bSchool of Applied Physics,

Two series of samples with nominal compositions of Tl1-xPbxSr1.8Yb0.2CaCu2O7 (x = 0.1 – 0.6) and Tl0.5Pb0.5Sr2-yYbyCaCu2O7 (y = 0 – 0.6) were synthesized using solid-state method and characterized by electrical resistance (dc) measurements and powder X-ray diffraction analysis. Temperature dependent electrical resistance measurements on Tl1-xPbxSr1.8Yb0.2CaCu2O7 (x = 0.1 – 0.5) showed metallic normal state behaviors and increase in Tc zero from 61 K at x = 0.1 to a maximum value of 101 K at x = 0.5. At x = 0.6 the normal state behavior remained metallic but Tc zero slightly decreased to 98 K. Substitution of Yb at Sr-site of Tl0.5Pb0.5Sr2-yYbyCaCu2O7 for y = 0 – 0.2 caused an increase in Tc zero from 62 K (y = 0) to a maximum value of 93 K (y = 0.2). However, further substitution of Yb caused Tc zero to decrease from 64 K at y = 0.3 to 34 K at y = 0.5. Superconductivity was not observed down to 16 K for y = 0.6. Results of transport critical current density measurements and powder X-ray diffraction are presented. The effects of Pb and Yb substitutions on superconductivity of Tl1212 are discussed in terms of ionic radius of elements, Tl 1212 phase formation and the concept of average Cu valence