The preparation and structure of salty ice VII under pressure

It is widely accepted that ice, no matter what phase, is unable to incorporate large amounts of salt into its structure. This conclusion is based on the observation that on freezing of salt water, ice expels the salt almost entirely as brine. Here, we show that this behaviour is not an intrinsic physico-chemical property of ice phases. We demonstrate by neutron diffraction that substantial amounts of dissolved LiCl can be built homogeneously into the ice VII structure if it is produced by recrystallization of its glassy (amorphous) state under pressure. Such ‘alloyed’ ice VII has significantly different structural properties compared with pure ice VII, such as an 8% larger unit cell volume, 5 times larger displacement factors, an absence of a transition to an ordered ice VIII structure and plasticity. Our study suggests that there could be a whole new class of ‘salty’ high-pressure ice forms. Freezing water containing salts is believed to produce pure ice and a salt hydrate. Neutron-diffraction measurements of the ice phase obtained by recrystallizing the glassy state of LiCl salt solution at high pressure suggests something different. The data reveal an ‘alloyed’ ice VII structure incorporating Li and Cl ions.