Shao Yu Tsun, Zuo Jian Min
Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
Acta Crystallogr B Struct Sci Cryst Eng Mater. 2017 Aug 1;73(Pt 4):708-714. doi: 10.1107/S2052520617008496. Epub 2017 Jul 19.
Crystal charge density is a ground-state electronic property. In ferroelectrics, charge is strongly influenced by lattice and vice versa, leading to a range of interesting temperature-dependent physical properties. However, experimental determination of charge in ferroelectrics is challenging because of the formation of ferroelectric domains. Demonstrated here is the scanning convergent-beam electron diffraction (SCBED) technique that can be simultaneously used for imaging ferroelectric domains and identifying crystal symmetry and its fluctuations. Results from SCBED confirm the acentric tetragonal, orthorhombic and rhombohedral symmetry for the ferroelectric phases of BaTiO. However, the symmetry is not homogeneous; regions of a few tens of nanometres retaining almost perfect symmetry are interspersed in regions of lower symmetry. While the observed highest symmetry is consistent with the displacive model of ferroelectric phase transitions in BaTiO, the observed nanoscale symmetry fluctuations are consistent with the predictions of the order-disorder phase-transition mechanism.
晶体电荷密度是一种基态电子性质。在铁电体中，电荷受到晶格的强烈影响，反之亦然，这导致了一系列有趣的温度依赖物理性质。然而，由于铁电畴的形成，铁电体中电荷的实验测定具有挑战性。这里展示的是扫描会聚束电子衍射（SCBED）技术，它可同时用于成像铁电畴以及识别晶体对称性及其涨落。SCBED的结果证实了BaTiO铁电相的非中心四方、正交和菱面体对称性。然而，对称性并非均匀的；几十纳米的几乎保持完美对称性的区域散布在较低对称性区域中。虽然观察到的最高对称性与BaTiO中铁电相变的位移模型一致，但观察到的纳米级对称性涨落与有序-无序相变机制的预测一致。
