Microdiffraction patterns

Figure 3. Microdiffraction patterns obtained with an electron beam of diameter about 1o8 from particles of Ru and Au on a MgO support, (a) MgO crystal, (b) Ru crystal, is2 in diameter, on MgO. (c) Au crystal, 20A in diameter, on MgO.

Figure 1. Example of a Zone-Axis Microdiffraction Pattern.

Glides plane can also be identified from Microdiffraction patterns. Their presence is characterized by a periodicity difference between the reflections present in the ZOLZ with respect to the ones in the FOLZ. This effect is visible on figure 1 where the ZOLZ reflections are placed at the nodes of a centered square net while those of the FOLZ are at the nodes of a smaller non-center square net. The corresponding glide plane is identified from simulations given in reference [2]. 

The SAED patterns consist of an intense base set of a - PbO subcell reflections and weak superstructure reflections hV4, kV3, T/2 referring to the a - PbO cell due to a modulation of the structure. To determine the Pb and Mo positions in PbsMoOg on the base of the relative intensities of the SAED reflections a multislice dynamic calculation [12] was performed. The calculations were performed separately for each of the 6 experimental microdiffraction patterns. The thickness of the specimen was determined independently for each of the patterns. The performed procedure is close to the published idea by Bing - Dong et al. [13]. 

Microdiffraction patterns are taken from individual particles after the reduction treatment and are shown in Figs. 3a-d. Most particles with platelet shape and straight edges produce similar microdiffraction patterns, one of which is shown in Fig. 3a. It is indexed as PtsSi with CusAu structure on [100] zone axis. Figs. 3b and 3c show the diffraction patterns from the not reacted Pt on [100] and [310] zone axes, respectively. Particles with irregular forms show various diffractions and a considerable amount of them can be attributed to Pt Sis. One such pattern is shown in Fig. 3d, exhibiting Pt Sis on [152] zone axis. 

Figure 3. Microdiffraction patterns from four single particles after the reduction, indexed as (a) [100] PtsSi, (b) [100] Pt, (c) [310] Pt and (d) [152] Pt Sis.

The sensitivity of these small crystallites to the electron beam makes obtaining microdiffraction patterns very difficult. Usually apparatus which involves specialized optics and electronic monitoring devices such as those developed by Cowley (15) can reduce beam exposure time and thus provide some meaningful data which might be useful in studying the support-metal interaction or structure-reactivity relationship. 

Figure 5 Microdiffraction patterns from three second phase particles, with most probable indexing, from some second phase particles in the steam treated (700 °C for 4h) FeZSM-5 particles grown from a stirred gel with Si02/Fe203-ratio 200.

Figure 13 Microdiffraction pattern recorded with <20 A probe on a second-phase region during an omega-phase transformation in a zirconium-niobium alloy ...

Whereas microscopy observations have revealed the existence of metallic icosahedral clusters, diffraction techniques alone may provide information about their structure. Powder and microdiffraction patterns have been obtained from gold icosahedral clusters 100 to 200 A in diameter in order to determine whether their structure was rhombohedral or nondeformed fcc. The somewhat surprising result is that they always show the nondeformed fee structure. This proves that above some given size, stresses inherent in the icosahedral... 

Figure 7.20 High-magnification TEM image of the GaN/PSC (5 pm-thick) interface (a), and selected microdiffraction pattern of the interfacial region (b). Arrows in the inset show extra spots arising from the presence of a superlattice-like structure. Reproduced from M. Mynbaeva et al., J. Cryst. Growth, 303, 472-479. Copyright (2007), with permission from Elsevier...

Another important analytical signal is the microdiffraction pattern. Here, the rays are spread by coherent (Bragg) scattering, so the resolution will be given by... 

Fig. 15. Microdiffraction pattern obtained from each part of the knee shown in fig. 13.

This type of small angle patterns are termed as microdiffraction patterns (MDP). In practice, the convergence of the probe usually is controlled by the second condenser aperture [1, 2]. Table 1 gives the comparison between both diffraction procedures, SADP and CBED. 

Switch to the diffraction mode and observe the microdiffraction pattern. 

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