Elsevier

Ultramicroscopy

Volume 176, May 2017, Pages 194-199
Ultramicroscopy

Quantification by aberration corrected (S)TEM of boundaries formed by symmetry breaking phase transformations

https://doi.org/10.1016/j.ultramic.2016.12.022Get rights and content

Highlights

  • Quantification of picometer displacements at ferroelastic twin boundary in CaTiO3.

  • Quantification of kinks in meandering ferroelectric domain wall in LiNbO3.

  • Quantification of column occupation in anti-phase boundary in Co-Pt.

  • Quantification of atom displacements at twin boundary in Ni-Ti B19′ martensite.

Abstract

The present contribution gives a review of recent quantification work of atom displacements, atom site occupations and level of crystallinity in various systems and based on aberration corrected HR(S)TEM images. Depending on the case studied, picometer range precisions for individual distances can be obtained, boundary widths at the unit cell level determined or statistical evolutions of fractions of the ordered areas calculated. In all of these cases, these quantitative measures imply new routes for the applications of the respective materials.

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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