrahedral geometry of hydrogen bonding between water molecules breaks due to the square ice formation. By analyzing the average number of hydrogen bonds of water molecules N-h , it is found that the hydrogen bond interaction of the square ice relies on the confinement environment, where N-h is reduced in the CrOCl confinement and increased in the graphene confinement. Probability distribution functions of the dipole orientation angles between the x- or z-axis and the projection of the oxygen atoms of the water molecules are also calculated. It is observed that the square ice structure is paralleled with the x-axis (unit cell axis) in the CrOCl confinement and tilted with the x-axis (the zigzag direction of graphene) at an angle 30 degrees in the graphene confinement. Furthermore, the square ice formation is found to be insensitive to temperature. Our result reveals the peculiar ordered structure and thermodynamics behavior of water in different nanoscale confinement environments.

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