[GSAS-II] Rietveld refinement of electron powder diffraction

[Kamil Krawczyk]

Hello all,

I am studying the effects of photoexcitation on the structure of quantum
dot monolayers. Our samples are essentially monolayers of quantum dots, and
as such, we are sampling an ensemble of these dots (2 - 10 nm in diameter)
with our electron beam size (~120 um). These images are converted to 1D
diffractograms after center finding and background removal.

I am currently trying to refine a structure from a known starting material
(FM3m rock salt lattice). However, what is is interesting is that a peak
that is not accounted for in simulated diffraction is present in our
experimental data. Furthermore, to add intrigue to the situation, this peak
disappears after laser excitation (and returns, as it seems the process is
reversible), creating a new excited structure that seems to far better
match an FM3m lattice by refinement.

Apart from the loss of that one unaccounted for reflection, one other
reflection has a far greater peak shift compared to thermal expansion, and
all peaks obey more or less Debye-Waller-like intensity decreases upon
heating but one higher order reflection, which decreases far more than to
be expected (both the greater shifting peak and the greater intensity
dropping peak are related to the (100) family of planes).

I would imagine there is some sort of strain present and a change in space
group is occurring, but my Rietveld refinement experiments (of which I am
new to since I am no crystallographer, but an experimental physicist) is
not able to catch this peak.

In my refinement, I am adjusting simple things, like peak profile, lattice
parameters (a/b/c, alpha/beta/gamma), as well as March-Dollase preferred
orientation. This is all being performed in Materials Studio via the Reflex
module.

I imagine refinements that I had mentioned cannot account for the
appearance of another reflection, and perhaps even a different space group
altogether, as mentioned. Is there a means to refine to a space group or a
missing peak? Or will trial and error of various similar space groups be
required?

Lastly, the increased peak shift and intensity change in two related
reflections is strange to me. I am not sure if they signify a change in
space group/symmetry just feel. I have not been able to find anything in
the literature with similar results, but alas, my crystallography knowledge
is still burgeoning so I could have easily overlooked some work. If this
seems relatively trivial and I am overthinking it, I would be happy to hear
your thoughts!

Thank you for your time!
Kamil
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