On Dec 16, 2007, at 11:20 AM, David Lee wrote:
I'm trying to fit an experimental Si pattern to refine the zero
offset for out instrument. The
calculated spectrum has 2 extra lines, and three missing lines.
I've hand calculated the
expected location of lines in the experimental spectrum and account
for all of the lines,
so I know I don't have the wrong spectrum or contaminated Si.
Choice of space group origin will affect the computation of
intensities but not which reflections are included in the pattern, so
that is not the problem. The fact that the 222 reflection shows up in
reflist makes it seem likely to me that you do not have the space
group input correctly.
A common error in GSAS (since it derives extinctions from the
symmetry terms) is not to include spacing between the symmetry terms
in the H-M name. You want F d -3 m (F d 3 m works too), but not F d
-3m, for example. Checking the generated symmetry operators to see
they match in I.T. is always a good idea.
If you want an explanation some of this stuff related to symmetry, I
think I have a canned lecture on it in the APS web site on
crystallography training and insomnia prevention.
Brian
********************************************************************
Brian H. Toby, Ph.D. office: 630-252-5488
Materials Characterization Group Leader, Advanced Photon Source
9700 S. Cass Ave, Bldg. 433/D003 work cell: 630-327-8426
Argonne National Laboratory secretary (Marija): 630-252-5453
Argonne, IL 60439-4856 e-mail: brian dot toby at anl dot gov
********************************************************************
