Bismuth Dibromide, BiBr₂

The evidence for the existence of Bismuth Dibromide, BiBr₂, is as unsatisfactory as that for the existence of the corresponding chlorine compound. Similar methods have been suggested for its preparation, and it is stated to be a brown, or grey, substance, crystallising in needles. Muir thought it probable that a lower bromide was formed by the reduction of the tribromide by hydrogen, but was unable to isolate the substance owing to its instability. Attempts to elucidate this problem by means of thermal investigation have also been undertaken. Herz and Guttmann state that the dibromide is a greyish-black substance with a density of 5.9 and a melting point of 198° C.; while Marino and Becarelli contend that no compound is formed in the system Bi-BiBr₃, but a series of solid solutions only. The latter investigators find the system similar to that of Bi-BiCl₃, the solid solution undergoing transformation into first a β-form and then a γ-form, the melting point of the γ-form always being higher than that of either bismuth or bismuth tribromide. On fusion and cooling, the γ-crystals decompose and deposit α-crystals of different composition, and two liquid layers are formed.