Strong dependence of superconducting transition temperature (Tc) on the rare earth ionic size in REBaSrCu3O7 (RE = Y, Dy, Nd and La) series

The REBaSrCu3O7 compounds with RE = Y, Dy, Nd and La have been synthesized by the standard solid-state reaction route. Superconducting transition temperatures (T-c), as measured by ac susceptibility technique, are 81 K, 79 K, 62 K and 45 K for samples with RE = Y, Dy, Nd and La, respectively. The rare earth dependence of T-c in the REBaSrCu3O7 series is quite different from that observed in the REBa2Cu3O7 (RE:123) series where T-c is nearly independent of the choice of the RE (except when RE = Ce, Pr and Tb). Neutron diffraction studios have been carried out on the REBaSrCu3O7 compounds and structural details have been obtained from the Rietveld analysis of the room-temperature neutron diffraction data. The compounds with RE = Y, Dy are found to crystallize in the orthorhombic RE:123 structure (space group Pmmm) with orthorhombicity considerably smaller than that of the RE:123 compounds. In fact, the orthorhombicity reduces so much that the REBaSrCu3O7 compounds with light rare earths, RE = La and Nd, are tetragonal or almost tetragonal.