Electron microscopy
Polarization in Ferroelectrics Measured by CBED in STEM Mode
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Tsuda et al. [1] were able to observe the two-dimensional (2D) distribution of the small atomic displacements  (~10 pm) cased by the ferroelectric polarization in the rhombohedral phase of the tetragonal BaTiO3 nanostructures using the CBED method in STEM mode. They demonstrated that this is a highly sensitive technique for detecting such small atom displacements or symmetry breaking.

In the analysis of ferroelectric polarization with STEM-CBED technique, one needs first to make sure:
         i) The analyzing area is not locally bent because the bending will cause intensity variation in the CBED patterns.
         ii) The intensities of the reflections which are used for comparison should be symmetrically equivalent in ideal crystals without polarization.
         iii) The analyzing area should be free from lattice defects and domain boundaries. [1]
         iv) The specimen thickness should be uniform if the polarizations are compared from one location to another.

With simultaneous HAADF imaging and CBED patterning, Tsuda et al. [1] employed the intensity difference (or called symmetry breaking) between the 020 and 0-20 reflections, (I020-I0-20)/I020, to map the directions of polarizations in tetragonal BaTiO3 nanostructures because the two reflections give the highest sensitivity theoretically and experimentally.






[1] Kenji Tsuda, Akira Yasuhara, and Michiyoshi Tanaka, Two-dimensional mapping of polarizations of rhombohedral nanostructures in the tetragonal phase of BaTiO3 by the combined use of the scanning transmission electron microscopy and convergent-beam electron diffraction methods, Applied Physics Letters, 103, 082908 (2013).