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The crystal structure of a high T c superconductor, Ba 2 YCu 3 O 7-x, has been refined by the Rietveld analysis of TOF neutron powder diffraction data. Two polymorphs of Ba 2 YCu 3 O 7-x are contained in the sample: orthorhombic (74%) and tetragonal (26%) forms. The two forms have nearly the same structure closely related to that of perovskite-type compounds. In the orthorhombic form, the occupation factor of an oxygen atom on the a axis differed greatly from that on the b axis, which is the main reason why Ba 2 YCu 3 O 7-x crystallizes in the orthorhombic form.