Dear everyone
I have a question about powder X-ray diffraction measurement with MOF.
1. The starting angle is very low, such as 5-35 °, and the high angle side is not very large. What is the reason for making the angle extremely low? Also, why does the high angle side need not be large?
2. I noticed that the results of the simulations obtained from CIF are often different. Specifically, the actual peak is observed on the lower angle side of the simulation angle. Why is this? By the way, it is measured at normal temperature and pressure.
3. I want to calculate the lattice constant from the measured powder X-ray data using CellRef or the like. However, I am having a problem like Question 2. In this case, how should I calculate the lattice constant?
Dear Takeshi Shimizu ,
Metal-organic frameworks (MOFs) are, in general, crystalline, representing a variety of compositions. The main reason is the vast number of inorganic building blocks combined with virtually almost any organic linker.
As a desirable characteristic, a tunable pore structure with several backbones can be functionalized. The MOF particle size and crystallinity strongly influence the physicochemical properties.
In X-ray diffraction experiments, X-rays' elastic diffraction on a solid-sample uses the atoms' scattering to identify its atomic and molecular structure.
MOF powder X-ray analysis data is commonly performed using Pawley extraction. It compares a theoretical diffraction pattern derived from a structure model to the corresponding experimental data. It varies unit cell parameters and peak profiles (area) until the convergence criteria are fulfilled. Nevertheless, it disregards the atoms' arrangement inside the crystal structure.
By contrasting the traditional Rietveld method, Pawley fitting treats peak areas as variables, thus rendering this method applicable to patterns recorded in reflection geometry.
The Pawley method extracts the lattice parameter from the diffraction positions, and the average crystal domain size d from the peak broadening can be obtained.
The angle usually starts low, as they are organic frameworks; the prominent reflection peaks are mainly centered between 5 up to 450 2theta. Using the cited range 5-35 degrees, you are englobing almost 100 % of the expected diffraction peaks. Lower angle peaks are more accurately measured due to the lowest width. An intrinsic higher angle enlarged width is expected. Therefore, they are more sensitive to the reference used in the simulation.
The CIF's obtained in a repository are not standard structures. Instead, they are measured real samples, deposited by their authors subjected to any physical-chemical variations induced by size, stress, stoichiometry fluctuation, measuring conditions, even the used equipment. It is natural to have different simulated X-ray patterns if considered the possible diversity. Use them for comparison purposes only, unless strictly controlled by a standard synthetic route measured in the same equipment under the same conditions.
Best regards,
WNM
Dear Wagner Da Nova Mussel
Thank you for the details. I would like to use Pawley extraction. Is Pawley extraction easy to analyze with software?
Dear Takeshi Shimizu ,
Yes, it is easy to do. There are several free and proprietary software where the routine for Pawley extraction is already implemented. Check FullProf Suite (https://www.ill.eu/sites/fullprof/), TOPAS (http://www.topas-academic.net), Materials Studio (https://www.3ds.com/products-services/biovia/products/molecular-modeling-simulation/biovia-materials-studio/), GSAS (https://subversion.xray.aps.anl.gov/trac/pyGSAS), HighScore (https://www.malvernpanalytical.com/en/products/category/software/x-ray-diffraction-software/highscore), among many others.
Regards,
WNM
Wagner Da Nova Mussel I am appreciative of the response you provided. I was curious as to whether or not you have a comprehensive handbook for undertaking refinement and indexing from the ground up for structural simulations of MOF and COF compounds using raw PXRD data.
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