Ca-doping generates excessive holes and suppresses the superconducting transition temperature Tc of RBa2Cu3O7−δ (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−δ samples (for RGd, x=0, 0.1 and 0.2; for x=0.1, RY, 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.
Journal of Electron Spectroscopy and Related Phenomena 114-116, pp.689-692