Dear Stefano,
if a Primitive trigonal cell is constructed from the hexagonal setting
of R-3c, then a very simple recovery of a lower symmetry space group can
be derived.
The Primitive unit cell can then be triclinic or possibly, monoclinic,
depending on which/and how many/ a=b=c al=be=ga conditions you release..
Just check on the International Tables.. within the subgroups of R-3c..
I am presently away from my office, so I can't give you the correct
answers..so..try looking at the proper Table!
Regards
Norberto
[EMAIL PROTECTED] ha scritto:
Dear all
I need your kind help
I am investigating a trigonal system. I collected neutron diffraction
patterns at T=300K and T=4K.
The data at T=300K can be nicely fitted with the spg R-3c.
At T=4K again I can described the data with the spg R-3c, but I noticed that now
the peaks with a larger c-axis component (see the (006) peak picture in
attachment) are splitted in two: it is like as at low temperature there are two
phases with different c-axis (10.5838 and 10.566 Amstrong) and same a-axis.
I don't think that the sample is chemically phase separated because at room
temperature the (006) reflection is clearly a single peak. The splitting
appears only at low temperature.
Could anyone suggest me any possible explanation of this splitting (lattice
distortion, modulation, etc)? Could be possible a triclinic distortion?
I don't know how to fit the data at T=4K. Should I change the space group
because now I have two peaks while the R-3c gives me only one peak? Then by
which criteria should I choose the new space group?
thank you very very very much for your advices
best regards
Stefano Agrestini
Physics Department
The University of Warwick, UK
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