Direct observation ofB-site cation displacements in Pb-based complex perovskite relaxor oxides

Abstract

Electron diffraction patterns recorded using a scanning transmission electron microscope (STEM) from PbMg1/3Nb2/3O3 (PMN) crystallites and PbZn1/3Nb2/3O3 (PZN) crystals show weak and systematic continuous diffuse streaking along the 〈110〉 directions. Detailed high‐angle annular dark‐field (HAADF) images recorded via an aberration‐corrected STEM show that the B‐site cations in PMN and PZN undergo correlated and long‐range displacements towards the Pb2+ ions on the (110) planes. The planar B‐site displacement measured from the centres of the octahedra is about 0.3–0.5 Å in PMN and about 0.20–0.4 Å in PZN. In the HAADF images of the PMN crystallites and PZN crystals studied, there is insufficient evidence for systematic long‐range planar displacements of the Pb2+ ions. The observed Pb2+ ion displacements in PMN and PZN appear randomly distributed, mostly displaced along 〈110〉 towards the B‐site columns. There is also evidence of possible stress‐related distortion in certain unit cells of PMN. In the relaxors studied, two distinct types of displacements were observed: one is the long‐range planar B‐site spatial displacement on the (110) planes, correlated with the Pb2+ ions, possibly resulting in the observed diffuse streaking; the other is short‐range Pb2+ ion displacement on the (110) planes. The observed displacement status indicates a mutual attraction between the Pb ions and the B‐site cations in which the B sites undergo the largest spatial displacements towards the Pb ions along 〈110〉.

Puzzling Intergrowth in Cerium Nitridophosphate Unraveled by Joint Venture of Aberration-Corrected Scanning Transmission Electron Microscopy and Synchrotron Diffraction

“…The diversity of three-dimensional frameworks is, in theory, infinite, and a general task of modern synthetic chemistry is to combine new structural motifs and complex elemental compositions in order to realize increasingly sophisticated materials. But as the materials become more numerous, structure elucidation frequently encounters various challenges including microcrystalline heterogeneous samples, pseudosymmetry, and weak superstructures. − Through technological advances, methods for solving these latter problems like aberration-corrected transmission electron microscopy (TEM), application of third generation synchrotrons, and refractive X-ray lenses producing microfocused beams for X-ray diffraction became available. − Atomic resolution Z -contrast scanning TEM (STEM) now enables the direct observation of vacancies or superstructures to derive atom positions and first structure models. − …”

Polar nanoregions and diffuse scattering in the relaxor ferroelectric PbMg1/3Nb2/3OPaściak1, Welberry2, Kulda3 et al. 2012Phys. Rev. B

“…Over the last (few) years there have been many reported observations of the DS in lead-based relaxors. [13][14][15][16][17][18][19][20][21][22][23] The polar nature of correlations giving rise to DS is now widely accepted. It is also known, however, that the expected main component of PNRs, i.e., the frozen TO soft mode, cannot alone explain the distribution of DS intensities.…”