Hi Matt,
On Wed, Dec 4, 2013 at 1:17 PM, Matt Frith
Dear All,
I need some help in fitting an amorphous iron oxyhydoxide sample. I am having difficulty producing a good fit, particularly in the k=4-6 range. Fitting this region well is very important for me, because if I add another metal(+3 oxidation state) into my system, this is where I observe the most quantifiable changes (The shoulder @ 5 A^-1 and the min @ 5.6 A^-1). Thus far I have been unable to fit the shoulder well enough to make meaningful comparisons.
I have been fitting in kq with kmin=2.566 And kmax=10.877, and Rmin=1 and Rmax=3.5, and using the goethite O1.1, Fe.1, and Fe.3 paths. Attached is an Artemis file (P41_006_merge_norm_TRANS.fpj) for an amorphous Fe oxyhydroxide sample (Fe only, no other metals). The data was collected at the Fe K-edge.
Is there a way to fit just this region (k~4-6 range) in k? If so what is the best method for doing this? If not, does anyone have suggestions as to how I can improve my fitting? Should I fit the data in k since the shoulder is less evident in kq?
Thank you for your time.
Sincerely,
Matt Frith
I haven't looked at your project, but yes, you can restrict the fit k range. Just use kmin=4,kmax=6. That said, I would not recommend using such a short k-range. For one thing, if you get a fit that does match your data well in this region but doesn't match outside this region, would you call it a good fit? I sure hope not. In general, any "features" that appear in chi(k) are a mixture of many distances, and you may have to go out very far in R to accurately reproduce them. There are cases where you only have to consider a split second or third shell to reproduce a diagnostic k space feature for some mineral phase, but again, the better approach is to describe this as a structural (R space) feature that translates into a spectroscopic (k space) one. Someone may be able to come up with a good counter-argument, but I recommend not ever fitting in "k" space or "q" space -- just fit in R space. --Matt