Structure, Stoichiometry, and Morphology of Bromine Hydrate^†

The hydrate of bromine was one of the first clathrate hydrates discovered. It has played a significant role in the development of the solid solution theory of clathrate hydrates, yet its detailed structure remains unknown. This hydrate again has become a test case for two different views of clathrates:  the solid solution model, after van der Waals and Platteeuw, that sees clathrates as unstable lattices which derive stability from a minimum degree of cage filling and thus are nonstoichiometric, and a view promoted by Dyadin and Aladko that all large cages in a hydrate structure need to be filled. In light of the latter view, existing data obtained over the last ∼160-year period on the composition and morphology of bromine hydrate would require the existence of four different hydrate structures. Our single crystal diffraction study of 16 different crystals of distinct compositions (Br₂·8.62H₂O to Br₂·10.68H₂O) and morphologies showed that there is just a single structure (tetragonal, P4₂/mnm, a = 23.04 Å, c = 12.07 Å, the structure originally proposed by Allen and Jeffrey) with considerable variation in the degree of occupancy of the large cages. The results favor the solid solution model for clathrates, and settle the question of long standing regarding the structure(s) of bromine hydrate. The bromine atoms occupy the large 14- and 15-hedral cages with up to 15 different crystallographically independent sites per cage and fractional occupancies from 0.19 to <∼0.01. The bromine hydrate structure is unique, so far. ¹²⁹Xe NMR results suggest that when attempts were made to produce a double hydrate of bromine and xenon, a transient cubic structure II hydrate resulted, which slowly converted to the tetragonal form.