As a non-TEM user, I just came accross the following question: Assuming I have a phase that can be either space group 161, i.e., trigonal (-3m) crystal symmetry, or SG 176, i.e., hexagonal (6/m) crystal symmetry, can I differentiate between them based on SAED reflexes? Are there particular orientations of the zone axes where I can, and others where I can't?
An additional question: If you were to performe phase ID, what technique(s) would you use in case that high spatial resolution is required, i.e., bulk diffraction techniques cannot be used?
Hi Sven,
Usually, the lattice symmetry is unique to a particular space group. I mean each space group corresponds to a ONLY ONE Bravais lattice. The hexagonal can be further classified into two crystal systems, the hexagonal and the trigonal. The trigonal system is an exception. So, ideally speaking there should be 6 crystal class, instead of 7 crystal systems. Depending on your choice, trigonal crystals can be expressed using an equivalent hexagonal/rhombohedral setting. So, for a particular crystallographic phase, I guess it is all the same and only the convention changes.
Regarding the phase identification, if you can identify the zone axis, index the SAED pattern and know the lattice parameters of the phase, you can always identify the phase from an electron diffraction pattern.
Hope it helps.
Thanks, Well, what I'm actually trying to figure out is that I have done the discrimination between the two crystal symmetries using EBSD. I'm quite confident that the results are right, as the crystallographic information of the EBSD pattern is quite rich. Now I do have some analysis results stemming from SAED, where the material was indexed as hydroxylapatite, and I wonder whether you could index that as whitlockite as well (these are the two space groups I was referring to). In other words, are they distinguishable via SAED or not, or can one be misinterpreted for the other. Please see the indexed diffraction pattern below.
Could you attach the EBSD pattern as well? I could do a dynamical simulation in order to check whether both phases are distinguishable. Of course I need the phases :-).
@P. Sahu: There is still another option which you do not discuss: pseudosymmetry. Many phases are orthorhombic but pseudo-hexagonal. There are even triclinic phases in databases available which have a cubic, hexagonal, tetragonal, orthorhombic or monoclinic lattice. This means, the lattice symmetry can be clearly higher than the space group symmetry.
Regarding the trigonal and hexagonal point lattice you are certainly right. But we do not see only the points but also some fine structure which should display the specific point group or Laue group symmetry
Hi Volker,
That's what I thought. The data shown is not mine. The indexed electron diffraction pattern shown above stems from a publication in a high profile magazine, where it was commented that the pattern has been indexed as hydroxylapatite. I'm having trouble to follow that arguing, or is that anything that requires arguing?
I don't see from the pattern that it is hydroxylapatite, and I don't think the indexing can be correct, or is there some way of notation where lattice space and reciprocal space are interchanged, transformed or anything I don't understand?
I rather don't want to mention the paper on a public forum as I don't intend to discredit the authors, especially since it is a fantastic piece of work that I wish I had written myself. It's rather for me to see how my, mostly not too good quality EBSD and EDS data compares with that.
Hi Gert,
Thanks for the point you raised. My answer was a generalized one, without knowing what/where exactly the problem was. I certainly did not consider pseudosymmetry, whose presence may indeed lead to uncertainties in determining the correct space group.
Hi Sven, sorry for the delay but this is the intensity simulation of one pattern you sent. Whether it is really Whitlockite is hard to say since there are too small bands visible, but the lattice can be definitely described by this band arrangment and the intensity of the major reflexes tell us that the structure has to be very similar.
6 bfold and 3 fold symmetry can be distinguished with convergent beam electron diffraction in a TEM. In SAED Fedels law will create in the 001 zone axis always 6 fold symmetry. This you can overcome by dynamic scattering in CBED. Has the central beam a 3 or 6 fold symmetry will tell the story. For this you need a thick crystal < few 100nm at 200kV and align the 100 zone axis and illuminated the area with a spot and a convergence angle of
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