STRUCTURAL STABILITY OF BISMUTH-BASED SUPERCONDUCTORS UNDER HETEROVALENT SUBSTITUTION

The stability of the three CuO2 layers phase of Bi-Pb-Sr-Ca-Cu-O super conducting system (BSCCO) with the addition of several rare-earth (RE) oxides is considered. It is shown that if a trivalent (Y, Nd, Gd, Dy, Ho, Er) or tetravalent (Ce) cation is nominally substituted ''ab init io'' on the Ca site, the higher-T(c) phase of this system (the 2223 ph ase) does not form, at least for dopant molar concentration larger tha n or equal to 0.032 in the nominal formula of the 2223 phase. If RE ox ides are added to a preformed 2223, a solid-state reaction at a temper ature of about 840-degrees-C takes place giving rise to a substituted (Bi2-yPb(y))Sr2Ca1-xRE(x)Cu2O8 compound plus other oxides. The synthes is of RE doped 2223 phase was attempted in different ways without succ ess. We argue that this is due to thermodynamic rather than kinetic fa ctors. A possible explanation of 2223 instability under doping in the calcium site with a higher-valence ion is proposed. It highlights the difference with the Bi2Sr2CaCu2O8 structural stability under the same doping conditions.