Manifestation of T′ and 1-2-3 features in [Nd2/3(Ce1-xThx)1/3]2(Ba2/3Nd1/3)2Cu3O9-δ compounds:: a XANES study

The Cu K- and Ce L-3-edge regions have been measured in the hole doped hybrid structured [Nd-2/3(Ce1-xThx)(1/3)](2)-(Ba2/3Nd1/3)(2)Cu3O9-delta (2-2-3), 0 less than or equal to x less than or equal to 1.0, compounds which are superconducting for x less than or equal to 0.30. From the energy positions of the Cu K-edge features, it is clearly noted that 2-2-3 compounds incorporate the K-edge features seen in the oxygen deficient 1-2-3 and the T'-type 2-1-4 superconductors. On progressive Th substitution in 2-2-3, Ce continues to show mixed valence (3(+), 4(+) predominant), Cu remains mainly in 2 + state and no significant change is seen in the weak feature at Cu1+ position. However, noticeable changes in the split white line features at the Cu K-edge are observed. It is inferred from the analysis of the spectral features that a re-distribution of oxygen on the Cu(1) octahedral (defect) site in 2-2-3 occurs with increasing x, affecting adversely the carrier density. This plausibly causes a steep fall in T-c with Th substitution. (C) 1999 Elsevier Science B.V. All rights reserved.