SYNTHESIS AND MAGNETIC-PROPERTIES OF THE R1+XSR2-XCU2+XNB1-XO8-Y SYSTEM

We have carried out systematic studies involving synthesis of R1+xSr2-xCu2+xNb1-xO8-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 RSr2Cu2NbO8-y (RSCNO) phase does not form. For R=Pr and Nd, only an off-stoichiometric composition yields the RSCNO single phase, Pr1.2Sr1.8Cu2.2Nb0.8O8-y (Pr1.2SCNO) and Nd1.1Sr1.9Cu2.1Nb0.9O8-y (Nd1.1SCNO), respectively. In contrast, for R=Sm, Eu and Gd, the stoichiometric ratio results in monophasic material. Lattice parameters for Pr1.2SCNO, Nd1.1SCNO, SmSCNO, EuSCNO and GdSCNO are found to vary linearly with rare earth ionic size. Magnetic susceptibility, chi, for Pr1.2SCNO 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 chi-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, mu(eff), for the rare earth ion is deduced to be 2.95 mu(B) in Pr1.2SCNO and 7.97 mu(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 Nd1.1SCNO and SmSCNO compounds. The mu(eff) value is determined to be 3.42mu(B) for the former. Results on the abovementioned compounds are discussed in the context of the related RBa2Cu2NbO8-y and RBa2Cu3O7-y systems.