Synthesis, superconductivity and magnetism of RSr 2Cu 2NbO 7+δ compounds (R = Y, Pr and Gd)

Abstract

Single-phase compounds of the series RSr 2Cu 2NbO 7+δ with R = Y, Pr and Gd have been synthesized through a solid-state reaction route. These are found to crystallize in the orthorhombic (space group Imcm) structure which is analogous to that of the superconducting RBa 2Cu 3O 7−δ compounds with complete substitution of CuO chains with NbO networks. Low-field (50 Oe) magnetic-susceptibility measurements of RSr 2Cu 2NbO 7+δ compounds reveal that the Y compound of this series shows a superconducting transition around 13 K with a small Meissner fraction, while the compounds with R = Pr and Gd are non-superconducting down to 1.8 K. The GdSr 2Cu 2NbO 7+δ sample shows magnetic ordering, presumably of the Gd moments, with Néel temperature T N ≈ 2.2 K, which is about the same as the ordering temperature of the Gd moments in GdBaCu 3O 7−δ. However, unlike in PrBa 2Cu 3O 7−δ where Pr moments order antiferromagnetically with T N ≈ 17 K, no evidence of magnetic ordering of the Pr moments is observed in the susceptibility of PrSr 2Cu 2NbO 7+δ down to 1.8 K. The magnetic susceptibility of the RSr 2Cu 2NbO 7+δ compounds follows a Curie-Weiss behavior in the paramagnetic state with effective paramagnetic moments of 7.50 μ B, 3.11 μ B and 0.728 μ B for the Gd, Pr and Y compounds, respectively. The effective Pr moment in PrSr 2Cu 2NbO 7+δ is found to be intermediate between that of the Pr 3+ and Pr 4+ free-ion moment as in PrBa 2Cu 3O 7−δ.