Crystal chemistry of HgBa2CaCu208+ and HgBa2Ca2Cu308+ Single-crystal X-ray diffraction results

Crystal structures of HgBa2CaCu206 + ~ (Hg1212 ) and HgBa2Ca2Cu308 + a (Hg1223 ) have been determined on single crystals at room temperature conditions by X-ray diffraction techniques. Both tetragonal specimens are isomorphous with compounds in the T1-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 (1⁄21⁄20) 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.