[tulip11]

Hi all,

I came across the sentence below whilst reading an article detailing PXRD peaks and was wondering if anyone could help me make sense of it?

"The two low intensity 2θ peak values between 8 and 10° are attributable to a small amount of nonorientated solvent-inclusive MOF-508a and solvent-free MOF-508b, respectively."

From my understanding the presence of peaks in a PXRD diffractogram by definition requires orientation and each peak correlates to a specific crystal plane.... If this is the case how can a peak correlate to the ''non-orientated crystal form''. Surley non-orientated crystals would be amorpohus and thereby give no XRD peaks?

Any insight into the meaning of this phrase or any misunderstanding I have would be appreciated!


Article: M. Wilson et. a., Appl. Mater. Interfaces, 2018, 10, 4057

[Corribus]

I haven't read the paper in question, so it's possible they are talking about something completely different, but nonoriented may refer to the fact that (as in most powders and microcrystalline samples) there is no preferred orientation of the crystal planes of individual crystal grains with respect to the X-ray beam direction in the powder XRD experiment. If the crystals are preferentially oriented, you can get intensification of some peaks with respect to others, because those crystal planes can be more likely (compared to random orientation) to scatter X-rays to the detector.

Remember, unlike in single crystal XRD, power XRD looks at powders, and unless they are compressed or something you have almost by definition nonoriented crystal planes, because the crystal grains are oriented randomly in space. Think about, e.g., table salt. If you grind it up, the crystal planes are facing in lots of different direction with respect to the X-ray source, which is why you see scattering from all the possible crystal planes in the XRD pattern. Compare that to if you did the experiment on one single large perfect crystal, which would have a preferred orientation with respect to the X-ray source, so you would only see scattering from certain planes and no scattering from others.

[tulip11]

I haven't read the paper in question, so it's possible they are talking about something completely different, but nonoriented may refer to the fact that (as in most powders and microcrystalline samples) there is no preferred orientation of the crystal planes of individual crystal grains with respect to the X-ray beam direction in the powder XRD experiment. If the crystals are preferentially oriented, you can get intensification of some peaks with respect to others, because those crystal planes can be more likely (compared to random orientation) to scatter X-rays to the detector.

Thank you - that helps explain it alot! From that could you say that if you have a lot of randomly orientated crystals such as in a powder is it possible that a diffraction peak occurs across a lot of the orientations and that then rather than assigning that peak to e.g. the (100) face you could think of the peak as belonging to a lot of different, random orientations?

[Corribus]

Sorry,  not following the question.  Could you clarify?