Evidence for superionic H₂O and diffusive He-H₂O at high temperature and high pressure

Area of Science:

• High-pressure physics
• Materials science
• Planetary science

Background:

• Understanding the behavior of water under extreme conditions is crucial for planetary science.
• Previous studies have explored various phases of water at high pressures, but the formation of diffusive phases with noble gases remains less understood.

Purpose of the Study:

• To investigate the formation and structural properties of superionic water (H2O) and a novel diffusive H2O-Helium (He) phase.
• To elucidate the distinct transition dynamics and underlying mechanisms governing these phases.

Main Methods:

• Time-resolved X-ray diffraction experiments were conducted on laser-heated samples within diamond anvil cells.
• Analysis focused on lattice parameter evolution over time and temperature at gigapascal (GPa) pressures.

Main Results:

• Evidence for a superionic H2O phase and a newly identified diffusive H2O-He phase was obtained.
• Both phases exhibit a similar body-centered cubic (bcc)-like structure.
• Superionic H2O forms gradually (1350-1400 K at 23 GPa), while the diffusive H2O-He phase forms abruptly (1300 K at 26 GPa).

Conclusions:

• The faster transition dynamics and lower formation temperature of the H2O-He phase are attributed to the higher diffusion coefficient of interstitial He compared to bound H atoms.
• The findings support theoretical predictions of a He-disordered diffusive phase at lower temperatures in H2O-He mixtures.