Electron Diffraction

Work-Package 20

WP20 is a multi-institution joint research activity focussed on the development and application of electron diffraction to study a broad range of structural problems across materials science, engineering, chemistry and physics. With new instrumentation, modern computational tools and new algorithms, there has been a recent renaissance in electron crystallography methods, using both electron diffraction and atomic-resolution imaging as complementary tools. That resurgence of interest is reflected in the topics under investigation within this JRA which include ab initio structure determination, refinement of the crystal potential, orientation mapping and measurement of thermal vibration and phonons. After only one year, we have already made great headway with these topics and in this new sletter we highlight some of that progress.
Direct imaging of atomic columns is now possible using aberration-corrected electron microscopes. With the introduction of high-area X-ray detectors, not only can atomic columns be imaged but their chemistry probed also. Researchers in Krakow have investigated the atomic structure and composition of γ’/γ’’ interfaces in a nickel-based superalloy, IN718. The figure below shows a series of atomic-resolution chemical maps, highlighting, for example, the substitution of Nb for Al in the γ’ phase. This work was presented at the MC2013 conference in Regensburg, Germany.

 JRA1 - image 1


Although electron diffraction, parallel or convergent beam, fixed beam or with precession, has been used for decades to determine local crystallography, by adding the ability to raster the beam, ‘diffraction-imaging’, or ‘orientation mapping’, is becoming an increasingly popular and important technique. There has been significant instrumental development in recent years by the SME involved in WP20, Nanomegas, enabling precession electron diffraction (PED) patterns to be acquired pixel by pixel across a scanned area. Such 4D data sets can be analysed to provide high resolution (ca. 1nm) phase-identification and orientation maps. Researchers in Cambridge have been collaborating with Nanomegas to use a rapid scanning approach as a way to minimise beam-dose for beam-sensitive materials such as organic crystals and pharmaceuticals, whilst retaining key crystallographic information. In the figure below, we show an example of an orientation map recorded from an organic thin film. Conventional bright-field and dark-field imaging is possible, although challenging, but it does not contain the wealth of orientation information using this diffraction-imaging approach. In the image the remarkable ribbon or fibre-like microstructure of the films is clearly identified; the false colour indicates different orientations of the crystal structure.

JRA1 - image 2
A related ‘random-tomography’ procedure is also being developed within the JRA with potential for identifying, for example, polymorphs of pharmaceutical structures. Acquiring diffraction patterns from multiple orientations from the same crystallite is nearly impossible given the high beam-sensitivity of many pharmaceutical materials. Instead the beam can be rastered across multiple crystals (at different, initially unknown, orientations) and electron diffraction patterns acquired in an automated fashion. It was possible to reconstruct the 3D reciprocal lattice of polymorphs of aspirin and resorcinol allowing the unit cell to be determined; an initial structure determination is underway.

JRA1 - image 3


We are also investigating crystallographic disorder in materials. Researchers in Cambridge have used electron diffraction to study molecular motion in organic semiconductor thin films (reported in Nature Materials doi:10.1038/nmat3710) as well as to static disorder in material for lithium ion cell electrodes. In both cases the form of the disorder could be identified from diffuse scattering present in diffraction patterns, such as the streaking seen between rows of reflections in the pattern to the right, recorded from lithium vanadate. The atomic displacements associated with the disorder were refined through the simulation of large arrays of atoms using GPU programming. Crystallographic analysis of novel systems, such as these disordered structures, is one of the key goals of the JRA.

 

Deliverable: Progress report on the use of PED and STEM for the determination of starting model structures

Partners involved:

University of Cambridge (Work-Package leader); TU Delft (WP co-leader);
ER-C Jülich; Universidad de Cadiz; AGH Krakow; TU Dresden; NanoMegas SPRL

Contacts:

Paul Midgley (University of Cambridge)
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Henny Zandbergen (TU Delft)
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