Oxygen-content dependence of crystal structure and T-c of (Nd0.675Ce0.325)(2)(Ba0.664Nd0.336)(2)Cu3.00Oy

The crystal structure of (Nd0.675Ce0.325)(2)(Ba0.664Nd0.336)(2)Cu3.00Ogamma is described as a modified YBa2Cu3O gamma structure, in which the Y layer is replaced by a (Nd,Ce)(2)-O-2 block. While the dependence of T-c on oxyge n content differs among the rare-earth copper oxide superconductors, superc onductivity appears in a range of y = 9.12-9.14 in the present system. One of the reasons for this difference is related to the nature of defects asso ciated with oxygen in the plane contained Cu. We studied the crystal struct ure of single-phase (Nd0.675Ce(0.325))(2)(Ba0.664Nd0.336)(2)Cu3.00Oy sample s of different oxygen contents by neutron powder diffraction and the superc onducting properties by T-c measurements and chemical analysis. Over the y range where superconductivity is realized, the Cu valence changes from 2.32 -2.33 but the Ce valence remains ca. +4. From Rietveld refinements of the n eutron data, we found a direct correspondence between the oxygen content an d the occupation of oxygen sites in the CuOdelta plane. The average bond di stances of Cu-O in CuOdelta and CuO2 plane, (Nd,Ba)-O(161) and (Nd,Ba)-O(16 m) decrease with increasing oxygen content. Moreover, since the average Ce valence does not vary with respect to oxygen content, the average bond dist ances connecting the (Nd,Ce) site to its oxygen neighbors are almost consta nt. The calculated bond valence sum for the Cu(1) atoms in the CuOdelta pla ne and the average Cu valence increase markedly with oxygen content. (C) 20 00 Elsevier Science B.V. All rights reserved.