Thanks for the help. My space group was put in as F d -3 m, but the
origin does show a multiplicity of 16.
I'll figure out how to rewrite the .cif file, to get the right Si
atom position.
I don't see a 222 reflection in my experimental data, fortunately!
Also, I've worked through a number of Dr. Toby's tutorials and have
never found them to be a cure for
insomnia. Instead, they keep me up with new things to try.
Thanks again,
David Lee
On Dec 16, 2007, at 2:18 PM, [EMAIL PROTECTED] wrote:
The space group of silicon is Fd-3m (usually written with bar over
the three). There are two settings of that space group. (This
situation arises because the point of highest symmetry does not
coincide with the point of fewest equivalent Wyckoff positions.)
Have a look at the drawings in the International Tables for that
space group (#227).
Origin choice one has the origin at a point of -43m symmetry, which
has multiplicity 8. This is how Si is usually drawn in the text
books, as an fcc lattice with a basis of one atom at the origin and
another at (1/4, 1/4, 1/4).
Origin choice two has the origin at a point of -3m symmetry, which
is in the middle of a bond between two Si atoms. In that choice,
the atoms are located at 8 equivalent positions generated by (1/8,
1/8, 1/8).
You can easily imagine that if you put the atom at the wrong
position for the assumed origin choice, the intensities will be all
wrong. One quick check is to look in the GSAS atom list at the
number of equivalent positions for your Si atom. It should be 8,
and I bet you'll see 16.
Also, if the only charge in the unit cell is at the position of the
atom, reflections like (222) will not be observed. However, taking
account of either bond charge or anisotropic (non-spherical)
thermal displacements, that reflection will acquire a very weak
intensity.
Your table has a nonzero observed intensity for the (222). I
assume you don't actually see a peak there, do you? If there is no
peak in the raw data but GSAS wants to put one in, it is probably
an issue with the background. If you do observe such a strong
peak, there is an issue with your diffractometer. Bremsstrahlung
contamination getting by the discriminator?
I think that should answer all of the problems you describe.
(I see that Vincent Favre-Nicolin has written an equivalent answer
in the time I was writing this.)
^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~
Peter W. Stephens
Professor, Department of Physics and Astronomy
Stony Brook University
Stony Brook, NY 11794-3800
fax 631-632-8176
