Hydrate formation at extreme conditions

A search of the Inorganic Crystal Structure Database for hydrates of the group I binary halides shows that the most extensive series is formed by lithium, for which mono-, di-and trihydrates are known for the chloride, bromide and iodide. The most highly hydrated species are the pentahydrates formed by LiCl and LiBr (Sohr et al. , 2018). Below lithium, the hydrate chemistry drops off quickly. Sodium chloride, bromide and iodide form only dihydrates, and amongst the heavier elements, only KF (cid:3) 2H 2 O (Preisinger et al. , 1994), KF (cid:3) 4H 2 O (Beurskens & Jeffrey, 1964) and RbF (cid:3) H 2 O (Troyanov, 2005) are known. This series of materials contrasts with the halides of the more highly charged group II metals, which have an extensive hydrate chemistry, including some very highly hydrated examples, such as MgCl 2 (cid:3) 12H 2 O (Sasvari & Jeffrey, 1966). The propensity towards hydrate formation by lithium can be regarded as an example of a diagonal relationship in the periodic table. Interest in hydrates of simple salts stems from their importance as possible mineral phases on icy planets and moons. While the majority of work has been carried out at ambient pressure, characterizing the behaviour of hydrates at high pressure, yielding data such as bulk moduli, as well as new and perturbed crystal structures, is important for developing geophysical models of extraterrestrial environments (Grindrod