Jens,
Your effect might be more related to strain than size broadening. You
would have to check widths at various diffraction orders in a given
direction (i.e., 111, 222, 333, etc., vs 200, 400, 600, etc. for an fcc
material). If the widths increase roughly in proportion to diffraction
order, but with a different slope for the two directions, you have
anisotropic strain broadening.
This was noted by Stokes and Wilson (Proc. Phys. Soc. London 56, 174-181
(1944)) in cold-worked fcc metals, who had a model as a random distribution
of stresses. N. Popa and I have independently considered the effect more
recently from a phenomenological viewpoint (J. Appl. Cryst. 31, 176 (1998)
and ibid 32, 281 (1999), respectively). And there is a growing literature,
especially from the group of Tamas Ungar, on the effect of specific lattice
defects on strain-broadening in diffraction patterns.
Regarding your use of the anisotropic size broadening model in GSAS, as you
point out, "broadening axis" for a cubic material is a rather iffy concept.
If my understanding is correct, GSAS does not do the full symmetry
equivalents in that calculation, and so it's a matter of luck how the
calculation will be done. That is, if you list a (111) broadening axis,
and the reflection list contains (111), you'll get one answer, but if you
list (-1 1 1) broadening axis, the (111) reflection will be calculated
differently.
-Peter
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Peter W. Stephens, Professor
Department of Physics & Astronomy
State University of New York
Stony Brook, NY 11794-3800
