The compounds Ga2Br4 (α+β) have been prepared by reacting GaBr3 with Ga metal without a solvent or in a hydrocarbon medium. α-Ga2Br4 is isotypic with Ga2Cl4, and β-Ga2Br4 with Ga2I4. Both forms have to be formulated as Ga+(GaBr4)- according to their crystal structures. The first order phase transition α→β has been investigated by means of DTA and temperature dependent X-ray powder photographs.
The crystal structure of β-Ga2Br4 has been refined with single crystal data (R3c; a = 2163.4(8) pm; c = 869.2(4) pm; Z = 18; R = 0.037). Main building units are GaBr4- tetrahedra (d̄(Ga -Br) = 232.9 pm), connected in three dimensions by capped trigonal prisms (6+2+1) around Ga+. The crystal chemical relationships between β-Ga2Br4 and Ga2I4 are discussed.
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