I'm trying to apply a Rietvelt refinement on XRD data (theta-2theta) from a standard Si sample in order to model instrument contribution.
The Si sample is diamond cubic (Fd3m, or space group 227) with a lattice parameter of 5.4309 angstrom.
However, Fullprof doesn't automatically generate the right reflections :
-Forbidden reflection (222) appears
-Reflections (220), (422) and (620) do not appear even if they are supposed to be allowed according to extinction rules.
Any ideas how to remove the forbidden reflection and add the missing ones so that my experimental data fits the simulated data?
NB I have the same issue with GSAS-II
Best regards,
Simon Laliberté-Riverin
This is a standard issue with Fd3m. There are two possible settings (marked in ICSD as Fd3mS and Fd3mZ) and if the software uses one of them and the Si coordinates are from another one you will get the wrong intensities. If you entered Si as (0;0;0) try (1/8;1/8;1/8) and vice versa.
I attach to this comment the calculated patterns (GSAS, CuKa) with wrong and correct Si positions
1- In GSAS software there is a option for edit background where you can adjust the background up to a certain limit. maybe this can help you.
2- you can also try the "Le Bali" method (in the LS controls tab).
Dear Simon,
Can you attach your recorded XRD pattern for Si standard sample? I think there may be error in space group entering or in lattice parameter. It is not possible if correct value of lattice constant and space group are entered.
This is a standard issue with Fd3m. There are two possible settings (marked in ICSD as Fd3mS and Fd3mZ) and if the software uses one of them and the Si coordinates are from another one you will get the wrong intensities. If you entered Si as (0;0;0) try (1/8;1/8;1/8) and vice versa.
I attach to this comment the calculated patterns (GSAS, CuKa) with wrong and correct Si positions
Thank you Alexander, I changed the atom position to (1/8;1/8;1/8) and it solved the problem.
Best regards!
Simon
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