STRONG DEPENDENCE OF SUPERCONDUCTING TRANSITION TEMPERATURE (T c ) ON THE RARE EARTH IONIC SIZE IN REBaSrCu 3 O 7 (RE = Y, Dy, Nd AND La) SERIES

The REBaSrCu 3 O 7 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 REBaSrCu 3 O 7 series is quite different from that observed in the REBa 2 Cu 3 O 7 (RE:123) series where T c is nearly independent of the choice of the RE (except when RE = Ce, Pr and Tb). Neutron diffraction studies have been carried out on the REBaSrCu 3 O 7 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 REBaSrCu 3 O 7 compounds with light rare earths, RE = La and Nd, are tetragonal or almost tetragonal.