Single crystals diffraction from

Fig. 4 Transmission electron micrographs of a highly facetted mostly triangular gold particles, b a hexagonal particle, c electron diffraction pattern of the triangular particle showing that it is a single crystal. Diffraction from the (111), (220), (311), (331), (422) planes are identified...

No matter how advanced, every numerical data processing technique has intrinsic limitations, especially when the complexity of the data increases or when the required information is partially missing, as for example in single crystal diffraction from macromolecules and proteins or in powder... 

C. Diffraction-Crystal simulates powder, fiber, and single-crystal diffraction from crystalline models, which helps interpret the experimental data from molecular, inorganic, and polymeric crystalline materials. 

The Wealth of Information from Single-Crystal Determinations. The amount of information that is determined from a crystal stmcture experiment is much greater and more precise than for any other analytical tool for stmctural chemistry or stmctural molecular biology. Indeed, almost all of the stmctural information that has been deterrnined for these two fields has been derived from x-ray single crystal diffraction experiments. 

Bragg-Brentano Powder Diffractometer. A powder diffraction experiment differs in several ways from a single-crystal diffraction experiment. The sample, instead of being a single crystal, usually consists of many small single crystals that have many different orientations. It may consist of one or more crystalline phases (components). The size of the crystaUites is usually about 1—50 p.m in diameter. The sample is usually prepared to have a fiat surface. If possible, the experimenter tries to produce a sample that has a random distribution of crystaUite orientations. 

The possibility of obtaining single crystal diffraction patterns from regions of very small diameter can obviously be an important addition to the means for investigating the structures of catalytic materials. The difficulty arises that data on individual small particles is usually, at best, merely suggestive and at worst, completely meaningless. What is normally required is statistical data on the relative frequencies of occurrence of the various structural features. For adequate statistics, it would be necessary to record and analyse very large numbers of diffraction patterns. 

In addition to microwave plasma, direct current (dc) plasma [19], hot-filament [20], magnetron sputtering [21], and radiofrequency (rf) [22-24] plasmas were utilized for nanocrystalline diamond deposition. Amaratunga et al. [23, 24], using CH4/Ar rf plasma, reported that single-crystal diffraction patterns obtained from nanocrystalline diamond grains all show 111 twinning. 

The facial complexes (PMe3)3lr(CH3)(H)(SiR3), (55), (R = EtO, Ph, Et) result from the oxidative addition of the corresponding silane to MeIr(PMe3)4.69 On heating (55) in which R = OEt and Ph, reductive elimination of methane forms iridasilacycles, as shown in reaction Scheme 6. The structure of compound (55) in which R = Ph is confirmed by single-crystal diffraction studies. 

X-ray detector, 26 420 X-ray diffraction (XRD). See also X-ray single-crystal diffraction, 24 72 application in high throughput experimentation, 7 395, 420-421 Bragg s law and, 26 418-419 from a crystal lattice, 26 416-418 in fine art examination/ conservation, 11 406... 

Structure determination from single crystal diffraction... 

Most of the unknown structures is determined from single crystal diffraction and refined from powder diffraction. Refinement is done with the Rietveld method, which is a least square fitting of the computed pattern to the measured one, while structure parameters are treated as the primary fitting parameters. This is in contrast to the procedure in pattern decomposition, which is outlined above (where not the structure parameters, but the peak intensities were the primary fitting parameters). Beside the... 

Information content in a powder diffraction pattern is reduced as compared to that in single crystal diffraction, due to the collapse of the three dimensional reciprocal space into a one dimensional space where the only independent variable is the scattering angle. The poorer the resolution of the diffraction method, the less the information content in the pattern (Altomare et al. 1995 David 1999). As a consequence, structure of less complex phases can be determined from power diffraction alone (fewer atoms in the asymmetric unit of the unit cell). However, refinement of the structure is not limited so seriously with resolution issues, so powder diffraction data are used in Rietveld refinement more frequently than in structure determination. Electron powder diffraction patterns can be processed and refined using public domain computer programs. The first successful applications of electron diffraction in this field were demonstrated on fairly simple structures. 

Fig. 4 a The structure of [Co (rt -C5H4COO)2] [NH4]-3H20 as obtained from single crystals grown from solution, b The calculated powder diffraction pattern matches the one measured on the powder material recovered from the solid-gas reaction... 

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