Crystallographic techniques oriented samples

As illustrated by Eig. 4.13, an electron microscope offers additional possibilities for analyzing the sample. Diffraction patterns (spots from a single-crystal particle and rings from a collection of randomly oriented particles) enable one to identify crystallographic phases as in XRD. Emitted X-rays are characteristic for an element and allow for a determination of the chemical composition of a selected part of the sample. This technique is referred to as energy-dispersive X-ray analysis (EDX). 

First we study the surface structure and chemisorption characteristics of crystals cut along different crystallographic orientations. Then a well-chosen chemical reaction is studied at low pressure to establish correlations between reactivity and surface structure and composition. Below 10 4 Torr the surface can be monitored continuously during the reaction with various electron spectroscopy techniques. Then the same catalytic reaction is studied at high pressures (1-100 atm) and the pressure dependence of the reaction rate is determined using the same sample over the nine orders of magnitude range. Finally, the rates and product distributions that were determined at... 

For certain minerals with multiple element substitutions on lattice sites, electron channeling experiments can provide estimates of site occupancy using a similar thin-film analysis technique. This latter approach, termed ALCHEMI, utilises an orientational dependence of X-ray emission from specific elements on crystallographic sites. Conventional thin-film analyses, which measure the concentration of elements in a sample, do not require specific, known orientations of a sample, and are best obtained from randomly-oriented or non-Bragg diffracting crystals and with a convergent beam which minimises channeling effects. 

Selected-area electron diffraction (SAD) is a basic TEM technique to obtain diffraction information from a part of the specimen. A selected-area aperture is inserted below the sample holder and in the image plane of the objective lens. Only the area selected by the aperture on the screen contributes to the SAD pattern. In case of polycrystalline specimens, if more than one crystal contributes to the SAD pattern, it can be difficult or impossible to analyze. As such, it is useful to select a single crystalline region for analysis at a time. It may also be useful to select two crystals at a time, in order to examine the crystallographic orientation between them. 

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