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.