Quantification by aberration corrected (S)TEM of boundaries formed by symmetry breaking phase transformations
Introduction
Crystal defects in materials are well known for their importance for the behavior of the material. From point defects providing the colors in precious stones over moving dislocations dictating the mechanical behavior of metals to precipitates affecting the magnetic properties in alloys, in all cases the defect structures are primordial to the functionality of the material. To understand, predict and possibly tune these parameters it is crucial to understand the exact atomic structures of these defects is essential. Indeed, because of lattice relaxations, when compared with the perfect matrix, small displacements or rearrangements of atoms can be expected at the sites of the defects. Depending on the system, such effects can be seen at the nanoscale, but in some cases they are limited to the picorange or to minor changes in chemical concentration. In this context aberration corrected transmission electron microscopy (AC-TEM) with image resolutions below the Ångstrom and, more importantly, precisions in the picorange, becomes an important tool [19], [22], [4], [43], [48]. Also, Z-contrast quantification in an AC-TEM can yield information on individual atomic site occupations [26], [36], [46].
In the present work the focus will be on crystallographic boundaries generated by symmetry breaking phase transformations. Four examples of recent work in this field will be reviewed, two on polar oxide systems, CaTiO3 and LiNbO3, and two on metals, one with a diffusive (Co-Pt) order-disorder and one with a displacive martensitic (Ni-Ti) phase transformation. In the oxide systems as well as the martensite the aim is to measure local atom displacements next to the boundary, while for the order-disorder system the focus will be on determining the type and number of atoms in a given column. For more details on some of these examples the reader is referred to the recent literature [3], [15], [17], [44].
Section snippets
Experimental
The work was performed with two FEI Titan instruments. The first (Qu-Ant-EM) is equipped with a monochromator and a probe and image aberration correction system allowing scanning as well as conventional TEM imaging to be performed in aberration correction mode. The second (X-Ant-EM) is equipped with a monochromator and probe aberration corrector and focusses on analytical applications. Both instruments also contain an EELS spectrometer while the X-Ant-EM has an in-built ChemiSTEM detector. They
Ferroelastic twin boundary in CaTiO3
Functional interfaces are not simple structural juxtapositions of the adjacent matrices, but contain novel structural elements which do not exist in the bulk and which may be used as objects in the field of domain boundary engineering. Examples can be superconducting domain boundaries in insulating materials [1], [24], twin boundaries with high defect mobilities [2], [6], [7], [40] or a two-dimensional electron gas at interfaces [20], [21], [34], [35]. In the present case the focus is on twin
Conclusions
It is clear from all of the above examples that the quantification, including precision ranges, obtained using various types of aberration corrected electron microscopy, be it at the level of atomic positions or atomic site occupation, has increased our knowledge on the defect structures in various systems. Unfortunately, due to visible variations in the atomic resolution images of the bulk or matrix parts, averaging procedures needed to be used in all cases and conclusions on individual atom
Acknowledgments
The authors acknowledge financial support from the Fund for Scientific Research-Flanders (G.0064.10N, G.0393.11N, G.0374.13N, G.0368.15N, G.0369.15N) and the Flemish Hercules 3 program for large infrastructure as well as financial support from the European Union Seventh Framework Programme (FP7/2007–2013) under Grant agreement no. 312483 (ESTEEM2). EKHS thanks EPSRC (EP/K009702/1) and the Leverhulme trust (EM-2016-004) for support. DS and MN acknowledges financial support from the Japan Society
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