COMBINED NORMAL AND HIGH-RESOLUTION SYNCHROTRON X-RAY DIFFRACTION STUDY OF SURFACE LAYER PZN-4.5%PT FERROELECTRIC SINGLE CRYSTAL

An extremely broad lower 2θ peak adjacent to the major (002) rhombohedral peak has been frequently observed in standard XRD profiles of bulk PZN-PT single crystals at room temperature. To understand the original and nature of this lower 2θ peak, high-resolution synchrotron X-ray diffraction study was carried out. The synchrotron (002) mapping together with a fracturing technique confirmed that the lower 2θ peak arises from a polishing-induced surface layer of "highly stressed rhombohedral state of monoclinic symmetry" which, structurally, may be likened to one full of "incipient monoclinic nuclei" of a range of lattice parameters. These "incipient" monoclinic nuclei/phases are microscopic in dimensions and are placed under intense in-plane compressive stress in the surface layer. All these contribute to the extreme broadness of the lower 2θ peak. The present work shows that deformation of relaxor single crystals occurs readily by stress-induced phase transformation of the material from the rhombohedral state to other lower symmetry states. The lower 2θ peak, and hence the "incipient" monoclinic surface layer, is fairly resistant to annealing and low-field poling but can be largely eliminated when the as-polished sample was poled to sufficiently high field (i.e., 1.5 kV/mm) at room temperature. After annealing, however, the "incipient" monoclinic surface layer became more resistant to poling such that remnant of the lower 2θ peak persisted even after high-field poling.