Probing ice VII crystallization from amorphous NaCl-D₂O solutions at gigapascal pressures

Area of Science:

• Geophysics
• Materials Science
• Crystallography

Background:

• High-pressure phases of water ice are crucial for understanding planetary interiors.
• Investigating solute incorporation in ice phases provides insights into extraterrestrial ice compositions.

Purpose of the Study:

• To determine if sodium chloride (NaCl) can be incorporated into the ice VII crystal lattice.
• To explore the behavior of salt solutions under high-pressure conditions relevant to planetary science.

Main Methods:

• Neutron diffraction experiments were conducted on a heavy water (D2O) and NaCl mixture at pressures between 2 and 4 gigapascals.
• Computational structure searches using density functional theory (DFT) were employed to model potential NaCl incorporation in ice VII.

Main Results:

• A high-density amorphous precursor crystallized into a structure closely resembling pure ice VII during high-pressure annealing.
• Experimental data showed no significant lattice expansion, which would be expected if NaCl ions substituted water molecules.
• Simulations predicted a substantial lattice expansion upon ion substitution, contradicting experimental observations.

Conclusions:

• The crystallized ice under the studied conditions is likely pure D2O ice or contains a very low concentration of NaCl.
• The ice VII lattice appears to resist significant incorporation of NaCl at pressures up to 4 gigapascals.