On Thursday 02 February 2006 23:58, Feng Wang wrote:

Hi Everyone, I had a trouble with the input for webatoms in trying to get feff.inp from it. I got the crystal structure information from papers, as attached below, would you please suggest what should I input for the items listed below:

Shift: Alpha= Beta= Gamma= The table of Crystallgraphic sites:

The purpose of the shift vector is explained in the Atoms document: http://cars9.uchicago.edu/~ravel/software/doc/Atoms/Atoms/node4.html#SECTION... The space group R-3c will be recognized as rhombohedral. This means that, for the hexagonal setting, you need to specify a and c and Atoms will know that gamma is supposed to be 120 degree.

Would Dr. Ravel or some of you can help with that? Thank you so much in advance.

Best regards,

Feng

Crystal structure information:

Iron trifluorine has a ReO3 type structure (hexagonal), with spac group of R- 3C, with two Fe atoms at (0 0 0) and (0.5 0.5 0.5)and 6 fluorines at (x, 0.5- x, 0.25), -(x, 0.5-x, 0.25), (0.5-x,0.25 x),-(0.5-x,0.25 x), (0.25 x, 0.5- x), -(0.25 x, 0.5-x), and with x=-0.164.

The point of crystallographic notation and so the point of Atoms is that you can specify all the atom positions with a very small number of parameters. In this case, you need to specify the a and c lattice constants and tell atoms that there is an Fe atom at (0,0,0) and a F atom at (-0.164, 0.336. 0.25). Finally you tell it the space group symbol, which is sufficient information to generate all the rest of the positions. There are ample examples of how this works on my web sites and other XAS sites, including Matt's atoms input database web site. Here is an example that is directly relevant -- it's even the same space group: http://cars9.uchicago.edu/cgi-bin/atoms/atoms.cgi?file=V2O3.inp although, in this example, the rhombohedral setting (specifying a and alpha) is given. B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/