[Ifeffit] Again _ for help with Webatoms input_sorry for incomplete information

[Bruce Ravel]

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#SECTION00414000000000000000

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 at anl.gov

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