Synthesis and magnetic properties of the R_1+xSr_2−xNb_1−xO_8−y system

Abstract

We have carried out systematic studies involving synthesis of R_1+xSr_2−xCu_2+xNb_1−xO_8−y (x = 0.0-0.2), R = La, Pr, Nd, Sm, Eu, Gd, Dy and Y, and have investigated their magnetic properties. For R = La, Dy and Y, the expected RSr₂Cu₂NbO_8−y (RSCNO) phase does not form. For R = Pr and Nd, only an off-stoichiometric composition yields the RSCNO single phase, Pr_1.2Sr_1.8Cu_2.2Nb_0.8O_8−y (Pr_1.2SCNO) and Nd_1.1Sr_1.9Cu_2.1Nb_0.9O_8−y (Nd_1.1SCNO), respectively. In contrast, for R = Sm, Eu and Gd, the stoichiometric ratio results in monophasic material. Lattice parameters for Pr_1.2SCNO, Nd_1.1SCNO, SmSCNO, EuSCNO, and GdSCNO are found to vary linearly with rare earth ionic size. Magnetic susceptibility, χ, for Pr_1.2SCNO and GdSCNO, exhibits a magnetic ordering of the R sublattice. This occurs at about 6 K for the former and at about 2.15 K for the latter. The peak in the χ-T curve at 2.15 K for GdSCNO agrees well with the cusp (2.17 K) reported in heat capacity vs. temperature data. The effective paramagnetic moment, μ_eff, for the rare earth ion is deduced to be 2.95μ_B in Pr_1.2SCNO and 7.97 μ_B in GDSCNO. This result for GdSCNO is consistent with that of heat capacity measurements which indicate an entropy value corresponding to a Gd spin of 3.3. Magnetic susceptibility results do not indicate a magnetic transition down to 1.8 K for the Nd_1.1SCNO and SmSCNO compounds. The μ_eff value is determined to be 3.42μ_B for the former. Results on the abovementioned compounds are discussed in the context of the related RBa₂Cu₂NbO_8−y and RBa₂Cu₃O_7−y systems.