UNDERSTANDING THE PHASE-RELATIONS AND CATION DISORDER IN LRE(1+X)BA(2-X)CU(3)O(7+DELTA)

Unlike Y123 which forms only a stochiometric compound, the light rare earth elements (LRE) form a solid solution LRE(1+x)Ba(2-x)Cu(3)O(7+delta) (LRE123ss), with increasing substitution of the LRE(3+) for the Ba2+ with increasing ionic radii of the LRE. Charge balance is maintained by increasing oxygen occupation on the anti-chain sites. The range of solubility is partially controlled by the oxygen partial pressure (PO2). The peritetic decomposition temperature also increases with increasing ionic radii. At doping levels of 0 < x < 0.1, there is an increase in T-c when the high temperature annealing (T similar to 940 degrees C) is performed in low PO2 (< 0.1 bar). The maximum Te occurs at a doping level of similar to x = 0.05 for Nd and Gd. When annealing is performed in 1 bar PO2, there is a gradual decrease in T-c with increasing x. These phenomenon can be understood in terms of the number and distribution of the LRE which substitute on the Ba site.