X-ray diffraction
X-ray diffractometry (XRD), or X-ray diffraction is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract in specific directions.
Why do you do XRD measurement?
The main purpose of XRD is to confirm whether your crystal is formed with the correct simulated structure or in agreement with previous works.
Pre-requisite understanding
Before using the pointers on how to interpret XRD results, there are some pre-requisite knowledge on crystal structure that must be well-understood, in the following order.
If you are unable to check all the boxes, you can explore many open resources on the internet. Chemistry on Libretexts mostly addresses all the scopes listed above.
Verifying your XRD results
To verify your XRD result there are two main methods: (1) compare the peak position and intensity with previously published works, and/ or (2) do a spectral fitting with theoretical files. For publication quality, refinement is highly advised whenever possible. I had the experience of the refinement being requested when it is not presented by a reviewer during submission stage.
Comparison with XRD database
There are some organisations that provides diffraction data, either open source or for a fee. These are some reliable sources for such XRD references.
If you are familiar with Python programming language, you can also generate your own once you have a crystal structure, many tools exist to generate a diffraction pattern, for example pymatgen using the XRDCalculator, though these will need to be artificially broadened if you want something that looks like an experimental pattern. You can find the tutorial to calculate diffraction pattern with pymatgen package in this MSc in MSE - MLE5219 Materials Infomatics course by Dr Zeyu Deng from NUS in the link below. You can find the material lattice parameters on Materials Project as well. These are all open source and free to access.
Fitting/ refinement
In an oversimplification, fitting is forcing a mathematical equation into an observational spectral curve. Two of the most prominent techniques are Rietveld and LeBail fitting. Both techniques provide a way to extract the lattice parameters and are widely accepted for publication. When possible, Rietveld should be tried first before the other alternatives.
Software: Profex
One of the best software to carry out Rietveld/ LeBail fitting is Profex. It is open source and free, and has been successfully used by numerous work. You can download and watch tutorials from their website on the links below.
More tutorials can be found on the website or their YouTube channel.
Writing interpretation
While interpretation of XRD differs on each project, it generally follows similar flow. To write the crystallography section of the main text, you can write something in this order:
The crystal space group, calculated & observation (fitting),
The primary peaks, as matched with other previous works and/ or theoretical calculations,
(If applicable) changes in peak positions, or comments on peak widths.
In the supplementary section, figures and tables regarding Rietveld/ LeBail refinement and its extracted lattice parameter should be made available.
FAQs
Practice sheet
You may use this practice set taken from the dataset of CsCu2I3 perovskite powder, J. Mater. Chem. C, 2024,12, 2398-2409. The refinement parameter is included in the last page for your reference.
More resources
Textbook/ overview
High-resolution XRD
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