Zusammenfassung
A systematic study of Pb-doping and annealing effects in vacuum (2×10
-3Â Pa) on T c and the microstructure of Bi 2- x Pb x Sr 2 CaCu 2
O 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 CuO 2 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 Pb 2+ and Bi 3+ cause the reduction of extra oxygen,
resulting in the increasing positive charge in the BiO double planes.
The distortion of CuO 2 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 CuO 2 plane is
induced by Pb-doping and annealing in vacuum. Both the distortion
of CuO 2 planes and the charge-transfer control the carrier concentration
and carrier distribution in CuO 2 planes, which is related to the
superconductivity of the Pb-doped Bi-2212 phase.
Nutzer