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

We probe the possible inclusion of salt (NaCl) in the ice VII lattice over the pressure range from 2 to 4 gigapascal. We combine data from neutron diffraction experiments under pressure and from computational structure searches based on density functional theory. We observe that the high density amorphous precursor (NaCl·10.2D₂O) crystallises during annealing at high pressure in the vicinity of the phase boundary between pure ices VII and VIII. The structure formed is very similar to that of pure ice VII. Our simulations indicate that substituting water molecules in the ice VII lattice with Na⁺ and Cl^- ions would lead to a significant expansion of the lattice parameter. Since this expansion was not observed in our experiments, the ice crystallised is likely to be pure D₂O or contains only a small fraction of the ions from the salt solution.