Two Dimensional Ice from First Principles: Structures and Phase Transitions

Abstract

Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here, we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression, the pentagonal structure becomes the most stable and persists up to ∼2 GPa, at which point the square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and the width.

Multilayer Two-Dimensional Water Structure Confined in MoS2

“…3,[14][15] Theoretical investigations of the structures of the two-dimensional (2D) water confined in between the flat walls have suggested puckered rhombic monolayer ice, planar hexagonal, or amorphous phases depending on the conditions and models employed in the simulations. [16][17][18][19][20][21][22][23][24][25][26][27][28][29] Although the structures of confined water have been predicted for a variety of dimensions and materials using molecular dynamics (MD) simulations, the first experimental observation of the 2D water in between the two graphene sheets was obtained very recently using high-resolution transmission electron microscopy measurements (TEM). 30 This observation revealed the formation of a monolayer of planar "square" ice with a high packing density and, depending on the inter-graphene distance, the formation of bi-and trilayer crystallites of water.…”

Evidence for stable square ice from quantum Monte Carlo

“…However, these experiments have been questioned [2], with it even being suggested that it is sodium chloride contamination and not ice that is responsible for the square symmetry observed. So far, it is not clear under what conditions (if any) square ice is stable.Theoretical investigations of the stability of confined 2D ice at high lateral pressures can, in principle, help in disentangling this issue and in complementing experimental findings [3][4][5][6][7][8][9][10]. From a theoretical perspective, the prediction of square 2D ice can be traced back to Nagle's 1970's "unit model" of ice [3].…”

“…However, later atomistic force field (FF) simulations found that 2D ice prefers a buckled rhombic structure [4,5]. More recently, density functional theory (DFT) based investigations [6][7][8][9] have been performed. However, these have produced qualitatively different results depending on the precise details of the calculations and, in particular, on the choice of exchange-correlation (XC) functional.…”

“…However, these have produced qualitatively different results depending on the precise details of the calculations and, in particular, on the choice of exchange-correlation (XC) functional. For instance, Chen et al [7] found hexagonal and pentagonal structures to be stable phases at low pressures ( Fig. 1).…”

Perspective: Role of structure prediction in materials discovery and design

“…For example, one can concentrate searches on particular space groups that have been found by experiment or previous searches to be common in certain types of structure, such as in inorganic or organic crystals. 83 Structure searching is also very flexible and can readily be adapted for discovering point defect structures, 84,85 surface and interface structures, [86][87][88] etc. The most important aspects of searching are illustrated in Fig.…”