Dynamic powder diffraction

Dynamic powder diffraction where time-resolved experiments are performed to follow materials during chemical or physical reactions and processes. 

The desirability of using a non-invasive in-situ probe has already been discussed. There is, however, a problem, in that standard characterisation techniques are unable to penetrate bulky reaction vessels. As a result of this, little is known about the reaction dynamics or kinetics of intercalation reactions. A non-invasive probe which can interrogate a typical intercalation process is required. It is also necessary to employ short data collection times in order that kinetic information may be obtained. X-ray powder diffraction is a highly appropriate tool. It is non-invasive, and is a powerful characterisation technique when used in combination with ex-situ analyses. 

Recently, the PDF method was extended to describe the local dynamics of disordered materials (Dmowski W, Vakhrushev SB, Jeong I-K, Hehlen M, Trouw F, Egami T (2006) Abstracts American conference on neutron scattering, St. Charles, IL, 18-22 June 2006, unpublished). The total PDF is obtained by the powder diffraction method so that S(Q) includes both elastic and inelastic intensities. To determine the dynamics we have to use an inelastic neutron scattering spectrometer and measure the dynamic structure factor, S(Q,a>), over a large Q and co space, and Fourier-transform along Q to obtain the dynamic PDF (DPDF). While the interpretation of the DPDF is a little... 

Such defect-driven structural transformations are effectively investigated by powder diffraction analysis of samples kept in reactive atmospheres. As solid catalysts are dynamic systems, the phase inventory and the defect ordering (real structure) may well change as a result of changes of chemical potential of a constituent in a reactive environment. Some of the changes are irreversible and can be detected by pre- and postoperation analysis of catalysts, but many are reversible and will not be evident in such experiments. 

Extinction effects, which are dynamical in nature, may be noticeable in diffraction from nearly perfect and/or large mosaic crystals. Two types of extinction are generally recognized primary, which occurs within the same crystallite, and secondary, which originates from multiple crystallites. Primary extinction is caused by back-reflection of the scattered wave into the crystal and it decreases the measured scattered intensity Figure 2.51, left). Furthermore, the re-reflected wave is usually out of phase with the incident wave and thus, the intensity of the latter is lowered due to destructive interference. Therefore, primary extinction lowers the observed intensity of very strong reflections from perfect crystals. Especially in powder diffraction, primary extinction effects are often smaller than experimental errors however, when necessary they may be included in Eq. 2.65 as ... 

The first two-dimensional detector in X-ray diffraction was conventional film. It remained for decades the detector of choice for both single crystal as well as powder diffraction experiments. In the field of two-dimensional detection it was surpassed initially by image plates and later by CCD cameras (Figure 14.1). Today virtually no film is in use, with perhaps the exception of Polaroid used for single-crystal images. To be able to compare various detectors with one another, and to select the most appropriate detector for a specific experiment, certain key technical qualities are important. These are in general the detective quantum efficiency, the spatial response characteristics, the size, speed and dynamic range. ... 

Additional assays may also be conducted on this material to more comprehensively characterize its thermal properties. The following may be considered variable temperature XRD (VT-XRD) studies may be conducted to observe changes in the powder diffraction patterns of the material as a function of temperature. Selection of appropriate temperatures depends on several factors the DSC transition temperatures, corroborating TGA and hot-stage evidence, and the accuracy of the heating controller for the XRD unit. It should also be noted that although XRD data may be collected at several different temperatures to simulate a variable temperature experiment, the follow-up VT-XRD assay is essentially a series of isothermal experiments, and the DSC is a dynamic assay. It is therefore reasonable to expect some variation in transition temperatures. 

P-13 - Dehydration dynamics of mordenite by in-situ time resolved synchrotron powder diffraction study a comparison with electrostatic site energy calculations. 

The front-line analytical techniques of X-ray powder diffraction (XRPD), vibrational (infra-red and Raman) spectroscopy and thermal analysis, and their application to solid state issues are discussed elsewhere in this book. Solid state NMR is a very sensitive reporter of molecular conformation, mutual interaction, dynamics and form. In this section, we will discuss the basics of solid state NMR and in particular the methods that can be used in the study of this state of matter. 

Rietveld method, has been applied to describe chemical bonding, dynamic disorder, and anharmonic thermal vibration. Computer programs such as Rietan for the Rietveld analysis and PRIMA for the MEM analysis, which incorporate MEM and optimized for powder diffraction are available. 

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