Charge-transfer induced by Pb-doping and annealing in Bi-2212 phase superconductor

A systematic study of Pb-doping and annealing effects in vacuum (2x10-3 Pa) on Tc and the microstructure of Bi2-xPbxSr2CaCu2Oδ compounds has been carried out. It is found that the structure belongs to the Bi-2212 phase. The c- and b-axes almost keep unchanged while Bi is replaced by Pb. With increasing annealing temperature, c- and b-axis parameters monotonically increase while the a-axis parameter is unchanged. The modulated period becomes longer and finally disappears with increasing Pb content and annealing temperature. Meanwhile, the distortion of CuO2 planes becomes serious. There is an optimum Tc with increasing annealing temperature, which reveals that the carrier concentration p changes from overdoped to underdoped. It is suggested that the differences of both the valences and the ionic sizes of Pb2+ and Bi3+ cause the reduction of extra oxygen, resulting in the increasing positive charge in the BiO double planes. The distortion of CuO2 planes is partially caused by the increase of positive charge in BiO double planes. A charge-transfer between the carrier reservoir (BiO-SrO) and the conducting CuO2 plane is induced by Pb-doping and annealing in vacuum. Both the distortion of CuO2 planes and the charge-transfer control the carrier concentration and carrier distribution in CuO2 planes, which is related to the superconductivity of the Pb-doped Bi-2212 phase. (author)