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

Electron diffraction patterns recorded using a scanning transmission electron microscope (STEM) from PbMg_1/3Nb_2/3O₃ (PMN) crystallites and PbZn_1/3Nb_2/3O₃ (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>.