Effects of Ca-doping on local structure in RBa2Cu3O1-δ systems (R=Y, Cd and Nd)

Ca-doping generates excessive holes and suppresses the superconducting transition temperature Tc of RBa2Cu3O7-delta (R123) due to the over doping effect. Relatively, the normal state resistivity of R123 systems dependent on the ion size of rare earth (R). Both Ca-doping and rare earth replacement cause structural modification on the CuO2 plane. In order to study the effects on local structure around CuO2 plane due to Ca-doping and rare earth ion size in R123 system, we have performed extended X-ray absorption fine structure (EXAFS) measurements on two series of R1-xCaxBa2Cu3O7-delta samples (for R=Gd, x=0, 0.1 and 0.2; for x=0.1, R=Y, Gd and Nd) at room temperature. Our results show that in the Gd system the Cu-O bond distance and the static disorder, as indicated by Debye-Waller factor, increases with Ca concentration. For samples with different rare earths but the same Ca content (x=0.1), we found the static oxygen disorder of the surrounding atoms increases with the radius of the R3+ ion. No significant changes in the coordination number of O surrounding Cu were observed. We conclude that the decrease of superconducting temperature due to Ca-doping and the increase of the normal state resistivity with R3+ ion size are both related to the increased static oxygen disorder in the lattice. (C) 2001 Elsevier Science B.V. All rights reserved.