The superconductors (Tl, Pb)Sr<SUB>2</SUB>CaCu<SUB>2</SUB>O<SUB>7</SUB> and (Tl, Pb)Sr<SUB>2</SUB>Ca<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>9</SUB>: neutron powder diffraction, high resolution electron microscopy and x-ray absorption studies

The structures of (Tl, Pb)Sr2CaCu2O7 (Tc=85 K) and (Tl, Pb)Sr2Ca2Cu3O9 (Tc=122 K) have been investigated using neutron powder diffraction and electron microscopy. They are isostructural (14/mmm) with the corresponding thallium/barium end members of compositions (TlBa2Can-1CunO2n+3), possessing two or three CuO2-sheets separated by (Tl, Pb)O- and Sr2O2-sheets. X-ray microanalysis and neutron diffraction are consistent with fully occupied strontium sites in both materials. However, the diffraction results suggest considerable site disorder in the (Tl, Pb) and Ca-sites. High resolution electron microscope images indicate occasional defects and, in the case of (Tl, Pb)Sr2Ca2Cu3O9, occasional intergrowth of a structure containing 4-layers of copper. X-ray absorption measurements are consistent with formal oxidation states of 4+ for Pb and 3+ for thallium in these materials