Structure and dynamics of water plastic crystals from computer simulations

Area of Science:

• Physical chemistry
• Materials science
• Geophysics

Background:

• Water exhibits a complex phase diagram with various crystalline structures.
• High-pressure and high-temperature water phases are crucial for understanding Earth's mantle and exoplanetary interiors.
• Computational simulations have predicted novel plastic crystal phases of water, yet these remain experimentally unconfirmed.

Purpose of the Study:

• To fully characterize the structure, dynamics, and thermodynamics of simulated plastic crystal phases of water.
• To investigate the interplay between structure, dynamics, and thermodynamics in these phases.
• To compare simulated plastic crystal phases with high-pressure liquid and Ice VII phases.

Main Methods:

• Molecular dynamics simulations were employed to study water's behavior.
• The two-phase thermodynamic method was utilized for comprehensive analysis.
• Reorientational correlation functions were calculated to determine relaxation times.

Main Results:

• The study characterized hypothetical body-centered-cubic (bcc) and face-centered-cubic (fcc) plastic crystal phases of water at 440 K and 8 GPa.
• Relaxation times for different reorientational correlation functions were determined for these phases.
• Entropy analysis indicated that the fcc plastic crystal phase is more stable than the bcc phase under the studied conditions.

Conclusions:

• The computational findings provide a detailed understanding of the structure, dynamics, and thermodynamics of plastic crystal water phases.
• The results offer insights into the behavior of water under extreme conditions relevant to planetary science.
• The fcc phase's greater stability suggests its potential significance in high-pressure water systems.