Evidence and Stability Field of fcc Superionic Water Ice Using Static Compression

Area of Science:

• High-pressure physics
• Materials science
• Geophysics

Background:

• Understanding the behavior of water ice under extreme conditions is crucial for planetary science and fusion energy research.
• Previous studies on hot dense water ice have provided limited insights into its structural phases.
• Ab initio simulations offer theoretical predictions but require experimental validation.

Purpose of the Study:

• To investigate the structural transformation of hot dense water ice.
• To identify phase transitions and determine the stability fields of different ice phases.
• To experimentally validate theoretical models of warm dense water ice.

Main Methods:

• Synchrotron X-ray diffraction
• Laser-heating diamond anvil cell
• High-pressure and high-temperature experiments

Main Results:

• Observed a transition from body-centered-cubic (bcc) to face-centered-cubic (fcc) oxygen sublattices from 57 GPa and 1500 K to 166 GPa and 2500 K.
• Identified the structural signature of the transition to face-centered-cubic superionic (fcc SI) ice.
• Determined the sign of the density discontinuity at the transition and revealed an extended fcc SI stability field.
• Constrained the stability field of body-centered-cubic superionic (bcc SI) ice up to at least 100 GPa.

Conclusions:

• The study provides experimental evidence for the existence and stability of fcc SI ice.
• The findings refine the phase diagram of hot dense water ice.
• The results offer critical data for validating and improving ab initio simulations of warm dense water ice.