I have the diffractogram of a compound with 3 different types of polymorphs, form A, form B and form C.
Looking at the diffractogram, the 2-theta peaks for form A and form C looks identical, however the absolute intensity (counts/cps) is different. Both diffractogram for form A and form C are from two different parties. My questions will be:
1) If the peaks look similar for form A and form C but having different absolute intensity, are these two different form?
2) Is there are possibility that the difference in absolute intensity caused by sample preparation or due to different XRD machine?
Please look at the following below links which may help you in your analysis:
https://www.rigaku.com/downloads/journal/RJ29-2/RigakuJournal29-2_8-15.pdf
http://www.dxcicdd.com/05/PDF/M_Davidovich_1.pdf
https://perso.univ-rennes1.fr/franck.artzner/publi/16_Allais_JTAC.pdf
Thanks
also the Raman spectra of polymorphs are different. The differences are small but detectable, also in organic molecules, such as for drugs. Pietro
Thanks for the great recommendation. I will look into that. I would like to ask if the XRD pattern is highly similar, would synchotoron XRD be necessary? How likely is it that we require synchotoron XRD to distinguish 2 polymorphs that are highly similar?
As a matter of fact I do not know nor composition neither symmetry of your study case, difficult to say something which make sense. Also, you should define better what you mean by "similarity". Normally to distinguish between two polymorphs it is important to spot especially their characteristic reflections, say supercell reflections or other stuff. You should plan your data collection on this ground. About synch collection, if you have an easy access it is always a good idea.
The difference in peak intensities is correlated to the cation distribution in the occupation sites of the structure. If the positions of the peaks are identical, it is quite acceptable to be considered as identical structure.
same peak positions, different intensity? grind the powder much more and prepare the specimens again: it can simply be that you have just one polymorph with residual preferred orientation. To confirm this, do a DTA/TG and you can immediately see what's going on
I agree with Matteo Leoni reply. This is the first thing you have to do. If your doubts persist, try different techniques as well, as suggested.
As far as I know different phases display different XRD pattern, in terms of peak positions, so exactly as written preferred orientation can justify the observation if the grinding is unsatisfactory you can move in two directions taking an XRD pattern under sample rotation or new crystallizzation attempt
WP
Hi all,
The hypothesis of two coexisting polymorphism co-synthesized in the same run and with different physical properties leading to a quite different response to grinding is quite intriguing. I would be quite interested if you could provide me with some example!
The phenomenon of allotropism is well documented e.g.
Neumann M.A., Tedesco C., Destri S., Ferro D.R., Porzio W.
J.Appl. Cryst. (2002) 35, 296.
“Bridging the Gap - Structure Determination of the Red Polymorph of
Tetrahexylsexithiophene by Monte Carlo Simulated Annealing, First Principles DFT
Calculations and Rietveld Refinement.”
- Sassella A., Borghesi A., Tubino R., Destri S., Porzio W.
Synth. Met. (1998) 98, 83-86.
"Orientation and order of b-tetrahexyl-sexithiophene films deposited by organic molecular beam epitaxy"
Destri S., Ferro D.R., Khotina I.A., Porzio W., Farina A.
Macromol. Chem. Phys. (1998) 199, 1973-1979.
“Tetrahexylsexithiophene: crystal structure and molecular mechanics calculations.”
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