Standard powder patterns

The last method for producing standard patterns for phases not in the PDF is more involved. In many instances single crystals of unknown phases can be removed from reaction mixtures. If this is the case, a full three dimensional crystal structure analysis will yield the positions of all atoms in the structure. Once the crystal structure is known, it can be used to calculate the X-ray powder diffraction pattern for the phase. This powder diffraction information can then be used with confidence as a standard powder pattern. 

Qualitative and quantitative analytical applications of X-ray diffraction both require reference diffraction patterns to identify and quantify the different polymorphic modifications. Experimental powder patterns may be suspect for their use as standards as a result of experimentally induced errors or aberrations or the lack of polymorphic purity in the sample itself (which may even result from the sample preparation). The availability of full crystal structure determinations for any or all of the polymorphic modifications can considerably facilitate generation of standard powder patterns. A variety of public domain software is now available for calculating powder diffraction patterns from single crystal data (ICDD 2001, lUCr 2001)." ... 

When dealing with organic pharmaceutical systems, an organic internal standard is preferred. The X-ray powder patterns of organic compounds, however, often contain numerous lines. As a result, it might not be possible to identify lines unique to the analyte and the internal standard that are completely separated from one another. Therefore, inorganic compounds are often used as internal standards. 

Fig. 4.4.5 X-ray diffraction patterns of Agl powder ot samples 1-V1 in Table 4.4.2. Locations g and h on the abscissa show the position and relative intensity of the standard XRD pattern of ( - and y-Agl. respectively. (From Rel. 10.)...

Two papers reported powder pattern crystallographic results. The paper by Santos et al. (7) stood out from the rest because it presented a collection of more classical physical chemistry experiments. In this paper the authors described the use of micro-combustion calorimetry, Knudsen effusion to determine enthalpy of sublimation, differential scanning calorimetry, X-ray diffraction, and computed entropies. While this paper may provide some justification for including bomb calorimetry and Knudsen cell experiments in student laboratories, the use of differential scanning calorimetry and x-ray diffraction also are alternatives that would make for a crowded curriculum. Thus, how can we choose content for the first physical chemistiy course that shows the currency of the discipline while maintaining the goal to teach the fundamentals and standard techniques as well ... 

H.E. Swanson et al, Standard X-Ray Diffraction Powder Patterns , NBS Monograph 25-Section 7, Dept of Commerce, Washington,... 

Differential thermal analysis was performed with the DuPont 900 differential thermal analyzer the heating rate was usually 10°C. per minute. To determine heats of reaction, the calorimeter attachment to the Du Pont instrument was employed. Planimeter determinations of peak areas were converted to heat values by using standard calibration curves. For the infrared spectra either a Beckman IR5A instrument or a Perkin Elmer 521 spectrophotometer with a Barnes Engineering temperature-controlled chamber, maintained dry, was used. Specimens for infrared were examined, respectively, as Nujol mulls on a NaCl prism or as finely divided powders, sandwiched between two AgCl plates. For x-ray diffraction studies, the acid-soap samples were enclosed in a fine capillary. Exposures were 1.5 hours in standard Norelco equipment with Cu Ko radiation. For powder patterns the specimen-to-film distance was 57.3 mm. and, for long-spacing determinations, 156 mm. 

Powder X-ray diffraction (XRD) patterns of the catalysts were obtained using a Philips APD X-ray diffraction spectrometer equipped with a Cu anode and Ni filter operated at 40 kV and 20 mA (CuKa = 0.15418 nm). Iron phases were identified by comparing diffraction patterns of the catalyst samples with those in the standard powder XRD file compiled by the Joint Committee on Powder Diffraction Standards published by the International Center for Diffraction Data. 

This web site also contains an interactive powder pattern calculator that allows the user to change the input and variables for a powder pattern calculation. This book was typeset using UTj X with the standard computer modern typeface. The LT X file and associated postscript plots were generated with a C++ program written by M. M. J. Treacy. We wish to acknowledge the assistance and collaboration of the members of the IZA Structure Commission for proofreading the manuscript and providing additional information. We are indebted to our employers (Arizona State University, and Air Products Chemicals, Inc.) for support of this project, and to our wives Laura and Carol for their patience and support. 

Raffinose crystallizes from water, or from aqueous alcohols or acetic acid, as the pentahydrate, of m.p. 78°, [a]20D + 105.2° (c 4, in water). No crystalline, characterizing derivatives of raffinose are known. Raffinose may be exactly identified by comparison of its x-ray diffraction powder pattern with that of an authentic standard.28 Other information of value in the identification of raffinose is its extreme ease of acid hydrolysis... 

Toraya s WPPD approach is quite similar to the Rietveld method it requires knowledge of the chemical composition of the individual phases (mass absorption coefficients of phases of the sample), and their unit cell parameters from indexing. The benefit of this method is that it does not require the structural model required by the Rietveld method. Furthermore, if the quality of the crystallographic structure is poor and contains disordered pharmaceutical or poorly refined solvent molecules, quantification by the WPPD approach will be unbiased by an inadequate structural model, in contrast to the Rietveld method. If an appropriate internal standard of known quantity is introduced to the sample, the method can be applied to determine the amorphous phase composition as well as the crystalline components.9 The Rietveld method uses structural-based parameters such as atomic coordinates and atomic site occupancies are required for the calculation of the structure factor, in addition to the parameters refined by the WPPD method of Toraya. The additional complexity of the Rietveld method affords a greater amount of information to be extracted from the data set, due to the increased number of refinable parameters. Furthermore, the method is commonly referred to as a standardless method, since the structural model serves the role of a standard crystalline phase. It is generally best to minimize the effect of preferred orientation through sample preparation. In certain instances models of its influence on the powder pattern can be used to improve the refinement.12... 

In Figure 3.13, the XRD powder patterns of the transformation products, Na-FCSW(6h) (Figure 3.13a), Na-FCSW(12h) (Figure 3.13b), and the pattern of the Na-Y commercial sample CBV100 (Figure 3.13c), with Si02/Al203 = 5.2, provided by the PQ Corporation Malvern, PA, USA, which was used as a standard for comparison, are shown [126], The XRD powder patterns show that the transformation products are Na-faujasite zeolites, with 80% crystallinity compared with the standard (CBV-100). The analysis by x-ray diffraction of all the samples lets us confirm that the maximum yield of the hydrothermal transformation procedure was achieved at approximately 9h. 

Quantitative analysis of field samples must, of course, be performed under identical conditions to those used in analyzing the standards Prior to the actual analysis, a powder pattern of a bulk sample such as rafter dust, an area sample, or a heavily loaded filter is obtained to determine information about the matrix. This information may necessitate some modification to the analytical method, such as... 

Figure 2. X-ray powder pattern of ZSM-23 formed from the standard gel composition after ca. 1 day at 1B0°C.

Powder Patterns. Powder patterns on titanium dioxide were obtained using a North American Philips x-ray machine and a standard powder camera. This work was done by A. Caron. 

There will always be a question of when to stop carrying out the refinement cycles. Perhaps the best way to decide on this is to make sure the variations in the parameters involved in the refinement change the model within acceptable standard deviations. Also, the model, in addition to explaining the observed powder pattern, must also make absolute chemical sense (e.g. bond distances, angles,... 

Because of the general similarities in the diffraction patterns, and the lack of clearly resolvable distinguishing peaks, they employed the Rietveld method (Young 1993). In the Rietveld method, the entire experimental diffraction pattern for each solid phase is used as a basis for comparison. For structure determination using powder diffraction, this comparison is made with a structural model used to generate a calculated pattern. In quantitative analysis of polymorphic phases, the known crystal stmctures are used to generate the standard diffraction patterns and these are then refined against the experimental powder pattern of the mixture to obtain the relative amounts of the polymorphs. 

One way to obtain highly resolved, selective, solid state information plus uniquely defined information on the orientation-dependencies of spin interactions is to perform NMR experiments on oriented single crystals. However, for various reasons, such single crystal NMR experiments appear impractical in standard applications to organometallic chemistry. The vast majority of solid state NMR experiments on organometallic compounds are— and probably will be— performed on polycrystalline powder samples. In principle, also for polycrystalline powders, all relevant information on spin interactions is contained in the shape of the powder patterns obtained imder static conditions. The problem then is the extraction of well-defined single parameters from such lineshapes resulting from either a multitude of resonances and/or the simultaneous presence of multiple spin interactions. In practice, it turns out that only very rarely is this possible. 

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