HOLE FILLING, CHARGE-TRANSFER AND SUPERCONDUCTIVITY IN ND1+XBA2-XCU3O7+DELTA

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. However, the superconducting properties decrease nonlinearly for increasing x, and T(c) can also be varied by lowering the annealing temperature without changing the low-temperature oxygen soak. The changes in microstructure and T(c) with increasing x are analogous with Y123 with increasing delta, yet for fully oxygenated Nd123ss samples, electron energy loss spectroscopy (EELS) demonstrates that the total unoccupied density of states below the unfilled band is constant for all x. The neutron diffraction and the inert gas fusion analysis support an overall charge balance for all x. T(c) is modeled in terms of a forced 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 heat treatments can be explained by the number of four-fold coordinated Cu's on the chains which can be varied by differing the amounts of paired and unpaired Nd3+ substituting for Ba2+, resulting in more or less charge transfer from the planes to the chains.