Remarkable influence of heat treatment on the structural and superconducting properties of (Y1-xPrx)(BaSr)Cu3O6+z

We report here on the preparation, X-ray diffraction (XRD), Seebeck effect, AC magnetic susceptibility, resistivity measurements and effect of heat treatments in the high Tc superconductor (Y1-xPr x)(BaSr)Cu 3O 6+z. Each sample was subject to two types of heat treatment: oxygen annealing [O] and argon annealing followed by oxygen annealing [AO]. When x increase, the oxygen content (6+z) of the samples decrease slightly from 6.9 to 6.8 as determined by our iodometry measurements. The orthorhombicity ε = (b -a)/(b +a) decreases to zero at x t [O] = 0.6 and x t [AO] = 0.75. These indicate a structural phase transition from orthorhombic to tetragonal at x t. Whereas, T c decrease to zero at x c [O] = 0.7 and x c [A,O] = 0.75. Therefore the [A,O] heat treatment increase x c. Note that ε decrease with T c as foreseen. However, for a given x the [AO] heat treatment makes increase ε (T c). Independently of heat treatment, a increase with x, whereas b decreased to its minimum at x t. For a given x < x t, the heat treatment [AO] makes decrease a and increase. A remarkable correlations were observed between T c (x) and ε (x); and between the variation δT c (x) = T c [AO]-T c [O] and δε (x). A combination of several factors such as decrease in the Cu (1)-apical oxygen distance; cationic order of Pr x on the (Sr,Ba) site along c axis, chain oxygen ordering on (O 4,O 5) sites; increase in p and in-phase purity for the [AO] samples may qualitatively account for the observed data. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)