Novel High‐Pressure Potassium Chloride Monohydrate and Its Implications for Water‐Rich Planetary Bodies

Abstract Saline water is a common fluid on the Earth‘s surface and in ice planets. Potassium chloride (KCl) is a common salt and is expected to be a ubiquitous solute in salt water in the Universe; however, few studies investigated the behavior of KCl‐H 2 O system at high pressures and temperatures. In this study, powder and single‐crystal X‐ray diffraction (SC‐XRD), Raman and Brillouin scattering combined with diamond anvil cells were used to investigate the phase relation in the KCl‐H 2 O system for different KCl concentrations at 0–4 GPa and 298–405 K. The results of powder X‐ray diffraction and Raman scattering demonstrate that a novel KCl hydrate is formed when KCl aqueous solutions transform to solid ice‐VI and ice‐VII at high pressure. Simultaneously, the single‐crystal of KCl hydrate is synthesized from a supersaturated KCl solution at 298 K and 1.8 GPa. The structure is solved by SC‐XRD, indicating a KCl monohydrate with the P 2 1 / n space group is formed. We have verified the phase stability of KCl monohydrate by using Raman spectroscopy and density functional theory. Our results indicate that KCl monohydrate is a stable phase under pressure and temperature conditions between 1.6 and 2.4 GPa and 298–359 K. By considering the thermal profile and composition of icy moons, we hypothesize that the formation and decomposition of KCl monohydrate might induce mantle convection in these moons.