Relation of the superstructure modulation and extra-oxygen local-structural distortio

The modulated structure of (Formula presented)(Formula presented)(Formula presented)(Formula presented)(Formula presented) (y=0.2, n0.4;n0.1⩽x⩽1.0) was studied by means of electron diffraction. The experimental data show that the modulated structure changes from a mixed Bi/Pb-type to a pure Bi-type modulation with an increasing amount of La content in both the y=0.2 and 0.4 series and that the periodicity of the Bi-type modulation decreases correspondingly. When x=1.0, the modulation exhibits nearly the same characteristic in the samples with y=0.2 and 0.4. In addition, the extra-oxygen behavior, together with the local-structural distortion characteristic, related to the change of the modulated structure was also examined with Raman scattering, x-ray photoelectron spectroscopy and high-resolution electron micro- graphy. The experiment results reveal that there are more extra-oxygen atoms intercalating in the (Formula presented)(Formula presented) layers with a decrease in the modulation periodicity in the y=0.2 series. However, for the y=0.4 series, no more extra oxygen is incorporated into the (Formula presented)(Formula presented) layers with a decrease in the modulation periodicity, but the degree of local-structural distortion strengthens markedly in the (Formula presented)(Formula presented) layers. Based on these experiment results, we discuss the relationship of the superstructure modulation, extra oxygen, and the local-structural distortion.