RARE-EARTH MAGNETISM AND SUPERCONDUCTIVITY IN RBASRCU3O(7-X) (R=GD, PR)

We present specific-heat, susceptibility, X-ray absorption (EXAFS and XANES) and Gd-155 Mossbauer experiments of RBaSrCu3O7-delta (R=Gd, Pr) in order to examine the effect of isoelectronic substitution of Ba by Sr upon superconductivity and magnetism with respect to the parent re ference compounds RBa2Cu3O7-delta. The RBaSrCu3O7-delta compounds offe r the possibility to vary the oxygen content without changing the long -range tetragonal structure. From Gd-155 Mossbauer spectroscopy a smal l local orthorhombic distortion is observed for tetragonal GdBaSrCu3O7 with a superconducting transition at 69 K. The XANES spectra of PrBaS rCu3O7-delta indicate that Pr is mainly trivalent with a considerable extent of hybridization, in agreement with a recent description of the specific-heat and susceptibility data in terms of Pr3+ crystal-field effects. From the comparison of the isoelectronic Ba/Sr substitution w ith hole doping (oxygen removal or hydrogen absorption) we deduce that the hybridization of the Pr 4f states with the Cu-O valence bands app ears to be important for the suppression of superconductivity and the high antiferromagnetic ordering temperature in these Pr compounds.