On Sunday 11 January 2009 12:23:26 pm Eugenio Otal wrote:
I am simulating the XANES spectra of a erbium atom in an interstitial site of ZnO. I needed a SCF radius of 9.75 (316 atoms) and FMS radius of 11.90 (599 atoms) to obtain no change in the simulated spectra (differences of 0.01% or less) Now I am trying to simulate Er(OH)3 and it is using SCF 10.90 (622 atoms) and I still haven found convergence, I will need more even larger radius for FMS. Does the XAS phenomena have this distances? Has this distances physical meaning?
Eugenio,
The mean free path is quite long just above the Fermi energy. It is conceivable that, in a well ordered material, the cluster size required for convergence can be quite large. I believe that a very large cluster -- something like 13 A -- is needed for pure silicon. Of course, I doubt
erbium hydroxide is that well-ordered!
So, are you are seeing physics associated with the mean free path or a
Bruce and Eugenio, It is generally believed that the cluster size (or called spatial resolution) is mainly limited by MFP, to around 1 nm (in the near-edge region), but further spatial limitation may come from elastic scattering (i.e. through phase cancellation). This is what we tried to understand through FEFF calculations in one paper. I put the abstract here in case you want to check: We have investigated the factors that determine the degree of localization of the information obtainable from electron energy loss or x-ray absorption fine structure. Inelastic scattering of the excited core electron limits the volume of specimen contributing to the backscattered intensity to a diameter in the range of 12 nm, dependent on the excited-electron energy and the composition and crystal structure of the sample. Phase cancellation between the backscattered waves further reduces the effective diameter that determines the observed fine structure to below 1 nm. Since the spatial resolution attainable by transmission electron microscopy can approach 0.2 nm or can even be below 0.1 nm (with aberration correction), we predict that delocalization arising from the excited-electron range may limit the resolution of images based on changes in core-loss fine structure. (J. Appl. Phys. 104, 034906 (2008)). Feng that problem
in Feff's construction of the potentials? I'm afraid I don't know how to answer that, but hopefully John or one of his gang will weigh in. Do you see the changes larger than your threshold over the entire data range or only very close to the edge?
Good luck, B
--
Bruce Ravel ------------------------------------ bravel@bnl.gov
National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973
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