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The continuing controversy about the true space group and structure of koechlinite y-Bi,O,-,MoO, due to uncertainties in the light atoms positions, has been resolved by neutron powder profile refinement. The original structure of Van den &en is correct, but contrary to his coordinates there are no unusually short oxygen-oxygen distances.The structure may be related to a series consisting of Bi,O, layers interleaved with perovskite like layers having the formula Bin-lMnX3,,+l (Aurivillius, 1952;Wells, 1975). For n = 1, M = Mo and X = 0 the perovskite layer is reduced to MOO,, which corresponds to the actual composition of the koechlinite Bi,MoO,. Complex oxides and oxyhalides of bismuth formed according to that principle have a pseudetetragonal symmetry. Deviations from tetragonal symmetry are due to MOO, octahedra tilts. Further distortions are revealed by octahedra rotations and displacement of molybdenum atoms from the centers of the octahedra.

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