Phase separation, microstructure and superconductivity in the Y1-xPrxBa2Cu3Oy compounds

High resolution synchrotron x-ray powder diffraction combined with micro-Raman spectroscopy are used to investigate the effect of Pr substitution for Y in optimally doped or overdoped polycrystalline YBa2Cu3Oy (Y123) compounds. The spectral analysis of the Raman-active B-1g-symmetry mode indicates a phase separation into coexisting nanoscopic environments consisting of pure Y123 and Pr123 and a mixed (Y-Pr) 123 phase of almost the nominal amount of Pr. The Y123 phase disappears at x approximate to 0.6 where superconductivity is suppressed, while the formation of the pure Pr123 phase is correlated with the increase of local lattice distortions at the Cu and Ba sites, the presence of crystal defects and the increase of microstrains, as obtained by analyzing the anisotropic XRD peak broadening. The comparison of the Ba A(g)-symmetry phonon shift for the Y1-xPrxBa2Cu3Oy and YBa2-zPrzCu3Oy compounds as well as lattice dynamic calculations proves that, when Pr substitutes for Y, it also occupies an amount of Ba sites. The data from Pr, La or Ca substitution for Y indicate that loss of superconductivity is correlated with the substitution of Pr, La for Ba and Ca for Y, though the underlying effects may not be the hole filling by these occupancies.