Superconducting La 1.5− xBa 1.5+ x−ySr yCu 3O z solid solutions. Stoichiometry, structure and properties

Abstract

The stoichiometry range for Sr-doped La-123/336 solid solutions has been determined by means a phase diagram study at ∼950°C. The solid solutions have general formula La 1.5− x Ba 1.5+ x−y Sr y Cu 3O z and form over the entire range 0⩽ x⩽0.5 and 0⩽ y⩽0.5 apart from compositions with high x and y. The solid solutions are tetragonal under all conditions used; lattice parameters vary linearly with x and y. T c passes through a maximum of 65 K at x∼0.5, y∼0.3, which is ∼ 10 K higher th the previous maximum reported for this system. Oxygen content determinations by TGA indicate two mechanisms for increasing the hole concentration: with increasing x, the substitution La 3+⇔Ba 2++h + occurs, whereas with increasing y, the substitution mechanism is Ba⇔Sr+ mO 2−+2 mh +. Rietveld refinement of powder XRD data shows that Sr substitutes preferentially onto Ba sites, in contrast to Ca substitution where Ca occupies La sites.