A relationship between synthesized phases and starting compositions has been studied in the Pb-Bi-Sr-Ca-Cu-O system. The reaction step of forming the high-T
c phase (T
c∼110 K) has been presumed as follows. Firstly, the high-T
c phase and Bi2Sr2CuO
x
are obtained from the disproportionation reaction of the low-T
c phase. Bi2Sr2CuO
x
and residual Ca and Cu atoms melt into the PbO flux. The low-T
c phase is precipitated from the melt. The formed low-T
c phase causes a further disproportionation reaction. The high-T
c phase is produced in large quantities by repeating the cycle.
The addition of Pb to the superconducting Bi-Sr-Ca-Cu-O system is found to increase the volume fraction of the high-T
c phase (T
c>100 K) determined by the ac susceptibility and the X-ray powder diffraction pattern. It also lowers the optimum firing temperature to produce the high-T
c phase. The peaks attributed to the high-T
c phase in the X-ray diffraction pattern become sharper, which indicates that the addition of Pb promotes crystallization. It is found by differential thermal analyses that Pb also acts as a flux.
The reaction mechanism to produce the high-T
c phase was investigated using X-ray powder diffractometry and thermogravimetry and differential thermal analysis (TG-DTA). The high-T
c phase was produced through the low-T
c phase (Bi2Sr3-x
Ca
x
Cu2O
y
, 1<x<2) which was formed as an intermediate reaction product and also as a precursor of the high-T
c phase. In order to produce the high-T
c phase, it is necessary that the melting point of the low-T
c phase be lower than that of the high-T
c phase. Increase of Ca content in the low-T
c phase or decrease of ambient partial oxygen pressure during calcination are quite efficient to decrease the melting point of the low-T
c phase, and hence to enhance the formation of the high-T
c phase.
The crystal structure of the high-T
c phase (T
c∼107 K) in the Pb-Bi-Sr-Ca-Cu-O system has been analysed by the Rietveld method on the basis of the X-ray powder diffraction data. The crystal structure belongs to the space group Bbmb and its lattice parameters are a=5.407(6) Å, b=5.407(6) Å and c=37.051(7) Å. The crystal structure is quite similar to that of the high-T
c phase (T
c∼125 K) of Tl2Ba2Ca2Cu3O10. A flat Cu-O plane and two zigzag Cu-O planes are sandwiched between two Bi-O planes in the crystal structure. It was found that Pb atoms occupy the Bi, Sr and Ca sites by 9%, 7% and 3%, respectively.
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