When comparing a PXRD of MIL-100 MOF with known literature it was observed that there are peaks on the same 2θ values but in different intensity ratios. Can I still say that I have synthesized the same compound? What more can I infer from varying intensity ratios ?
Below document contains the observed and the literature PXRDs.
It may not be a different atomic structure, but an effect of preferred orientation.
A comment: the background levels are very different. I notice your 'Fe powder' sample has a very high background that is rising, if you are using Cu radiation this will be from Fe fluorescence in the sample. If this is the case then it will be well worth trying to reduce this fluorescence by using a Co anode or some form of energy discrimination at your detector.
Returning to matter of intensities, it may be that the 'known literature' diffractogram has the incorrect intensities - was that from a published crystal structure or was it a similar experiment to your own? The best way to determine what they should be is to find a published crystal structure of this material and generate a synthetic diffractogram fr comparison.
HI,
yes, you have synthesized ... the intensity does not matter for your case , as your all synthesis procedure details might not be the same (I am talking about the reproducibility).... However, you can talk about if you see any change during your own work.
Regards,
Mohammad
It means, that you have the same crystal lattice parameters (cell lengths and angles), but the atomic structure is different. Did you use the same solvent for preparation, than in this article (where you have found this PXRD)? Different solvents in the voids change the intensity ratios
It may not be a different atomic structure, but an effect of preferred orientation.
A comment: the background levels are very different. I notice your 'Fe powder' sample has a very high background that is rising, if you are using Cu radiation this will be from Fe fluorescence in the sample. If this is the case then it will be well worth trying to reduce this fluorescence by using a Co anode or some form of energy discrimination at your detector.
Returning to matter of intensities, it may be that the 'known literature' diffractogram has the incorrect intensities - was that from a published crystal structure or was it a similar experiment to your own? The best way to determine what they should be is to find a published crystal structure of this material and generate a synthetic diffractogram fr comparison.
I can agree with Ian J Slipper too, it can be a background scatter at low 2 theta values. The other intensity ratios seems to be similar. So don't worry you prepared succesfully the MIL :). Maybe there are some difference in the solvent in the voids, or maybe background scatter. If you have the same lattice, the framework should be the target compound.
there are always some differences most likely presence of solvent in the pores and preferred orientation or too large crystals. But do not worry you got what you want if cell parameters are very similar to the reported, this is most important:)
Thanks for your feed back. But I'm confused regarding a small fact: in theory I have read that θ depends on braggs law. so low θ values could be from high d spacing (indicating more porosity). and my product have shown low intense peaks at low θ values compared to the literature values. So cann't this mean that the MOF I have synthesized has less amount of pores of high d spacing values? indicating that the synthesized MOF is less porous compared to the literature value? Because the more the amount of a particular d spacing, more the diffraction and more the intensity right ?
I think you have used variable slit during the experiment and this is the problem. try to measure it again just with constant slit.
Miss Ian, Thank you for pointing out the background scattering. It sounds a very plausible explanation. reported data has been observed by many published journals in the same intensity ratios.
And same solvent system has been used. But the purity could vary. Thanks for pointing me out that solvents could vary the diffraction intensity.
Sir Bartlomiej, how can the intensity vary with the size of the crystal ? Is there a basis for it ?
Vihanga, it is not correct to simply equate large d-spacing with high porosity, all it means is that you have some large d-spacings within the unit cell. It has nothing to do with porosity as I understand the term (my background is geological) however, if you want to interpret the distance between the atomic lattice planes as 'pores' then fine. But where do you draw a limit? What is your definition of a pore? The size of the largest d-spacing in your diffractogram is around 25A, and that is not counting any peaks that may have been missed below 3 degrees 2-theta (since I cannot see the background below 3 degrees I would always wonder if there are more low angle peaks to be discovered).
It is also not correct to correlate the intensity of particular peak with the 'amount of a particular d-spacing'. While it is true that as the diffracting volume increases, the overall intensity of the pattern will increase, it is not true for individual peaks; these will not vary independently. These relative intensity variations are due to the arrangement of the various atoms in space within the unit cell, they will be fixed for given atomic structure. Only when the powder has non-random orientation (preferred orientation) will the relative intensities deviate from the expected values.
Dear Vihanga Munasinghe, maybe u cud povide ur raw data with the detail of it compound. But if it strictly confidential u may send via inbox. This is the better way, not only for above experts comments but also can reveal other information.
I had the graph plotted from the raw data attached. And now I attached my X/Y data along with the cif. file I took the information from. Thanks for your consideration. :)
Hi, if you have too large crystals you can have effect like preferred orientation but in this case it is not enough crystallizes to have equal representation of each crystal orientation, but if your sample was ground properly it should not be a problem.
What about the slit? and fluorescence?
you can also try to change measurements geometry (e.g.. use capillary) this usually reduce preferred orientation problem but there might be some x-rays absorption
good luck
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