Hi,
I'm trying to use the trigonal (rhombohedral) formula to determine the lattice values of a unit cell (R-3c, 167) using the 2thetha values extracted from a XRD powder diffraction pattern. However, the rhombohedral formula does not contemplate a != b != c . The unit cell i'm trying to extract it the Calcite Hexagonal Scalenohedral
Is there any XRD equation that contemplates such scenario? or should I use the trigonal scenario and solve 6 equations with 6 unknowns?.
This is indeed the case (see Xuej Tan's answer), the question is formulated not clear.
The trigonal s.g. R-3c admits two sets: 1) Hexagonal, which you have and
2) Rombohedral where A=B=C and Apha=Betta=Gamma.
The parameters of both cells linked by equations (Caps for Rombohedral set):
A=a/3*sqrt [3+(c/a)2]; sin(Alpha/2)=3/2 / sqrt [3+(c/a)2 ].
Vector equations between both sets allows to transfer any geometrical parameters from one basis to another:
a=A-C; b=C-B; c=A+B+C
Use PowderCell. It enables the transformation of both settings into each other. If you have the correct structure descriptions your equations will hopefully work. I only wonder why do you want use the rhombohedral setting?
FullProf is useful to calculate lattice parameter for your diffraction pattern.
Dear Emerson,
please find, in the attachment, the easy way to change from one reference system to the other one, for indexes of the planes (faces) and directions
Sincerely yours
Dino
Dear Dino,
It seems, that in 2 of your equations, the vectors (a hbhch) and (arbrcr) should be mutually exchange their places. How is your opinion?
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