Dear Jens, Peter Sthephens is right, try first to see if you have an anisotropic strain effect. But if not, it doesn't mean that you have not a simple size effect, not necessarily staking faults. The size anisotropy model in GSAS is in fact the rod (or plate) model (I wonder why the needles model was not introduced - sin(phi) in place of cos(phi)) and you have to give apriori the "broadening axis". For non cubic is easy to guess because frequently is the n-fold axis (n=2,3,4,6) and the average over equivalents has no effect. But as Peter said the guess is ambiguous for cubic (and not only). Nevertheless you have an approach for size anisotropy that needs no apriori information (except the Laue group), the spherical harmonics approach. For details see the same (J. Appl. Cryst. 31, 176 (1998)). In spite of some skeptical opinions (not clearly argued) the approach is enough robust and you can expect to obtain accurate volume averaged column length as function of direction.
Best wishes, Nicolae Popa > > 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 > > ~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~ > Peter W. Stephens, Professor > Department of Physics & Astronomy > State University of New York > Stony Brook, NY 11794-3800 >
