CHANGES IN DOPING STATE AND TRANSPORT CRITICAL CURRENT DENSITY OF (Tl,Pb)(Sr,Yb)2CaCu2O7 CERAMICS

Abstract

Two series of samples with nominal compositions of Tl_1-xPb_xSr_1.8Yb_0.2CaCu₂O₇ (x = 0.1 – 0.6) and Tl_0.5Pb_0.5Sr_2-yYb_yCaCu₂O₇ (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- _xPb_xSr_1.8Yb_0.2CaCu₂O₇ (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 T_c zero slightly decreased to 98 K. Substitution of Yb at Sr-site of Tl_0.5Pb_0.5Sr_2-yYb_yCaCu₂O₇ for y = 0 – 0.2 caused an increase in T_c zero from 62 K (y = 0) to a maximum value of 93 K (y = 0.2). However, further substitution of Yb caused T_c 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.