Remarkable Influence of Heat Treatment on the Structural and Superconducting Properties of LnSrBaCu3O6+z

We report here on the preparation, X-ray diffraction with Rietveld refinement, AC magnetic susceptibility measurements and effect of heat treatments in LnSrBaCu₃O_6+z (Ln = rare earth = Eu, Sm, Nd). Each sample was subject to two types of heat treatment: oxygen annealing [O] and argon annealing followed by oxygen annealing [AO]. When the ionic radius r(Ln⁺³) increase, the orthorhombicity epsiv = (b - a)/(b + a) decrease with T_c[O] and T_c[AO]. For each r(Ln⁺³), the [A,O] treatment increases epsiv (except for Nd), T_c, the distance d[Cu(2) - Ln] and decrease d[Cu(1) - O(1)]. T_c increases remarkably by 10 K, 6 K and 6 K to T_c[AO] = 78 K, 85 K and 87 K, respectively for Ln = Nd, Sm and Eu. Remarkable correlations were observed. A combination of several factors such as the r(Ln⁺³), decrease in d[Cu(1) - O(1)]; increase in cationic and chain oxygen ordering; and in-phase purity for the [AO] samples may account for the observed data.