Combing electron diffraction techniques for structure solution

The last few years have seen a revolution in the field of 3D electron diffraction or diffraction tomography. We have moved from only acquiring a few low index zone axis patterns to full tomographic data sets recording all accessible areas of reciprocal space. These new larger data sets have made it easier for structure solution techniques such as direct methods from the x-ray world to be applied to the electron diffraction data for structure solution. While structure solution with tomographic electron diffraction is non trivial when compared to the x-ray case it is significantly easier than it was a few years ago. Mugnaioli et al. We are now in a situation where the most difficult and time consuming step can be the assignment of the space group to a data set. Electron diffraction has many advantages over the x-ray case in terms of the manner in which we can manipulate the electron beam. This allows the collection to convergent beam diffraction (CBD) or large angle convergent beam diffraction (LACBED) patterns, via the recently developed technique by Beanland et al. These techniques can make the assignment of space group significantly easier affair, and the path to structure solution a lot smoother. We will present the combination of data from tomographic, selected area (SA) and nano-beam (NBD) datasets, with diffraction from tomographic LACBED experiments where using the strengths of each technique can be leveraged for a much quicker route to structure solution.

Using the TEM Condenser Lens to Switch between Image and Diffraction Modes

Central to the operation of the transmission electron microscope (TEM) (when used with crystalline samples) is the ability to go back and forth between an image and a diffraction pattern. Although it is quite simple to go from the image to a convergent-beam diffraction pattern or from an image to a selected-area diffraction pattern (and back), I have found it useful to be able to go between image and diffraction pattern even more quickly. In the method described, once the microscope is set up, it is possible to go from image to diffraction pattern and back by turning just one knob. This makes many operations on the microscope much more convenient. It should be made clear that, in this method, neither the image nor the diffraction pattern is “ideal” (details below), but both are good enough for many necessary procedures.

One of my Failures: Diffraction in the TEM

There are two principal techniques for obtaining diffraction patterns in the transmission electron microscope (TEM). They are selected-area diffraction (SAD) and convergent-beam diffraction (CBED). CBED is quicker and easier to use, and it provides a much richer characterization of the sample. Thus, it is clear that CBED should be used in the vast majority of cases. It should be the diffraction technique that students learn first, and students should be taught to consider it the standard method of doing diffraction in the TEM.

Synthesis of BaAl2Si2O8 from solid Ba–Al–Al2O3–SiO2 precursors: Part III. The structure of BaAl2Si2O8 formed by annealing at ≤650 °C and at 1650 °C

Analytical TEM and HREM have been used to examine the structure of BaAl2Si2O8 crystals produced within oxidized Ba–Al–Al2O3–SiO2 precursors upon annealing: (i) at ≤650 °C and (ii) up to 1650 °C. A BaAl2Si2O8 polymorph with a c-axis parameter of 15.6 Å was detected after annealing at ≤650 °C. Stacking faults and antiphase boundaries were detected within this polymorph after the 650 °C treatment. After a 15 h heat treatment at 1650 °C, convergent beam diffraction patterns and HREM confirmed that the predominant phase was β–hexacelsian. Although antiphase boundaries were absent in the β–hexacelsian crystals, dislocations and stacking faults were detected after the 1650 °C anneal. The generation of defects in BaAl2Si2O8 crystals within specimens annealed at ≤650 °C and at 1650 °C is discussed in light of prior structural analyses.