Diffraction unit cell dimensions

Fig. 7. A typical X-ray diffraction pattern of the Fepr protein fromZJ. vulgaris (Hil-denborough). The pattern was recorded on station 9.6 at the Synchrotron Radiation Source at the CCLRC Daresbury Laboratory using a wavelength 0.87 A and a MAR-Research image-plate detector system with a crystal-to-detector distance of 220 nun. X-ray data clearly extend to a resolution of 1.5 A, or even higher. The crystal system is orthorhombic, spacegroup P2i2i2i with unit cell dimensions, a = 63.87, b = 65.01, c = 153.49 A. The unit cell contains four molecules of 60 kDa moleculEu- weight with a corresponding solvent content of approximately 48%.

The crystallinity of organic pigment powders makes X-ray diffraction analysis the single most important technique to determine crystal modifications. The reflexions that are recorded at various angles from the direction of the incident beam are a function of the unit cell dimensions and are expected to reflect the symmetry and the geometry of the crystal lattice. The intensity of the reflected beam, on the other hand, is largely controlled by the content of the unit cell in other words, since it is indicative of the structural amplitudes and parameters and the electron density distribution, it provides the basis for true structural determination [32],... 

These crystals of reduced horse haemoglobin had some interesting properties which had not been observed previously. Normally when crystals of reduced haemoglobin are exposed to air they break up and most of their X-ray diffraction pattern disappears indicating that the crystals arc disordered. However when these crystals were left to become oxygenated or were slowly oxidised characteristic, well ordered lattice changes appeared. The unit cell dimensions were unchanged except for a... 

CCD detectors use a phosphor to convert the incoming X-ray to visible light, which is in turn detected by the CCD chip. During this conversion process, the apparent size of an X-ray reflection increases, a phenomenon known as the point-spread factor. Typically, this change in reflection size does not present a significant challenge. In extreme cases (i.e. large unit cell dimensions or a very short sample to detector distance), however, it can lead to overlaps between adjacent reflections. When overlaps do occur, diffraction data can be recorded... 

The amount of Fe203 supported on zeolite and the Si02/Al203 molar ratio (S/A ratio) of the prepared catalysts were obtained by X-ray fluorescence spectrometry (Rigaku Denki, 3080E). Specific surface areas were measured by BET method (Yuasa, QUANTACHROME). Unit cell dimension (U.D.) was determined from the diffraction angles of (642) with an X-ray powder diffractometer (Rigaku Denki, RU-200). Silicon was used as the reference. 

The compound crystallizes in the monoclinic system with unit-cell dimensions of a = 6.64 A b = 8.58 A c = 4.54 A (3 = 102.8°. These data agree well with those reported by Lukaszewicz.4 Manganese diphosphate can be identified according the following powder diffraction lines ... 

What are the spacings of the 100, 110, and 111 planes in a cubic crystal system of unit cell dimension a In what sequence would you expect to find these reflections in a powder diffraction photograph ... 

The theoretical density of a crystal can be obtained from the volume of the unit cell and the mass of the unit cell contents. The results of an X-ray diffraction structure determination gives both of these data, as the unit cell dimensions are accurately measured and the type and number of formula units in the unit cell are also determined. An example of this type of calculation for FeO follows ... 

Th ree different, X-ray diffraction patterns were observed, depending on the condition of the sample. The base plane is hexagonal in all cases. For the as-spun polymer (A), After annealing in water, the hydrated form (B) is obtained, with a = b = 1.571 nm and c = 1.882 nm. Upon dehydration (form C), the unit-cell dimensions are a = b = 1.438 nm and c = 0.579 nm. A triple helix, with each strand corresponding to a 6(0.289) helix, was proposed for the C form. Upon hydration, the triple-helical symmetry... 

N 18.67% yel ndls (from Nitrobenz), mp 316° (dec), mp — explodes sol in dimethyl form-amide si sol in hot acet, methyl ethyl ketone glac acet acid. This compd vas prepd by Shipp (Ref 5) from 2,4,6TTr initrobenzyl chloride in methanol by heating on a steam bath with methanol contg KOH. Since this product melts at 105° higher than that.reported by Reich. et al (Refs 1 2), its identity was confirmed by mw detn, elemental analysis, and by an unambiguous independent synthesis. Its X-ray diffraction measurements were also made its unit cell dimensions reported. The product of Reich et al [mp 211°(dec), correct value 218—20°] is probably 2,4,6,2, 4, 6 -Hexanitrobibenzil... 

The x-ray diffraction powder pattern for Mg2NiH4 was indexed on the basis of a tetragonal crystal system with unit cell dimensions of a = 6.464 A and c = 7.033 A. The measured density was 2.57 g/cm3, compatible with four formula units of Mg2NiH4 per unit cell. 

Interpretation of the diffraction patterns of mixed crytals, as far as the determination of unit cell dimensions, may be desirable. This is described in Chapter VI. 

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