| Water is one of the most common materials on the earth,which is praised as the source of life.It exhibits a variety of crystalline phases due to the extraordinary ability of water molecules to form diverse network of hydrogen bonds in response to different external pressure and temperature.The physical and chemical properties and dynamic features of nanoconfined water are very different from those of bulk states.Besides,water molecules possess dipole moments,which can be easily polarised under electric field.So researching on nanoconfined or electric-field water,including phase behavior,physical properties,chemical properties and flow property,is in favor of us to understand the structure of water.We can develop more important role of water in physics,chemistry,energy,environment and material if we understand the structure property of wate more deeply.The main work of this paper is studying water phase behaviors under different conditions,and finding out unknown properties of water by MD simulations.The main content is summarized as follows:(1)An ideal hydrogen bonding network of water should satisfy three requirements:Four hydrogen bonds connected with every water molecule,nearly linear hydrogen bonds and tetrahedral configuration for the four hydrogen bonds around an O atom.However,under nanoscale confinement,some of the three requirements have to be unmet,and the selection of the specific requirement(s)leads to different types of hydrogen bonding structures.According to molecular dynamics(MD)simulations for water confined between two smooth hydrophobic walls,we obtain a phase diagram of three two-dimensional(2D)crystalline structures and a bilayer liquid.A new 2D bilayer ice is found and named as the interlocked pentagonal bilayer ice(IPBI),because its side view comprises interlocked pentagonal channels.The basic motif in the top view of IPBI is a large hexagon composed of four small pentagons,resembling the top view of a previously reported"Coffins" bilayer ice.However,there are fundamental differences between the two bilayer structures due to the difference in the selection among the three requirements.The IPBI sacrifices the linearity of hydrogen bonds to retain locally tetrahedral configurations of the hydrogen bonds,whereas the coffins structure does the opposite.The tradeoff between the conditions of an ideal hydrogen bonding network can serve as a generic guidance to understand the rich phase behaviors of nanoconfined water.(2)Despite recent experimental evidence of the 2D square ice in graphene nanocapillaries,based on transmission electron microscope(TEM)imaging,the AA-stacked bilayer square ice structure has not been observed in all previous classical MD simulations,nor found in recent unbiased first-principles structure searches.Herein,we report MD simulations of 2D bilayer ice formation for water confined between two parallel hydrophobic walls.We find a bilayer ice whose simulated TEM imaging resembles that of bilayer square-like ice.However,the realistic structure of this bilayer ice consists of two hexagonal monolayers with the AB stacking order,where the hexagonal rings are slightly elongated with two unequal inner angles,107° and 146°(rather than 120°).Based on extensive MD simulations,structural properties of BHI-AB and related phase behavior are investigated.BHI-AB can be derived by slowly reducing the nano-slit width,with BHI-AA as the initial structure in the nano-slit.It also can transform to BL-VHDI by compression effect of lateral pressure.In both two courses of phase transitions,a solid-to-liquid-to-solid phase transition behavior(or Oswald staging)can arise.The new insights obtained from this systematic study of bilayer ices can be useful for understanding many intriguing phase behaviors of 2D water/ice.(3)Water is ubiquitous in nature and can freeze into diverse ice polymorphs depending on the external temperature(7)and pressure(P).Herein,we show simulation evidence of a very-high-density(p>1.25 g/cm3)bulk ice phase,termed ice "XX",that can form spontaneously from liquid water at room temperature under high pressure and high external electric field.Ice "XX" exhibits an orthorhombic structure with 56 molecules per unit cell.Most surprisingly,ice "XX" is a "missing"phase in the state-of-the-art T-P phase diagram,as the free-energy computation shows that ice "XX" is the most stable structure in the high-pressure/low-temperature region,located between ice Ⅱ and ice Ⅵ and next to ice V,thereby giving two triple points(6.063 kbar/131.23 K and 9.450 kbar/144.24 K).A possible explanation for the "missing" ice phase in the T-P phase diagram is that ice "XX" is a rare polarized ferroelectric phase,whose nucleation/growth occurs only under very high electric fields. |
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