77-K superconductor Pb0.8SrBa1.2Y0.7Ca0.3Cu3O7+ delta : Comparison with other Pb-based superconductors.

Abstract

Bulk superconductivity at 77 K has been induced in the ``Pb-based 1:2:1:3 compound'' with the composition ${\mathrm{Pb}}_{0.8}$${\mathrm{SrBa}}_{1.2}$${\mathrm{Y}}_{0.7}$${\mathrm{Ca}}_{0.3}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7+\mathrm{\ensuremath{\delta}}}$ by annealing in ${\mathrm{O}}_{2}$ at 450 \ifmmode^\circ\else\textdegree\fi{}C; high oxygen pressure increases the diamagnetic fraction. Superconductivity in Pb 1:2:1:3 is very sensitive to synthesis conditions and post annealing, as are the other previously reported Pb-based superconducting compounds Pb 2:2:1:3 and Pb 1:2:1:2. Each compound, however, requires different conditions of synthesis and annealing oxygen pressure. The oxidation states of Pb and of Cu in these three compounds have been determined by iodiometric titration and by thermogravimetric analysis. Superconducting Pb 1:2:1:3 and Pb 1:2:1:2 contain mainly ${\mathrm{Pb}}^{4+}$, while Pb 2:2:1:3 superconducts when ${\mathrm{Pb}}^{2+}$ predominates. The formal copper valency is \ensuremath{\approxeq}2.2 in all three. We conclude that the oxidation state of Pb is not critical to the superconductivity of Pb-based cuprates.