Crystal structures of HgBa2CaCu2O6+δ (Hg-1212) and HgBa2Ca2Cu3O8+δ (Hg-1223) have been determined on single crystals at room temperature conditions by X-ray diffraction techniques. Both tetragonal specimens are isomorphous with compounds in the Tl-Ba-Ca-Cu-O system. We observe excess oxygen in both samples on two sites near the z=0 plane: at ( 1 200) and close to ( 1 2, 0.4, 0) - but no oxygen at ( 1 2 1 20) as reported in earlier studies. Refined Hg occupancies for both samples indicate a substitution of Cu for Hg of more than ten percent. If the substitution of Cu for Hg and excess O at ( 1 200) are coupled, then the effect is to insert a perovskite-like six-coordinated copper site at (000). No evidence was found for a split atom at the barium sites in these compounds.
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