Metastable Dihydrate\nof Sodium Chloride at Ambient\nPressure

Sodium chloride (NaCl)\nplays an important role in geochemistry,\nbiology, industry, and food production, and it is among the most common\nsalts in the solar system. Here, we report the discovery of a metastable\nNaCl dihydrate formed through rapid freezing (10<sup>1</sup>–10<sup>2</sup> K s<sup>–1</sup>) of a NaCl solution at ambient pressure.\nUsing synchrotron X-ray and neutron powder diffraction, we show that\nit transforms irreversibly to hydrohalite and ice Ih above 190 K upon\nheating and propose it is structurally related to hydrohalite with\na 3 × 1 × 3 supercell as its unit cell. Calorimetric analyses\nreveal that the new hydrate transforms to hydrohalite with a heat\nrelease of −3.47 ± 0.55 kJ mol<sup>–1</sup>. The\nidentification of this new NaCl dihydrate on the surfaces of icy worlds\nsuch as the moons of Jupiter and Saturn could indicate regions of\nrecent activity where subsurface brines have frozen rapidly, priority\ntargets for upcoming planetary missions.