Isovalent Substitution and Heat Treatments Control of T c, Chain Oxygen Disorder and Structural Phase Transition in High T c Superconductors (Y1−x Nd x )SrBaCu3O6+z

Abstract

We report here on the preparation, X-ray diffraction with Rietveld refinement, AC magnetic susceptibility measurements and effect of heat treatments in (Y1−x Nd x )SrBaCu3O6+z . Each sample was subject to two types of heat treatment: oxygen annealing [O] and argon annealing followed by oxygen annealing [AO]. For each x, the [AO] heat treatment increases the orthorhombicity e=(b−a)/(b+a) (for 0≤x<1), T c (for x>0.2) and the distance d[Cu(1)–(Sr/Ba)] (decrease T c) for x<0.25 and decrease it (increase T c) for x>0.25. When x increase from 0 to 1, e decreases to 0 with transition from orthorhombic to tetragonal structure. e[O] decreases with T c[O]. However, T c[AO] decreases with e[AO] until x=0.2, increases for x=0.4 and after it decreases by 9.8 K to 77.2 K for x=1 [AO]. Remarkable correlations were observed between T c(x) and the volume of the unit cell V(x); and between δT c(x)=T c[AO]−T c[O] and δe(x). A combination of several factors such as decrease in d[Cu(1)–(Sr/Ba)]; increase in cationic and chain oxygen ordering; the number p sh(x) of holes by Cu(2)–O2 superconducting plans and in-phase purity for the [AO] samples may account for the observed data.