X-rays powder photographs

An X-ray powder photograph taken with a focusing camera and X rays monochromatized by crystal reflection would probably show more than 100 powder lines, providing a more rigorous text of the proposed structure. Accurate values of predicted intensities of the lines will become available only after coordinates have been assigned to all of the atoms, a formidable task with such a complicated structure. This task is now being attempted (S. Samson, personal communication). 

Both chondrosamine and D-glucosamine are usually isolated as hydrochlorides but rotational and mutarotational differences between these derivatives are not sufficiently sharp to distinguish between them. The x-ray diffraction pattern is quite distinct for each and the x-ray powder photograph method21 would appear to be the best for identifying small amounts. 

Probably the most widely employed technique now used in phase studies is powder X-ray diffraction. The X-ray powder pattern of a compound can be used as a fingerprint, and data for many compounds are available. This can be illustrated with reference to the sodium fluoride (NaF)-zinc fluoride (ZnF2) system. Suppose that pure NaF is mixed with a few percent of pure ZnF2 and the mixture heated at about 600°C until reaction is complete. An X-ray powder photograph will show the presence of two compounds (or phases), NaF, which will be the major component, and a small amount of a new compound (point A, Fig. 4.1a). A repetition of the experiment, with gradually increasing amounts of ZnF2 will yield a similar result, but the amount of the new phase will increase relative to the amount of NaF until... 

The authors wish to thank Ms. E. Sherry for providing us with the X-ray powder photographs. 

It has been shown 37) that the alpha radiation displaces every atom in the lattice of a simple crystalline compound, such as polonium tetrabromide, at least once a day. It is therefore not surprising that X-ray powder photographs of polonium compounds are generally poor. 

Chapter V, on identification by X-ray methods, is concerned with the practical details of taking X-ray powder photographs, and also includes elementary diffraction theory, taken as far as is necessary for most identification problems. 

The optical properties of crystals are usually quite reliable criteria for identification but occasionally crystals have sifbmicroscopic cracks and cavities, and although appearing quite normal, give refractive indices lower than those of an entirely solid crystal. This phenomenon, which is obviously very misleading, is fortunately very rare, but has been observed in anhydrite (calcium sulphate) and calcite (calcium carbonate) prepared in the laboratory. In cases erf doubt, X-ray powder photographs should be taken—see Chapter V. 

X-ray powder photographs of n iron, copper, sodium chloride, and potassium chloride. The photograph of a iron was taken with cobalt K[Pg.234]

No structural data are available for silver(H) pyridine salts. Based on the evidence500 of ease of doping at all levels of the silver and copper persulfate complexes into the cadmium spedes, isomorphism was assumed. X-Ray powder photographs have since shown this not to be the case.499... 

X-ray investigations of the structure of benzene began in 1923 when Broome took the first X-ray powder photographs of the molecule. Later, Cox (1928) determined the cell dimensions and space group and showed that the molecule was at least centrosymmetric. The development of the work on the benzene structure has been reviewed by Cox (1958). A more detailed paper on the crystal structure of benzene at — 3°C (Cox et al., 1958) has established that the benzene molecule does not deviate significantly from the 6jmmm symmetry predicted by chemical theory, the maximum deviation of the carbon atoms from the mean molecular plane being 0-0013 A. 

Apart from that of the recently synthesized gold pentafluoride (2), which was reported to be amorphous, the structures of all the transition metal pentafluorides are known. For all the compounds, cell dimensions have been derived, either from X-ray powder photographs or from single-crystal data. For seven of them structural analyses, at various levels of accuracy, have been performed on single-crystal data, and the remainder have unit-cell dimensions closely similar to a compound of known structure (see Table V). 

It has been suggested (79) that polymorphism may occur for transition metal pentafluorides as it does for oxide tetrafluorides (91). X-Ray powder photographs of products from the reduction of hexafluorides in anhydrous hydrogen fluoride (79) showed that the samples of rhenium and osmium pentafluorides had different structures from those previously reported, but no unit-cell dimensions could be derived. 

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