Crystal substructure and physical properties of the superconducting phase Bi4(Sr,Ca)6Cu4O16+x

Abstract

We have isolated a high-$T_c$ phase in the Bi-Sr-Ca-Cu-O system of composition ${\mathrm{Bi}}_4{(\mathrm{S}\mathrm{r},\mathrm{C}\mathrm{a})}_6{\mathrm{Cu}}_4{\mathrm{O}}_{16+x}$. The crystal substructure has a tetragonal unit cell $a=3.817\mathrm{}$ Å, $c=30.5\mathrm{}$ Å) with similarities to both the oxygen-defect perovskites $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ and the ${\mathrm{K}}_2$Ni${\mathrm{F}}_4$ structure of ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$. The oxygen content, determined by titration and thermogravimetric analysis experiments, corresponds to a formal oxidation state Cu(2.15). Oxygen can be reversibly depleted in an argon ambient in an amount corresponding to the reduction of the Cu(III) into Cu(II). The compound has a metalliclike resistance above its $T_c$ near 85 K. Processing this precursor compound by heating to temperatures near its melting point (885 °C) produces a sharp resistivity drop near 110 K that we show by ac susceptibility and Meissner effect is due to a superconducting transition.