Abstract. Potassium heptabromodialuminate, KA12Br7, is monoclinic with a= 12.373 (2), b= 10.822 (2), c= 14.444 (3) &, fl= 133.87 (1) °, Z=4, space group P2Jc and calculated density 3.107 g cm -3. The crystals were prepared by zone refinement. The AlzBr7 anion consists of two AIBr4 tetrahedra sharing one corner, in staggered arrangement with a bent A1-Br-AI bridge (109.3°). The potassium ion is surrounded by nine bromine ions at distances of 3.3 to 4.0 .~. Introduction.AIBr3 was prepared by reacting HBr with A1 (purity 99.999%, Vigelands Brug, Norway) at 450°C. The product was purified by repeated vacuum distillations. The dry HBr used, containing only traces of Br2, was prepared by reacting 85 % H3PO 4 with a mixture of NaBr and PzO~ in the weight ratio 1:3.

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