Single crystals of ${\mathrm{Bi}}_2{M}_3{\mathrm{Co}}_2{\mathrm{O}}_y$ phases with $M=\mathrm{Ca},$ Sr, and Ba were synthesized by the flux method. The undoped Ca and Sr analogs are semiconducting while the Ba analog has a metal to semiconductor transition at ≈80 K. Pb-substituted single crystals in which Pb partially substitutes for Bi were prepared by a similar method. Transmission electron microscopy carried out on single crystals of the undoped phases reveals a superstructure similar to that of superconducting bismuth-2212. This superstructure disappears when the Sr and Ba compounds are Pb doped. Transport measurements show that Pb doping induces a clear increase in the metallic character of the samples for Sr and Ba analogs. The metal to semiconductor transition in ${\mathrm{Bi}}_2{\mathrm{Ba}}_3{\mathrm{Co}}_2{\mathrm{O}}_y$ is suppressed by Pb doping, and ${(\mathrm{B}\mathrm{i},\mathrm{}\mathrm{P}\mathrm{b})}_2{\mathrm{Ba}}_3{\mathrm{Co}}_2{\mathrm{O}}_y$ is metallic down to 30 mK. The magnetic-susceptibility data do not show any evidence of ordering, and the magnetic moment Co atom was found to be $\approx 1{\mu }_B.$ Resistivity measurements carried out up to 20 GPa show that the samples become more semiconducting with increasing pressure.

• Received 22 February 2000

DOI:https://doi.org/10.1103/PhysRevB.63.094109