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

A systematic study of Pb-doping and annealing effects in vacuum (2×10^-3 Pa) on T_c and the microstructure of Bi_2-xPb_xSr₂CaCu₂O_δ 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 T_c 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.