Site-selective doping and superconductivity in (La 1-y Pr y )(Ba 2-x La x )Cu 3 O 7+δ

Samples in the quaternary system $({\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Pr}}_{y})({\mathrm{Ba}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}){\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ have been prepared and characterized using x-ray and neutron diffraction, thermogravimetric analysis, and transport and magnetic measurements. Pr substitutes on the oxygen-depleted La layers for $yg0.0$, while La substitutes on the Ba sites for $x=0.0$. The effect of doping on each site is inferred to be primarily local, affecting immediately adjacent Cu-O layers. The similar suppression of superconductivity that accompanies doping on each of the two distinct sites apparently correlates with the degree of oxidation of the Cu-O sheets (and not the chains), indicating that the sheets support the high-temperature superconductivity. Comparison of orthorhombic and tetragonal samples with similar Ba:La ratios (and $y=0$) demonstrates that the orthorhombic phase yields the largest Meissner signals and highest transition temperatures in the $\mathrm{La}({\mathrm{Ba}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}){\mathrm{Cu}}_{3}{\mathrm{O}}_{7+\ensuremath{\delta}}$ system. The effect on superconductivity of oxygen-vacancy configuration in the Cu-O chain layers is proposed to derive, indirectly, from their influence on the Cu-O sheets. In addition, optimally superconducting $\mathrm{La}({\mathrm{Ba}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}){\mathrm{Cu}}_{3}{\mathrm{O}}_{7+\ensuremath{\delta}}$ samples exhibit interesting normal-state magnetic properties, with a paramagnetic susceptibility that decreases steadily with temperature between 350 K and ${T}_{c}$.