Diffraction patterns The appearance of the diffraction pattern can reflect the nature of the crystalline phases in the specimen. For example, if the material is microcrystalline or amorphous the diffraction pattern consists of a series of concentric rings rather than spots/discs. Spot patterns When the electron beam interacts with the sample when the sample is oriented with a zone axis pattern parallel to the electron beam, then the diffraction pattern form in the back focal plane of the objective lens is a regular array of reflections. This is seen projected onto the viewing screen as an array of reflections organized in a predictable manner based on the crystal structure of the sample. The image shows a classic spot diffraction pattern obtained from Nd 13CaO 7. Click to find out how to set up the TEM lenses in order to obtain such an image.
11.3 The analysis of electron diffraction patterns 279 Trace of reflecting planes Centre spot Diffracted spot R u 2u L Fig. Idealized representation of the direct and diffracted ray paths in the electron microscope between the specimen and the screen. L is the camera length and R is the distance between the centre spot (direct beam).
Convergent Beam Electron Diffraction (CBED) When the electron beam is converged on the sample to a point (method = convergent beam), instead of using a parallel stream of electrons through the sample, the diffraction pattern forms discs instead of spots in the back focal plane of the objective lens. These discs can contain detail that provides information about the crystal structure of the specimen. In these images we see almost perfect mirror symmetry in the patterns within the discs.
![Diffraction Diffraction](https://i.ytimg.com/vi/zxCWY7S0P6A/maxresdefault.jpg)
These symmetries can be used to determine the point and space group of the crystal.
CSpot Software CSpot is a computer program for simulation, indexing and analysis of three types of electron diffraction patters: spot, Kikuchi and ring (polycrystalline). CSpot user interface The program allows for manipulation of simulated diffraction patterns in real-time and in an interactive manner by changing and visualizing crystal orientation and adjusting simulation parameters (e.g., microscope operating conditions).
![Tem diffraction pattern Tem diffraction pattern](http://www.calidris-em.com/images/eldspot.gif)
It’s main feature is the ability to perform automatic indexing and determination of crystal lattice orientation. The program contains build-in crystallographic calculators providing additional abilities in the analysis of diffraction patterns. The data produced by CSpot can be saved in a form of bitmaps or reports (Word files). CSpot is mainly dedicated to be a laboratory tool helping in the analysis and interpretation of data. It can be also used as a teaching aid. The demo version of the software can be downloaded Figure 2.
Example of spot (a), Kikuchi (b) and ring (polycrystalline) (c) electron diffraction pattern Indexing TEM spot diffraction patterns using CSpot (template matching) Indexing TEM spot diffraction patterns using CSpot (followed by manual detection of diffraction reflection positions) Indexing TEM Kikuchi diffraction patterns using CSpot CSpot has the ability to work with the Crystallography Open Database. In order to take full advantage of this possibility, the COD has to be downloaded from the website and installed using CSpot build-in installation tools. Indexing integrated (XRD-like) TEM ring (polycrystalline) diffraction patterns using CSpot with the aid of Crystallography Open Database Searching and loading structures from Crystallography Open Database , and simulation of electron diffraction patters using CSpot.