Metastable hydrate of sodium chloride is a pentahydrate as revealed by 3D electron diffraction

Mixtures of water and NaCl, the archetypal salt, play important roles in industrial processes and natural environments on Earth and throughout the Solar System, yet their structural diversity at low temperatures remains poorly understood. Here, we determine the crystal structure of the metastable sodium chloride hydrate formed upon flash freezing using three-dimensional electron diffraction. Our analysis reveals a sodium chloride pentahydrate (NaCl∙5H2O), which consists of one-dimensional chains of hydrated sodium octahedra, with Cl− ions and water molecules located between the chains and linked by hydrogen-bonding interactions. NaCl∙5H2O introduces a new structural motif in the NaCl hydrate family and differs considerably from the thermodynamically stable and denser sodium chloride dihydrate that forms upon slow cooling. Fully dissociated ions in this metastable phase suggest that rapid freezing of saline solutions may provide a synthesis route to hydrates that retain local coordination environments resembling those of the parent solution.