X-rays powder, diffraction

Turley [35] has published a collection of X-ray diffraction patterns of polymers. X-ray diffraction methods have been applied to crystallinity and crystal structure studies of a wide range of polymers including cellulose [36, 37], Nylon 6,6 [38], 

Nylon 6 [39,40], Nylon 8 [39], ethylene-propylene copolymer [41], poly-4-methylpentene [42], polyacrylonitrile [43], polychloroprene [44], PS [45], PVOH [7,46], polyaniline [47], polycyclohexadiene [9], polylactide [8], polyazomethine [6], poly-a-methylstyrene [48], PET [10], PE-like polyesters [11], and rigid polyphenylene dendrimers [49]. 

Wide-angle X-ray diffraction (WAXD) scattering has been used widely in studies of the characterisation of crystal structure of polymers (poly(ethylene-2,6-naphthalate) [53], atactic polypropylene [14], poly(co-pentadecalactone) [54], and PP homopolymer [55]). 

Creep studies. These have been carried out on ultrahigh-strength PE fibre [56]. 

Effect on orientation in amorphous and crystalline regions. Cho and co-workers [57] studied this in the case of poly(glycolide-co-e-caprolactone). 

A mineral is defined by its structure, i.e. by the regular arrangement of its atoms in space. Only tliose methods, therefore, which reflect the structure are capable of providing unambiguous identification of a particular oxide. In general, diffraction methods fulfill this purpose and X-ray powder diffraction (XRD), the most common of these, is essential for identification and control of the purity of the product. The minimum size required for a crystal to diffract X-rays is of the order of a few unit cells (ca. 2-3 nm). Electron diffraction is another method often used for 

To eorreet for instrumental misalignment an internal standard, e.g. 10% silieon (Si) or eorundum (a-Al203) powder (1 am Fisher Comp. 122651 K) should be mixed with the sample. The peak width must be eorreeted for the instrumental broadening using a well crystalline standard, sueh as lanthanum boride LaB. As an example, a Rietveld fit of an X-ray pattern from a mixture of synthetic hematite and goethite is shown in Fig. 3-9. 

If the peak positions are used to determine the unit cell edge lengths, a further correction of the peak position (so far not incorporated in available Rietveld models) is needed for very small crystals the broad peaks of which may show an (apparent) shift A°20 due to inconstancy of the Lorenz-polarization and structure factor over the angular range of the peak. 

The corrected FWHHs can be used to ftirther characterize the crystal size (more precisely the size of the mean coherently scattering domain, MCD) in the various crystallographic directions if this is smaller than ca.lOO nm. For example, small hematite crystals are often less developed in the [001] 

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X-ray powder diffraction studies are perfonned both with films and with counter diffractometers. The powder photograph was developed by P Debye and P Scherrer and, independently, by A W Hull. The Debye-Scherrer camera has a cylindrical specimen surrounded by a cylindrical film. In another commonly used powder... 

The formation of such materials may be monitored by several techniques. One of the most useful methods is and C-nmr spectroscopy where stable complexes in solution may give rise to characteristic shifts of signals relative to the uncomplexed species (43). Solution nmr spectroscopy has also been used to detect the presence of soHd inclusion compound (after dissolution) and to determine composition (host guest ratio) of the material. Infrared spectroscopy (126) and combustion analysis are further methods to study inclusion formation. For general screening purposes of soHd inclusion stmctures, the x-ray powder diffraction method is suitable (123). However, if detailed stmctures are requited, the single crystal x-ray diffraction method (127) has to be used. 

Analysis. Excellent reviews of phosphate analysis are available (28). SoHds characterization methods such as x-ray powder diffraction (xrd) and thermal gravimetric analysis (tga) are used for the identification of individual crystalline phosphates, either alone or in mixtures. These techniques, along with elemental analysis and phosphate species deterrnination, are used to identify unknown phosphates and their mixtures. Particle size analysis, surface area, microscopy, and other standard soHds characterizations are useful in relating soHds properties to performance. SoHd-state nmr is used with increasing frequency. 

Many of the procedures used for technical analysis of aluminum hydroxides are readily available from the major producers of aluminum hydroxides. Phase Composition. Weight loss on ignition (110°—1200°C) can differentiate between pure (34.5% Al(OH)2) ttihydroxides and oxide—hydroxides (15% Al(OH)2). However, distinction between individual ttihydroxides and oxide —hydroxides is not possible and the method is not useful when several phases are present together. X-ray powder diffraction is the most useful method for identifying and roughly quantifying the phase composition of hydroxide products. 

Doping of alkali-metals into CNTs has been examined [11]. The X-ray powder diffraction (XRD) patterns of the K- or Rb-doped CNTs show that alkali-metals are intercalated between the CNT layers. The hexagonal unit cell is essentially the same as that of the stage-1 alkali-metal intercalated graphite ACg (A=K, Rb). For a sample doped with Rb, the observed lattice parameter of the perpendicular... 

According to X-ray powder diffraction data, compounds RF NbOFs, Cs2NbOF5 [174] and Cs2TaOF5 [176] have similar type structure and are similar to K2GeF6, whereas (NFL,)2NbOF5 crystal structure is similar with Rb2Mo02F4 [184]. The above-mentioned compounds contain isolated NbOF52" complex ions [185]. 

Lastochkina et al. [216] reported on the preparation of KTaOi.5F3-H20, but the X-ray powder diffraction pattern obtained for the anhydrous product, KTaOi.5F3, does not correspond with the pattern given for K2Ta203F6 in [215]. 

The compounds characterized by X Me = 3.5 have a common formula of M2Me205F2 and crystallize either in a pyrochlore [192] or a veberite [229] type structure. According to X-ray powder diffraction patterns, the structure of Na2Nb205F2 can be regarded as a super-structure of pyrochlore, which is made up of octahedrons connected in layers and arranged in the (111) direction. The layers are linked via octahedrons so that each octahedron in one layer shares three vertexes with an octahedron in the adjacent layer. 

Fig. 43 shows fragments of X-ray powder diffraction patterns of compounds with rock-salt-type structures. 

Fig. 43. Fragments of X-ray powder diffraction patterns of compounds with rock-salt structures that underwent modification to a state of disordered ionic arrangement. 1 - Li3Ta04 2 - LiflbO 3 - Li4Ta04F 4 - Li3Ti03F 5 -LiiFeOiF 6 - LiNiOF (Reflections attributed to LiF are marked by an asterisk).

In all cases, broad diffuse reflections are observed in the high interface distance range of X-ray powder diffraction patterns. The presence of such diffuse reflection is related to a high-order distortion in the crystal structure. The intensity of the diffuse reflections drops, the closer the valencies of the cations contained in the compound are. Such compounds characterizing by similar type of crystal structure also have approximately the same type of IR absorption spectra [261]. Compounds with rock-salt-type structures with disordered ion distributions display a practically continuous absorption in the range of 900-400 cm 1 (see Fig. 44, curves 1 - 4). However, the transition into a tetragonal phase or cubic modification, characterized by the entry of the ions into certain positions in the compound, generates discrete bands in the IR absorption spectra (see Fig. 44, curves 5 - 8). 

X - Ray powder diffraction data of some important tantalum and niobium fluoride compounds (CuKa radiation)... 

Structural Studies. X-ray powder diffraction patterns for I indicate that the crystal structure is isomorphous to Zr2(0H)2-(SOO3 (H20)>. Figure 1 depicts the structure of the zirconium compound (5). The structure of I is identical to that of the zirconium analog except for variations in bond distances and angles which do not affect the overall structure. We have as yet been unable to obtain single crystals of I which are suitable for X-ray diffraction studies. 

X-Ray powder diffraction photographs suggest that NbSCl3 and TaSCl3 are isostructural (13). 

The crystal structures of four chlorinated derivatives of di-benzo-p-dioxin have been determined by x-ray diffraction from diffractometer data (MoKa radiation). The compounds, their formulae, cell dimensions, space groups, the number of molecules per unit cell, the crystallographic B.-factors, and the number of observed reflections are given. The dioxin crystal structures were performed to provide absolute standards for assignment of isomeric structures and have been of considerable practical use in combination with x-ray powder diffraction analysis. 

Correlation of the single-crystal structure results with the powder diffraction pattern, establishing x-ray powder diffraction as a convenient, powerful means of isomer identification and... 

Several cases have been encountered that show the utility of x-ray powder diffraction as an analytical method for identifying dioxins. An attempt was made to synthesize 2,8-dichlorodibenzo-p-dioxin by heating the potassium salt of a trichlorinated 2-hydroxydiphenyl ether (3). Surprisingly, the major crystalline product under the initial reaction condi-... 

The dichlorodibenzo-p-dioxin component was isolated by passing a dioxane solution of the mixture through acetate ion exchange resin to remove phenolics. The eluted product was recrystallized from benzene. The x-ray powder diffraction pattern of the precipitate was identical with that of 2,7-dichlorodibenzo-p-dioxin. Analysis of the mother liquor by GLC showed a singular peak consistent with 2,7-dichlorodibenzo-p-dioxin. The mother liquor was cooled to 5°C and yielded transparent crystals. This material had an x-ray diffraction pattern congruent to a sample of 2,8-dichlorodibenzo-p-dioxin obtained from A. E. Pohland (FDA). The two patterns were quite distinct from each other 14, 15). 

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