Effects of isovalent substitutions and heat treatment on structural and superconducting properties of high-critical temperature superconductors

Abstract

We have investigated the structural and superconducting properties Y0.5Ln0.5SrBaCu3O6+z of compounds (with Ln = rare earth = Eu, Sm and Nd) by X-ray diffraction with Rietveld refinement, resistivity and AC magnetic susceptibility measurements. Each sample was subject to two types of heat treatment: oxygen annealing [O] and argon annealing followed by oxygen annealing [AO]. For each ionic radius r of Ln, the [AO] treatment increases the orthorhombicity ε, Tc from 82.1 K [O] to 82.47 K [AO] for Y0.5Eu0.5SrBaCu3O6+z, from 80.98 K [O] to 82.02 K [AO] for Y0.5Sm0.5SrBaCu3O6+z and from 78.86 K [O] to 80.54 K [AO] for Y0.5Nd0.5SrBaCu3O6+z and decrease the distance d[Cu(1)-Sr/Ba]. For each r, in the normal state, the [AO] treatment reduced the linear resistivity parameters indicating a diminution of the interaction of carrier charges with phonons. There was a remarkable improvement in the shielding effect S (Amplitude of χ′(T)) in the case of the samples [AO] at all T < Tc and for any applied field Hdc. For each r and heat treatment, S(r) decreases when Hdc and the temperature increase. Remarkable correlations were observed. A combination of several factors such as decrease in d[Cu(1)-(Sr/Ba)]; increase in cationic and chain oxygen ordering; psh and in-phase purity for the [AO] samples may account for the observed data.