Enhancement of metallic behavior in bismuth cobaltates through lead doping

Single crystals of Bi2M3Co2Oy phases with M=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 Bi2Ba3Co2Oy is suppressed by Pb doping, and (Bi,Pb)2Ba3Co2Oy 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 ∼1μB. Resistivity measurements carried out up to 20 GPa show that the samples become more semiconducting with increasing pressure