Synthesis and characterization of YBa2(Cu1−xMx)3O7−δ (M=Co,Ni): Crystal symmetry, superconductivity, and the role of oxygen

Abstract

We have prepared and characterized a series of Co- and Ni-doped YBa2(Cu1−xMx)3Oy compounds and determined the oxygen content for 0.0≤x≤0.15 and 0.0≤x≤0.20, for M=Co and Ni, respectively. The oxygen stoichiometry in Co-doped materials does not vary for x<0.10q but increases to about 7.3 for 15% Co. The oxygen content of Ni-doped materials remains constant at 7.0±0.1 for all x. The solubility limits of the solid solutions are x≈0.15 for Co and x≈0.20 for Ni. The Co-doped material undergoes a transformation from orthorhombic to tetragonal symmetry at 0.025<x<0.05 with a concomitant drop in Tc; the Ni-doped material remains orthorhombic throughout the limit of the solid solution. Based on an empirical model calculation, Co was deduced to occupy metal sites only in the Cu(1)O3 chains. The structural transition to tetragonal symmetry is a result of cobalt's preference for octahedral coordination, and is attributed to a disorder of the oxygen atoms (0,½,0) into the sites (½,0,0). The rapid suppression of Tc in the Co-doped samples is ascribed to the structural phase transition. Also presented are magnetic susceptibility and critical-field data.

Source-reported materials — not catalogue approval

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