I collected X-ray powder diffraction data by using synchrotron radiation. I successfully carried out Rietveld refinement using a Pbcastructural model. All diffraction lines are well fitted, except a very faint one (its relative intensity is 3/100.); no secondary phases are observed.
The analyzed sample was growth in a form of crystals, but these crystals are strongly affected by mosaicity and twinning.
My doubt is: can this very faint peak be originated by twinning?
I’m not an expert of twinning, but I see that in the case of twinning by reticular merohedry the twin lattice and the individual lattice differ in both their point group symmetry and in their translational symmetry. So I guess that this could give rise to a diffraction effect (peak), isn’t it?
In the case, what could I do to confirm this hypothesis?
Thank you
Dear Alberto,
twinning boundaries can be regarded as planar defects, which typically leads to streaking perpendicular to the planar planes. In powder diffraction, this streaking is typically invisible, apart from the positions, where these streaks intersect the plane parallel to the fault planes intersecting the orgin of reciprocal space, see Fig. 6 in Article Crystal structure, layer defects, and the origin of plastic ...
This requires sufficient planar density of parallel planar faults (twins or stacking faults) and that the intersection points should not conicide with regular reciprocal lattice points with regular Bragg intensity. This yields peaks (occasionally referred to as Warren peaks; first described for faulted graphite) which are not predicted by the ordinary unit cell. Sometimes they suggest on the first view presence of a superstructure (e.g. Article On AgRhO2, and the new quaternary delafossites AgLi1/3M2/3O2...
). This effect is, however, not sufficiently strongly caused by twinning observed on a micrometer scale.Best regards
Andreas
Dear Alberto Martinelli
2 additional possibilities :
i) Synchrotron is a powerful tool to bring evidence of superstructures (modulation of atomic positions, occupancy, aperiodicities,..), for which the signature is often few peaks with really small (mostly hardly perceptible) intensities at low ° (below ~50°). Did you check for any evidences of additional ordering on your single crystal XRD datas ? Try to look at the precession images for any additional spots. Attached is an example of typical signature we found on our compounds. We also performed synchrotron experiment on these same compounds and we found some additional weak intensities satellites peaks.
ii) Is there any possible artifact coming from your synchrotron experimental setup ? Maybe try to discuss about it with your local contact !
Link to the paper for details (precessions are in the Supplementary file) : Article Magnetization Steps Promoted by Structural Modulation in BaC...
Best,
Dear Bastien,
Thanks for your reply. Actually I don't have single crystal data; twinning and mosaicity are observed under polarized optical microscope. In principle, this faint peak could be fit by a super-structural model, but other faint peaks should be observed that are not present in the pattern.
All the best,
Alberto
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