Synthesis microstructure and superconducting properties of Ce-Doped Ti-Sr-Ca-Cu-O

A new Tl_1-xCe_xSr₂CaCu₂O₇ compound with x=0.25, which forms a primitive tetragonal structure with a=b=3.816 A ̊ and c=12.112 A ̊, has been synthesized in nearly pure form. The structure has been confirmed for the heavy atoms at the microstructural level by high resolution lattice imaging techniques. Selected area electron diffraction did not indicate the presence of structural modulations. The presence of Ce has been confirmed by analytical electron microscopic methods and by X-ray photoemission spectroscopy; the latter also indicates that cerium is present as Ce⁺⁴ ions. SQUID magnetometry, AC susceptibility and four-probe resistance measurements indicate superconductivity with a T_c between 62 and 68 K. Meissner fractions greater than 10% of ideal diamagnetism signal the presence of bulk superconductivity in the samples. The nature of the substitution process, and the reasons for a 20 K drop in T_c from a value of near 85 K for the isostructural Tl_0.5Pb_0.5Sr₂CaCu₂O₇ superconductor, are discussed.