Structural disorder and superconductivity suppression in NdBa₂Cu₃O_z (z ∼ 7)

Structural features of a low-T_c nearly stoichiometric Nd_1+xBa_2-xCu₃O_z phase (x ≤ 0.05, z ∼ 6.9, a = 3.897-3.899 Å, b = 3.902-3.908 Å, c = 11.707-11.719 Å) have been investigated for the first time by XRD, EXAFS structure refinement techniques and Raman spectroscopy. It has been found that the abnormal lattice parameters and the low superconductivity transition temperature (55 K) of such a phase are related to structural disorder in the Ba and Nd sites resulting in oxygen disordering which cannot be avoided by a standard oxidation procedure. It is shown that longer annealing at high temperatures is an important factor for reducing the oxygen disordering and hence enhancing superconducting properties after oxidation.