Mosaic spreads

Figure 3 The variation of mosaic spread T with growth rate R for crystals of sodium chlorate, NaC103. (Reprinted with permission from Ref. 2.)... 

There is actually no sharp distinction between the crystalline and amorphous states. Each sample of a pharmaceutical solid or other organic material exhibits an X-ray diffraction pattern of a certain sharpness or diffuseness corresponding to a certain mosaic spread, a certain content of crystal defects, and a certain degree of crystallinity. When comparing the X-ray diffuseness or mosaic spread of finely divided (powdered) solids, the particle size should exceed 1 um or should be held constant. The reason is that the X-ray diffuseness increases with decreasing particle size below about 0.1 J,m until the limit of molecular dimension is reached at 1-0.1 nm (10-1 A), when the concept of the crystal with regular repetition of the unit cell ceases to be appropriate. 

Mosaic spread derived from the asterism of the diffracted X-ray beams in the Laue diffraction, mentioned above [2]... 

The analysis of a full tilt series of 2H NMR spectra not only allows the determination of the unique bond angle for a deuteriated methyl group, but also provides an internal check for the consistency of the spectral interpretation. In particular, simulations provide a means for the analysis of line-broadening effects, which arise from the sample mosaic spread as well as the intrinsic line width of the nuclear transition and instrumental factors. When line shapes are fitted to a full tilt series of spectra in a concerted manner and are also compared with the powder spectrum of an unoriented sample, the different contributions can be discerned. In that way an intrinsic line width of around 2 kHz is found for the spectra shown here, together with a mosaic spread between 8° and 10° for the three samples. 

No shift of peak with beam position on sample Mosaic spread... 

Figure 3.1 A schematic representation of the defects common in epilayer structures, (a) mismatch, (b) misorientation, (c) dislocation content, (d) mosaic spread, (e) curvature, (f) relaxation, (g) inhomogeneity...

Mitchell, E. P. and Carman, E. F. (1994). Hash-freezing of protein crystals investigation of mosaic spread and diffraction limit with variation of cryprotectant concentration. /. Applied Crystallogr. 27,1070-1074. 

In addition to the positional and thermal parameters of the atoms, least-squares procedures are used to determine the scale of the data, and parameters such as mosaic spread or particle size, which influence the intensities through multiple-beam effects (Becker and Coppens 1974a, b, 1975). It is not an exaggeration to say that modern crystallography is, to a large extent, made possible by the use of least-squares methods. Similarly, least-squares techniques play a central role in the charge density analysis with the scattering formalisms described in the previous chapter. 

Morula cell, 35 101 Mosaic spread, 47 471 [Mo,S4([9]aneN3),], structure, 37 154 Mo—S bridge, 38 55 M04S4 clusters, preparation, 38 26-27 [Mo,84] core, formation, 38 59 M04S4 cores... 

Fig. 8a-c. r rotation photographs of H. marismortui SOS crystals at 0 "C and at cryotemperature (obtained at XI1/EMBL/DESY and at SSRL/Stanford U.) a The hkO-orientation of a nearly perfectly aligned (although split) crystal reveals the mirror symmetry of the C-centred lattice plane. The severe overlap problem in this orientation caused by the large mosaic spread is obvious from this picture, b The Okl-orientation shows the extinctions of the twofold screw axis, c The best crystals have a Bragg resolution limit of about 6 A, which decreases to about 9 A in the course of a hundred exposures... 

However, most crystals nave a very large mosaic spread of up to 3° even in the first picture. This is indeed larger than the permissible rotation range at our current resolution. Hence there is a severe problem with overlap, in particular when the long 590 A axis is in the direction of the X-ray beam. What is worse, for many crystals we are left with no fully recorded reflexions to scale the partial intensities ( post refinement , These problems, taken together with the short lifetime render even a joint refinement of all the three axes of individual crystals nearly impossible. 

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