![]() ![]() Nonetheless, significant advances have been made in this area, and an increasing proportion of the new framework types are solved this way. The determination of the structure of a zeolite with a new framework type remains a challenge to the powder method. Similarly, the presence of an amorphous phase can be established very easily simply by looking at the diffraction pattern. POWDER DIFFRACTION FULLMore detail can be extracted from the powder pattern if a full Rietveld (whole-profile) structure refinement is performed, but even the simple qualitative evaluation of the pattern described above can be extremely useful. The high-angle region is usually less sensitive to the presence or absence of electron density in the channels and cages and more sensitive to distortions of the framework. For example, a calcined material will tend to have higher relative intensities in this region than the corresponding as-synthesized or loaded sample. In general, non-framework species have a pronounced effect on the low-angle region of the pattern. Changes in the relative intensities of the peaks indicate that a structural modification has occurred, changes in the positions of the lines indicate that the unit cell has deformed in some way, and broader (or narrower) lines indicate that the crystallinity has deteriorated (or improved). A simple method for the evaluation of whether or not a post-synthesis treatment has induced structural change is to look at the effect on the powder diffraction pattern. However, if there are unindexed lines, either the indexing is incorrect or there is a crystalline impurity present. If all lines can be indexed on a single unit cell, there is probably only one crystalline phase present. Indexing a pattern can also serve to establish whether or not a phase is pure. Of course, lattice parameters can also be used to study the effects of post-synthesis treatment (e.g., ion exchange, calcination, dealumination, sorption, etc.), to estimate Si/Al ratios in well-calibrated systems such as faujasite, to monitor a phase transition as a function of temperature, or to begin the structural characterization of a new material. The identification of an unknown phase can sometimes be facilitated if the lattice parameters are determined, and these are compared with those of known zeolites. Įven a cursory examination of the patterns in the Collection shows that different materials with the same framework type can have markedly different diffraction patterns, so direct comparison is not always straightforward. An up-to-date internet version of the latter that includes data for newly approved framework types is maintained at. For laboratories without such a set of patterns or for those whose set is incomplete, the Synthesis Commission of the IZA has published experimental patterns for some zeolites in the book entitled Verified Syntheses of Zeolitic Materials (also available on the internet at ), and the Structure Commission has published a book entitled Collection of Simulated XRD Powder Patterns for Zeolites, which contains at least one representative powder diffraction pattern for each known framework type. Ideally, a laboratory should have a set of “standard” zeolite patterns measured on the in-house instrument for direct comparison. Probably the most common application is the use of a powder diffraction pattern as a “fingerprint” in the identification of synthesis products. Powder diffraction techniques are used on a routine basis by many zeolite scientists. McCusker, Christian Baerlocher, in Studies in Surface Science and Catalysis, 2007 4.2 Common applications ![]()
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