SUPERCONDUCTING LA1.5-XBA1.5+X-YSRYCU3OZ SOLID-SOLUTIONS - STOICHIOMETRY, STRUCTURE AND PROPERTIES

The stoichiometry range for Sr-doped La-123/336 solid solutions has been determined by means of a phase diagram study at approximately 950-degrees-C. The solid solutions have general formula La1.5-xBa1.5+x-ySryCu3Oz and form over the entire range 0 less-than-or-equal-to x less-than-or-equal-to 0.5 and 0 less-than-or-equal-to y less-than-or-equal-to 0.5 apart from compositions with high x and low 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 almost-equal-to 0.5, y almost-equal-to 0.3, which is approximately 10 K higher than 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 La3+ double-line arrow pointing left and right Ba2+ +h+ occurs, whereas with increasing y, the substitution mechanism is Ba double-line arrow pointing left and right Sr + mO2- +2mh+. Rietveld refinement of powder XRD data shows that Sr substitutes preferentially onto Ba sites, in contrast to Ca substitution where Ca occupies La sites.