Hg-BASED SUPERCONDUCTING Cu MIXED OXIDES. E. V. Antipov¹, S. N. Putilin¹, E. M. Kopnin¹, V. A. Alyoshin¹, A. M. Balagurov², S. M. Loureiro³, J. J. Capponi³, M. Marezio⁴, ¹Dpt. of Chemistry, Moscow State University, Moscow 119899 Russia; ²Lab. of Neutron Physics, JINR, Dubna, Russia; ³Lab. de Cristallographie CNRS, BP 166, 38042 Grenoble France; ⁴MASPEC, Parma, Italy

The arrangement of the (CuO₂) layers in the HgBa₂Ca_n-1Cu_nO_2n+2+d structures is supposed to be optimal for the existence of superconductivity at high temperatures. The structure investigation of these compounds is important for understanding the superconductivity phenomenon among layered Cu mixed oxides.

The six members of the series were isolated and characterized. The third member (Hg-1223) undergoes superconducting transition at the highest T_c, while the other members exhibit lower superconducting transition temperatures due to several reasons such as underdoping of the higher members and alterations of the in-plane and apical Cu-O bond lengths.

The neutron powder refinement was made for monophasic HgBa₂CuO_4+d samples prepared in sealed tubes with different extra oxygen content (d=0.055, 0.12 and 0.18) and T_c (71K, 98K and 83K, respectively). No substitution on the Hg site and additional extra oxygen except the one in the middle of the mesh in the Hg-layer was found. The dependence of T_c vs. d will be discussed. The substitutions of Hg and O atoms in the (HgO_d) layer of the HgBa₂CuO_4+d structure by carbonate (sulphate) group and fluorine, respectively, were studied. The different geometry or formal valence of substituents causes different variations of the structure and properties of

the Hg-1201 superconductor.

This work was partially supported by ISF (M1G300), the Russian Scientific Council on Superconductivity (Poisk), INTAS (N 93-2483) and JNICT/PRAXIS XXI/BD 3328/94.