The superconductors (Tl, Pb)Sr₂CaCu₂O₇ and (Tl, Pb)Sr₂Ca₂Cu₃O₉: neutron powder diffraction, high resolution electron microscopy and x-ray absorption studies

Abstract

The structures of (Tl, Pb)Sr₂CaCu₂O₇ (T_c=85 K) and (Tl, Pb)Sr₂Ca₂Cu₃O₉ (T_c=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 (TlBa₂Ca_n-1Cu_nO_2n+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.