Zero resistance was recorded at temperatures up to 162 K for multiphase samples with nominal composition $\mathrm{Tl}{\mathrm{Ca}}_4{\mathrm{Ba}}_3{\mathrm{Cu}}_6{\mathrm{O}}_x$. The compound with zero resistance at temperatures higher than 140 K could be repeatedly fabricated in a time span of about two months. The formation of the compound was found to be critically dependent on the starting stoichiometry, sintering temperature, and time as well as cooling rate. The compound was tentatively identified to be $\mathrm{Tl}{\mathrm{Ca}}_2{\mathrm{Ba}}_3{\mathrm{Cu}}_4{\mathrm{O}}_x$, tetragonal in structure with $a=b=0.394\mathrm{}$ nm and $c=3.95\mathrm{}$ nm and of $\frac{P4\mathrm{}}{\mathrm{mcc}}$ space group by transmission electron microscopy (TEM) and electron energy dispersive spectrometry analysis. High-resolution TEM images showed that the phase is primarily of a tensubcell structure which is consistent with a structure model with four Cu-O layers interposed between Tl-O layers.

• Received 6 July 1988

DOI:https://doi.org/10.1103/PhysRevB.39.2792