Structural disorder and superconductivity suppression in NdBa2Cu3Oz (z similar to 7)

Structural features of a low-T-c nearly; stoichiometric Nd1+xBa2-xCu3Oz pha se (x less than or equal to 0.05, z similar to 6.9, a = 3.897-3.899 Angstro m, b = 3.902-3.908 Angstrom, c = 11.707-11.719 Angstrom) have been investig ated for the first time by XRD, EXAFS structure refinement techniques and R aman spectroscopy. It has been found that the abnormal lattice parameters a nd the low superconductivity transition temperature (55 K) of such a phase are related to structural disorder in the Ba and Nd sites resulting in oxyg en disordering which cannot be avoided by a standard oxidation procedure. I t is shown that longer annealing at high temperatures is an important facto r for reducing the oxygen disordering and hence enhancing superconducting p roperties after oxidation. (C) 2000 Elsevier Science B.V. All rights reserv ed.