R2Ba4Cu7O15−δ: A 92-K bulk superconductor

Abstract

Superconductivity is investigated in ${\mathit{R}}_2$ ${\mathrm{Ba}}_4$ ${\mathrm{Cu}}_7$ ${\mathrm{O}}_{15\mathrm{-}\mathrm{\delta }}$ (R=Y or Er) as a function of oxygen stoichiometry. High-quality single-phase ceramic samples prepared at 869 °C in oxygen at one atmosphere were annealed under various oxygen partial pressures and temperatures then rapidly quenched into liquid nitrogen to fix the value of δ. ${\mathit{T}}_{\mathit{c}}$ was found to decrease monotonically with δ from a maximum of 92 K when δ=0, to a minimum of 30 K when δ=10. The normal-state carrier concentration has a one-to-one correspondence with δ. There is no significant difference between the oxygen unloading curves for δ as a function of temperature and pressure for ${\mathrm{Y}}_2$ ${\mathrm{Ba}}_4$ ${\mathrm{Cu}}_7$ ${\mathrm{O}}_{15\mathrm{-}\mathrm{\delta }}$ and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$; i.e., the unloading in the latter is twice that of the former. The volume change per oxygen vacancy is 3.1 Å per formula unit compared to 3.6 Å for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. The material remains orthorhombic for all values of δ.