Formation pathways in the synthesis and properties of (Tl0.5Pb0.5)(Sr0.9Ba0.1)2Ca2Cu3Oz and (Tl0.5Pb0.5)(Sr0.8Ba0.2)2Ca2Cu3Oz-1223 superconductors

The formation pathway of (Tl0.5Pb0.5)(Sr0.9Ba0.1)2Ca2Cu3Oz and (Tl0.5Pb0.5)(Sr0.8Ba0.2)2Ca2Cu3Oz was studied by neutron and x-ray diffraction. The following reaction pathway was proposed: thallium oxide and lead oxide react with Sr-rich (Sr1-xCax )CuO2 and Ca-rich (CaxSr1-x)O to form Sr4Tl2O7 and (CaxSr1-x)PbO3, respectively. The thallate and the plumbate compounds then form the (Tl0.5Pb0.5)-1212 phase starting at a temperature of 600 ◦C. Finally, between 850 and 900 ◦C, the 1223 phase is formed from (Tl0.5Pb0.5)-1212, (CaxSr1-x)2CuO3 and CuO. Parallel experiments to fabricate the (Tl, Pb)-1223 superconductor resulted in specimens with critical temperatures of 117.5 K and 116 K, respectively, and transition widths of 2 K. Differences between the pathways for the formation of Pb-doped, Sr-rich and Pb-free, Ba-rich Tl-1223 superconductors are discussed.