Dehydration of Magnesium Bromide Hexahydrate Studied by in situ X‐ray Powder Diffraction

Laboratory X‐ray powder diffraction data were used to investigate the dehydration process of magnesium bromide hexahydrate in the temperature range 300 K ≤ T ≤ 420 K. By heating of the as synthesized hexahydrate (MgBr₂ · 6H₂O, observed in the temperature range 300 K ≤ T ≤ 349 K), three lower hydrates can be obtained in overlapped temperature regions: MgBr₂ · 4H₂O (332 K ≤ T ≤ 367 K), MgBr₂ · 2H₂O (361 K ≤ T ≤ 380 K) and MgBr₂ · H₂O (375 K ≤ T ≤ 390 K). Although the crystal structure of the hexahydrate was published almost eighty years ago, there are no data on the structures of the lower hydrates. The crystal structures are reported and are found to be isotypical with the structures of the respective chlorides. The structure of MgBr₂ · 6H₂O is characterized by discrete Mg(H₂O)₆ octahedra and is the only hydrate of this group that contains unbonded Br^– anions. MgBr₂ · 4H₂O is composed of discrete MgBr₂(H₂O)₄ octahedra, and the structure was found to be disordered. The crystal structure of MgBr₂ · 2H₂O is formed by single chains of edge‐sharing MgBr₄(H₂O)₂ octahedra, while in the case of MgBr₂ · H₂O double chains of edge‐shared MgBr₅H₂O are formed. By increasing the temperature, as expected, positive thermal expansion was evidenced. Thermal expansion coefficients, based on the changes of the unit cell parameters, were derived for the following hydrates: MgBr₂ · 6H₂O, MgBr₂ · 4H₂O, and MgBr₂ · 2H₂O.