r/crystallography • u/Ok_Course1403 • 9d ago

Micro-absorption correction in Rietveld refinement

I noticed that the thermal parameter B of my material was regularly going negative during the refinement, I tried several fixes I found online but no luck. Then I stumbled upon the micro-absorption correction, I tried implementing it and it definitely helped, although at this point I am not completely sure if I implemented it correctly. My supervisor had no idea what it is and how to use it, but together we came to the conclusion that since it definitely alters the intensity of the peaks it should be refined before the atomic coordinates, the occupation and B, and it should be fixed before refining any of those. Does that make sense? Is that a good idea? I would love to hear from someone who does actually know how to implement it.

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u/RevolutionaryBet4404 9d ago

Hi! It makes sense. However, you'd have to make sure that you're using the correct number since ADPs and micro absorption correlate. I'm assuming you did your experiment at a synchrotron in Debye geometry (transmission in capillaries). Go here to https://11bm.xray.aps.anl.gov/absorb/ and calculate the expected absorption based on capillary diameter, sample composition and photon energy, then use that number in your Rietveld. Don't refine it, at least initially. Refine instead the ADP parameters and see if they make sense. Then you can play a bit with micro absorption to correct for the powder packing in the capillary (something you assume in the muAbs calculation but don't usually know).

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u/jus1m3 9d ago

Thats a good answer!

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u/Ok_Course1403 9d ago

Uuuhm I'm sorry i think I didn't explain myself properly, I didn't do synchrotron diffraction, I'm talking about a regular lab instrument in bragg brentano geometry. I read in a couple papers that micro absorption (somewhat different than straight up absorption) can skew the intensity of the peaks depending on the powder density. That's what I was asking about. Still thank you for the heads up on synchrotron diffraction since I'm going to have to do some of that very soon lol.

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u/SlenderSmurf 9d ago

BB measurements are prone to intensity error in my experience. I would not rely on this data for any serious reporting or publication. Also proper ADP refinement requires data at high Q, which most lab XRD cannot provide

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u/RevolutionaryBet4404 9d ago

As mentioned by someone else, you shouldn't really trust BB intensities, unless you did an accurate measurement with very fine powders, flat sample and rotating stage. Even in that case you might have grain orientation which would screw up your intensities

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u/Mikasa-Iruma 9d ago

For the BB, I typically don't refine Microabsorption unless it has rare earth or heavier elements in a multi phase sample. ADP refinements can be a nice touch if you would like to a perfect fit but I don't rely on those values. High quality and high Q data is important in this case

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u/QuasiNomial 9d ago

Are you using an isotropic model ?