Use of X-ray Anomalous Dispersion: the Superstructure of PbZrO₃

Intensity measurements of several superlattice reflections of lead zirconate, PbZrO₃, have been made on single-crystal specimens as a function of incident X-ray energy around the Pb L_III and Zr K absorption edges. Anomalies in the squared structure factor |F(hkl)|² of superlattice reflections with l odd, and of those with l even, are observed at both the absorption edges. This indicates that both Pb and Zr atoms show an out-of-phase type of displacement, not only on the xy plane but also along the z axis in the orthorhombic cell. The results of the Zr displacement are in contradiction with the models in the literature. It is shown that the use of X-ray anomalous dispersion is effective to obtain information on the atomic displacement and to investigate the superstructure.