Melting Curve and Isostructural Solid Transition in Superionic Ice.

Area of Science:

• Earth and Planetary Science
• Materials Science
• Physical Chemistry

Background:

• The behavior of water ice under extreme conditions is crucial for understanding planetary interiors.
• Previous studies have explored various phases of ice, but high-pressure melting curves remain incompletely characterized.

Purpose of the Study:

• To experimentally determine the phase diagram and melting curve of water ice up to 45 GPa and 1600 K.
• To identify and characterize new high-pressure, high-temperature phases of water ice.

Main Methods:

• Synchrotron X-ray diffraction was employed using a diamond anvil cell (DAC).
• Resistive and laser heating techniques were utilized within the DAC to achieve high temperatures.
• Ab initio molecular dynamics simulations were performed for comparison and phase identification.

Main Results:

• A triple point was identified at 14.6 GPa and 850 K.
• Evidence for a first-order solid-solid transition from dynamically disordered ice VII (ice VII^{'}) to a high-temperature phase (ice VII^{''}) was observed.
• The high-temperature phase possesses a body-centered cubic (bcc) oxygen lattice, larger volume, and higher entropy than ice VII^{'}.

Conclusions:

• The experimental data confirm the existence of a triple point and a new high-temperature phase of water ice.
• The identified high-temperature phase aligns with theoretical predictions of superionic ice VII^{''}.
• This research advances our understanding of water ice under extreme conditions relevant to planetary science.