Preferred orientation anomaly

jlstapper

Hello everyone,

This is a question not so much related to Topas itself, but rather an issue with the XRD patterns I'm collecting, although I'm using Topas to analyze the patterns.

The samples I'm analyzing are hydrating cement minerals, and so they contain crystalline calcium hydroxide or portlandite. This mineral shows strong preferred orientation with the c axis perpendicular to the sample surface, so mainly the (001) reflection is enhanced.

Interestingly, it is just the (001) reflection which shows PO, while for instance the (002) reflection does not. In my understanding that shouldn't be the case because the whole family of planes (00n) would suffer from the preferred orientation. I was wondering if any has every come across this issue, knows what may cause it, or knows of a way to tackle it?

The main problem that arises is that if I apply a correction in Topas, it will increase the calculated intensity at the (001) reflection to match the observed intensity, but this automatically increases the calculated intensity at (002) which is not actually observed enhanced, thus leading to a considerable overestimate.

Thank you in advance for any help and advice!

rowlesmr

Is this in a bragg-Brentano, constant divergence instrument? Was the specimen spinning?

jlstapper

rowlesmr Yes, this was a Bragg-Brentano geometry with a fixed divergence slit. However, the specimen was not spinning. These were scans collected on a paste sample (powder + water) covered with a thin Kapton film to prevent excessive moisture loss.
After scanning for a certain amount of time, the sample reacts and hardens and I've already powdered it for other analyses, so I can't return it to the original state in which it was measured. This phenomenon did occur in multiple samples, however!
Please let me know if any other information about the setup can help solving the issue. I can also provide a scan.

Kind regards,
Julian

rowlesmr

Can you repack and recollect the data?

rowlesmr

My first thought is that there was a large oriented crystal near the edge of the beam for the 001 reflection. By the time the angle has changed to the 002 reflection, the footprint has gotten smaller and is no longer seeing the original oriented crystal.

It may be able to mitigate the issue by spinning or using constant illumination length.

johnsoevans

The only other reason I can think of is if the chemistry isn't quite what you think. e.g. with some clays exchange of interlayer cations will significantly affect 001/002 intensity ratios. If something like this was happening then all the 00l intensities would potentially be affected. In Mathew's case of a single oriented crystal (or a few) it might just affect one of the 00ls.
If your diffractometer is misaligned then the low angle 001 reflection intensity could be wrong - this normally leads to a weaker peak than expected though.
I agree with Mathew's suggestion of getting data with different experimental conditions if that's possible.

jlstapper

Thank you both for your suggestions, Mathew and John. I plan on repeating this experiment and the method multiple times, so this will hopefully help me troubleshoot the issue!
In the 2Th range that I'm scanning, only the (001) and (002) reflections appear, but on the next run, I will do a final scan with a much wider range to capture other (00l) reflections to get more input about what exactly is causing this.

rowlesmr

Yeah, you need all the angles to so anything definitively with PO.