STRUCTURAL TRANSITIONS IN THE SUPERCONDUCTING OXIDES BA-PB-BI-O

The crystal structures at room temperature and the structural transitions in BaPb1-xBixO3 (Ba-Pb-Bi-O) have been investigated in detail by means of electron diffraction. From an analysis of the crystal structures on the basis of electron diffraction patterns obtained experimentally, it is shown that, as the temperature is lowered, Ba-Pb-Bi-O undergoes a cubic-to-tetragonal transition resulting from the condensation of the triply degenerate R25 mode, R25x, in the lower-Bi-content range 0 less-than-or-equal-to x < 0.35, and successive transitions from the cubic to the tetragonal and to a monoclinic structure in the intermediate-Bi-content range 0.35 < x < 0.90. In the successive transitions, the cubic-to-tetragonal transition is understood to be due to the first condensation of the R25 mode, R25x, and the tetragonal-to-monoclinic transition is due to the second condensation of the R25 mode, R25y. The soft-phonon-mode behavior of the R25 mode in the cubic and tetragonal phases was actually detected as a change in an intensity of diffuse scattering appearing at the R point in the first Brillouin zone of the cubic perovskite structure. In addition, the crystal structure at room temperature in the higher-Bi-content range 0.90 < x less-than-or-equal-to 1.00 is confirmed to involve the breathing displacements resulting from the appearance of charge-density waves as well as the R25x + R25y displacements. Moreover, a simple explanation of the physical origin of the superconductivity in Ba-Pb-Bi-O is presented on the basis of the present experimental results.