Shift of the apical oxygen of the CuO₆ octahedron and the superconductivity in Bi_1.8Pb_0.2Sr_2-xCuO_y systems

The Raman scattering, electron diffraction (ED), and resistivity are measured for a nominal composition of Bi_1.8Pb_0.2Sr_2-xCuO_y system. It is found that the peaks of A_lg vibrational modes of the oxygen atom in a SrO layer along the c axis are broadened, the intensity is reduced, and the position of the peak shifts toward low frequency in Raman spectra as Sr content decreases. The reduction of the incommensurate modulation wave-length is observed by ED measurements with the decrease in Sr content. This suggests that the results in Raman spectra and ED are attributed to the shift of the apical oxygen in CuO₆ octahedron induced by the decrease of Sr content in Bi_1.8Pb_0.2Sr_2-xCuO_y system. The resistivity measurements show that the shift of the apical oxygen not only brings about the disappearance of superconductivity, but also induces a metal-semiconductortransition.