Room temperature electrofreezing of water yields a missing dense ice phase in the phase diagram

Abstract

Water can freeze into diverse ice polymorphs depending on the external conditions such as temperature (T) and pressure (P). Herein, molecular dynamics simulations show evidence of a high-density orthorhombic phase, termed ice χ, forming spontaneously from liquid water at room temperature under high-pressure and high external electric field. Using free-energy computations based on the Einstein molecule approach, we show that ice χ is an additional phase introduced to the state-of-the-art T-P phase diagram. The χ phase is the most stable structure in the high-pressure/low-temperature region, located between ice II and ice VI, and next to ice V exhibiting two triple points at 6.06 kbar/131.23 K and 9.45 kbar/144.24 K, respectively. A possible explanation for the missing ice phase in the T-P phase diagram is that ice χ is a rare polarized ferroelectric phase, whose nucleation/growth occurs only under very high electric fields.

Fig. 1

Room-temperature electrofreezing of liquid water. Snapshots of ice χ obtained at T = 270 K and E = 2.3 V nm⁻¹: view in the (a) z-axis and (b) y-axis direction. c Computed radial distribution function (RDF) of oxygen atoms for ice χ. d Time-dependent potential energy per water molecule for the system at P = 5 kbar, T = 270 K, and E = 2.3 V nm⁻¹. Oxygen atoms are depicted as red balls, hydrogen atoms as white sticks, and hydrogen bonds as blue dotted lines

Fig. 2

A hysteresis loop of the dipole moment per water molecule for ice χ. A hysteresis loop (blue and red lines) of the dipole moment D_z per water molecule for ice χ, based on a MD simulation at P = 5 kbar and T = 200 K. The electric field E_Z is applied along or against the z-axis. The black square at E_Z = 0 corresponds to the value of 2.211 Debye, set as the initial polarization of ice χ. The black-square line illustrates the increasing trend of electric field strength E_Z. The red line illustrates the decreasing trend of E_Z along the z-axis until E_Z reaches at zero. Thereafter, E_Z increases again in the reverse direction (opposite to z-axis). The permanent electric dipole moment of a single TIP4P/2005 water molecule is 2.305 Debye

Fig. 3

P–E phase diagram and the density of ice phases. a A semi-quantitative P–E phase diagram of TIP4P/2005 water for T = 270 K. The error bar of the electric field strength is 0.05 V nm⁻¹. Polar ice B is denoted by the yellow region, ice χ is denoted by the green region, very-high-density amorphous (VHDA) ice is denoted by the pink region, and the liquid phase is denoted by the blue region. b The density of the ice phase vs. P (at T = 270 K and E = 3.0 V nm⁻¹). The different colour circles correspond to different ice polymorphs, denoted by the same colour in a

Fig. 4

Relative enthalpy per water molecule. Relative enthalpy per water molecule (based on vdw-DF2 calculations without including ZPE correction) versus P for ice χ, ice II, ice XI, ice B, and polar ice B, where ice VI is taken as the reference in the calculation

Fig. 5

Phase diagram for TIP4P/2005 water model. The T–P phase diagram for TIP4P/2005 water model, obtained from free-energy calculations