The crystal structure of Bi4O5I2 has been solved from a 3:1 twinned crystal; accurate lattice parameters have been determined by a Rietveld refinement. Bi4O5I2 crystallizes with space group P21 (no. 4), Z=4, a=14.944(1), b=5.6983(3), c=11.263(1) Å, β= 99.81(1)°. Its structure is isotypic to the structure of the homologous Bi4O5Br2 for which more reliable lattice parameters have been determined by a Rietveld refinement. The effects of replacement of Br by I on the structure are discussed and compared with corresponding effects in the BiOX family (X = Cl, Br, I). A structural reason for the deviation of the twin supercells of the two compounds from orthogonality is discussed.
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