Metallic and non-superconducting phases in LnBa₂Cu_3-xNi_xO_7+δ oxides

Abstract

A series of compounds LaBa₂Cu_3-xNi_xO_7±δ (0.0 < x < 0.3) and Ln Ba₂Cu_3-xNi_xO_7-δ (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 LaBa₂Cu₃O_7±δ(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 Ln Ba₂Cu_2.9Ni_0.1O_7±δ system. Electrical resistivity measurements show metallic behavior down to 4.2 K for LaBa₂Cu_3-xNi_xO_7±δ (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(1) sites in La123 stabilized by a larger La³⁺ ion and Ni²⁺ ion occupying the Cu(2) sites in the Ln 123 system of oxides.