METALLIC AND NONSUPERCONDUCTING PHASES IN LNBA2CU3-XNIXO7+DELTA OXIDES

A series of compounds LaBa2Cu3-xNixO7+/-delta (0.0 < x < 0. 3) and LnBa2Cu3-xNiO7-delta (Ln123; Ln = Pr, Nd, Sm, Gd, and Er; x = 0.1, 0.2) has been synthesized by a ceramic method and their structure characterized by X-ray and electron diffraction. The substitution of nickel in LaBa2Cu3O7+/-delta(La123) induces an orthorhombic to tetragonal structural transition as well reducing the activation energy for oxygen desorption. Orthorhombicity increased with decreasing lanthanide ion radius in the LnBa2Cu2.9Ni0.1O7+/-delta system. Electrical resistivity measurements show metallic behavior down to 4.2 K for LaBa2Cu3-xNixO7+/-delta (0. 1 < x < 0.3). The superconducting transition temperatures of Ni-doped Ln123 decreased with increasing rare earth ion radius, for the same nickel content. These properties are discussed in terms of the Ni3+ ion occupying the Cu(l) sites in La123 stabilized by a larger La3+ ion and Ni2+ ion occupying the Cu(2) sites in the Ln123 system of oxides. (C) 1994 Academic Press, inc.