CRYSTAL-CHEMISTRY OF HGBA2CACU2O8+DELTA AND HGBA2CA2CU3O8+DELTA - SINGLE-CRYSTAL X-RAY-DIFFRACTION RESULTS

Crystal structures of HgBa2CaCu2O6+delta (Hg-1212) and HgBa2Ca2Cu3O8+delta (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 (1/2 00) and close to (1/2, 0.4, 0) - but no oxygen at (1/2 1/2 0) 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 0 at (1/2 00) 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.