Dependencies of T-c on the composition of 1 : 2 : 3 superconductors (CaxLa1-x)(Bac-xLa2-c+x)Cu3Oy and their interpretation by the cluster component method

The temperature of superconductive transitions T-c was measured for a varie ty of compositions (x,c) of the 1:2:3 tetragonal superconductors (CaxLa1-x) (Bac-xLa2-c+x)Cu3Oy (so called CLBLCO or CLBCO or Ca-La-Ba-Cu-O) both in ox ygen under-doped and overdoped region. The maximal values of T-c (T-c,T-max ) were calculated directly from these T-c vs y graphs. We found that instea d of the 1:2:3 superconductors without calcium where substitution of Ba for La effectively decreases T-c,T-max Ca rich compositions of CLBLCO do not c hange T-c,T-max over the wide range of this substitution. This fact and als o the previously found increasing of T-c,T-max with the Ca-content (x) are explained by the cluster component method; we found that the compositions w ith unit cells containing La on both the Y and Ba-sites, are nonsuperconduc tive, while all compositions containing unit cells with Ca on the Y-site ha ve the highest T-c,T-max = 81 K. All the experimentally found T-c,T-max(x,c ) may be described by the varying of the ratios of these unit cells in the sample. The role of Ca in stabilizing of T-c,T-max at substitution of Ba fo r La, and also in the increasing of T-c,T-max with increasing the Ca-conten t is because of the much higher ability of Ca to bind La (from the Ba site) than the ability of La on the Y site for the same binding. This prevents t he formation of La(LaBa)Cu3Oy unit cells, the presence of which in CLBLCO d ecreases T-c,T-max. (C) 2001 Elsevier Science Ltd. All rights reserved.