Crystal chemistry of HgBa₂CaCu₂O_8+δ and HgBa₂Ca₂Cu₃O_8+δ single-crystal x-ray diffraction results

Crystal structures of HgBa₂CaCu₂O_6+δ (Hg-1212) and HgBa₂Ca₂Cu₃O_8+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.