Crystal directions

If the detection system is an electronic, area detector, the crystal may be mounted with a convenient crystal direction parallel to an axis about which it may be rotated under tlie control of a computer that also records the diffracted intensities. Because tlie orientation of the crystal is known at the time an x-ray photon or neutron is detected at a particular point on the detector, the indices of the crystal planes causing the diffraction are uniquely detemiined. If... 

A very efficient one-pot procedure for the production of 3-hydroxy-3-cephems (45) has been developed which gives the desired product in almost 80% overall yield from (43a) which is readily available from penicillin. TTie sequence of reactions is (1) mesylation to give (43b), (2) formation of enamine (43c), (3) bromination to afford (44) and (4) hydroly-sis/cyclization with hydrochloric acid in methanol to afford (45) which, in some cases, crystallizes directly from the reaction mixture (B-82MI51000). 

In this case furoin crystallized from the ethanolic solution upon cooling. The following somewhat simpler procedure may also be used. A solution of 13.4 g (0.05 mol) of catalyst, 96.1 g (1.0 moll of 2-furaldehyde, 300 mL of absolute ethanol, and 30.3 g (0.3 mol) of tri ethyl amine is stirred at room temperature for 12 hr. The product (84.5 g, 88%) crystallizes directly from solution and is isolated by filtration. 

Fig. 5.6. Direction indices for identifying crystal directions, showing how the [166] direction is defined. The lower part of the figure shows the family of (111 > directions.

Komrska Satava (1970) showed that these accounts apply only to the reaction between pure zinc oxide and phosphoric acid. They found that the setting reaction was profoundly modified by the presence of aluminium ions. Crystallite formation was inhibited and the cement set to an amorphous mass. Only later (7 to 14 days) did XRD analysis reveal that the mass had crystallized directly to hopeite. Servais Cartz (1971) and Cartz, Servais Rossi (1972) confirmed the importance of aluminium. In its absence they found that the reaction produced a mass of hopeite crystallites with little mechanical strength. In its presence an amorphous matrix was formed. The amorphous matrix was stable, it did not crystallize in the bulk and hopeite crystals only grew from its surface under moist conditions. Thus, the picture grew of a surface matrix with some tendency for surface crystallization. 

Cooling crystallization is also preferred. Here, there are options. The mixture can be crystallized directly in melt... 

Isotropic etching (i.e. etching of bulk material with equal etch rates in all material/crystal directions). 

For good aspect ratio and device geometry, anisotropic etching agents have to exhibit a strong difference in etch rate between crystal directions. In a typical KOH solution for single crystal Si, about a two orders of magnitude smaller etch rate in... 

The values of these autocorrelation functions at times t = 0 and t = 00 are related to the two order parameters orientational averages of the second- and fourth-rank Legendre polynomial P2(cos/ ) and P4 (cos p). respectively, relative to the orientation p of the probe axis with respect to the normal to the local bilayer surface or with respect to the liquid crystal direction. The order parameters are defined as... 

Yin, W.-J. Chen, S. Yang,). Gong, X.qG. Yan Y. Wei, S.qH., Effective band gap narrowing of anatase Ti02 by strain along a soft crystal direction, Appl. Phys. Lett. 2010,96 221901-3. 

A strong texture can be deduced from the XRD spectrum. The intensity of the [020] reflection as compared with those of the [120] and [031] reflections in boehmite is 10 times larger than can be expected for a random distribution of crystal directions and is in accordance with the model shown in Figure 2.7. 

Compounds crystallized directly onto the carbon grid or with a defined orientation, due to other preparation methods, normally exhibit a suitable initial zone close to 0°. Samples from insoluble compounds are almost statistically oriented only biased by the particle shape. In this case, it is difficult to find a single crystalline part of appropriate thickness oriented with a suitable zone parallel to the surface. The best flexibility, and therefore the best possibility to orient a zone correctly, is given by a recently developed rotation-double tilt holder (Gatan Inc.). Through the combination of rotation and additional tilt (beta tilt) it is possible to orient the tilt axis exactly even if the crystal is not sitting flat on the support film (see Fig. 4). The tilt range, dependent on the pole piece distance of the objective lens, should be at least 40°. 

Columnar. These types of grains are most common in compact thin films. They are the result of preferred growth in certain crystal directions. Randomly oriented grains are usually small compared to film thickness. 

The enhanced strength of whiskers and natural fibers, by comparison to the strength of materials of the same composition in another morphology, could be a coincidental in these crystalline synthetic and mineral fibers, a particular crystal direction is parallel to the direction of the applied stress. However, the inverse diameter-strength relationship indicates that factors other than crystal structure contribute to the mechanical strength of fibrous materials. 

The position of Ti and Zr is again important in this context. While the b.c.c. phase in these elements has long been known to indicate mechanical instability at 0 K, detailed calculations for Ti (Petty 1991) and Zr (Ho and Harmon 1990) show tiiat it is stabilised at high temperatures by additional entropy contributions arising from low values of the elastic constants (soft modes) in specific crystal directions. This concept had already been raised in a qualitative way by Zener (1967), but the... 

Take the crystals directly out of the oil using a loop and freeze. 

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