SYNTHESIS, SUPERCONDUCTIVITY AND MAGNETISM OF RSR(2)CU(2)NBO(7+DELTA)COMPOUNDS (R=Y, PR AND GD)

Single-phase compounds of the series RSr(2)Cu(2)NbO(7+delta) with R = Y, Pr and Gd have been synthesized through a solid-state reaction rout e. These are found to crystallize in the orthorhombic (space group Imc m) structure which is analogous to that of the superconducting RBa(2)C u(3)O(7-delta) compounds with complete substitution of Cu-O chains wit h Nb-O networks. Low-field (50 Oe) magnetic-susceptibility measurement s of RSr(2)Cu(2)NbO(7+delta) compounds reveal that the Y compound of t his series shows a superconducting transition around 13 K with a small Meissner fraction, while the compounds with R = Pr and Gd are non-sup erconducting down to 1.8 K. The GdSr2Cu2NbO7+delta sample shows magnet ic ordering, presumably of the Gd moments, with Neel temperature T-N a pproximate to 2.2 K, which is about the same as the ordering temperatu re of the Gd moments in GdBaCu3O7-delta. However, unlike in PrBa2Cu3O7 -delta where Pr moments order antiferromagnetically with T-N approxima te to 17 K, no evidence of magnetic ordering of the Pr moments is obse rved in the susceptibility of PrSr2Cu2NbO7+delta down to 1.8 K. The ma gnetic susceptibility of the RSr(2)Cu(2)NbO(7+delta) compounds follows a Curie-Weiss behavior in the paramagnetic state with effective param agnetic moments of 7.50 mu(B), 3.11 mu(B) and 0.728 mu(B) for the Gd, Pr and Y compounds, respectively. The effective Pr moment in PrSr2Cu2N bO7+delta is found to be intermediate between that of the Pr3+ and Pr4 + free-ion moment as in PrBa2Cu3O7-delta.