Comparison of the Sr substitution effect on Nd(Ba1−xSrx)2Cu3O6+δ and Pr(Ba1−xSrx)2Cu3O6+δ prepared under reducing atmosphere

We report on the results of systematic studies of Sr substitution effect for R(Ba1−xSrx)2Cu3O6+δ (where R = Nd, Pr) with 0 ⩽ x ⩽ 0.5, prepared under reducing atmosphere. Single phases were obtained for a wide range of x for both systems of Nd(Ba1−xSrx)2Cu3O6+δ and Pr(Ba1−xSrx)2Cu3O6+δ by means of synthesis at high-temperature under reducing atmosphere (Ar:O2 = 100:0.1). In particular, solubility as high as x = 0.5 was successfully achieved for the first time in the Pr(Ba1−xSrx)2Cu3O6+δ. For R = Nd, the superconducting transition temperature, Tc, decreases from 95 to 50 K as x increases from 0 to 0.5. From the Rietveld refinement of powder X-ray diffraction (XRD) data, occupancy of Sr at the Nd site is found, gSr/Nd (=2 × gNd/Ba), and the value of gSr/Nd increases with x from 0 (x = 0) to 0.11 (x = 0.5). Occupancy of O(4), gO(4), decreases and O(5), gO(5), increases with x. It is demonstrated that the depression of Tc with x is caused by the replacement of Ba2+ by Sr2+ and Nd3+ and also by the replacement of Nd3+ by Sr2+. Disorder in the BaO plane induce the disorder in the CuO plane, which results in the decrease of the orthorhombicity. For R = Pr, both of the electrical resistivity and antiferromagnetic ordering temperature, TN, increase with x until x = 0.3, then decreases with x. Refinement results do not show intermixing of Sr and Pr; Sr ions occupy only the Ba site and Pr ions does not occupy the Ba site. The dependence of the orthorhombicity on x is quite different from that of Nd-system. The crystal becomes tetragonal at x = 0.4, which is smaller than that of Nd-system. The hybridization between the Pr 4f and the O 2p orbitals which prefers the tetragonal symmetry is considered to advances the transition to the tetragonal structure.