ENHANCED SUPERCONDUCTIVITY IN ND1+XBA2-XCU3O7+DELTA BY LOW-OXYGEN PARTIAL-PRESSURE ANNEALING

Previous work has shown that unlike YBa2Cu3O7-delta (Y123), the Nd-Ba-Cu-O system exhibits a solid solution Nd1+xBa2-xCu3O7+delta (Nd123ss) for 0.04 less than or equal to x less than or equal to 0.6.(1-3) An earlier paper showed that although the superconducting properties decrease nonlinearly for increasing x, T-c can be varied by increasing the annealing temperature without changing the low temperature oxygen soak.(2) The changes in microstructure and T-c with increasing x are analogous with Y123 with increasing delta except that the total hole concentration remained constant. T-c was modeled in terms of oxygen disorder resulting from Nd3+ atoms on the Ba sites relocating chain oxygens to anti-chain sites. The variability in T-c as a function of x and processing conditions can be explained by the number of fourfold coordinated coppers on the chain sites. In this paper, the model has been further substantiated by processing in 1% O-2. The annealing in a reduced oxygen partial pressure followed by a 450 degrees C oxygen soak resulted in a marked increase in T-c compared to the 100% PO2 anneal. The low PO2 anneal favors pairing of Nd3+ substituting for Ba2+ to conserve oxygen ions, resulting in fewer disrupted fourfold-coordinated coppers thus increasing charge transfer from the planes to the chains.