Dear FullProf users,

I would like to make a pcr file for a Heusler alloy with so-called A2 
structure, in which three atoms are completely disordered.
(See Fig.1 in http://dx.doi.org/10.1088/0022-3727/48/16/164011
or this direct link 
http://cdn.iopscience.com/images/0022-3727/48/16/164011/Full/jphysd500490f01_online.jpg)

For example, a straightforward description of A2 Heusler alloy Co2FeSi would be 
like this.
  I m -3 m               <--Space group symbol
  !Atom   Typ       X        Y        Z     Biso       Occ     In Fin N_t Spc 
/Codes
  Co     Co      0.00000  0.00000  0.00000  0.50000   0.01041   0   0   0    0
                    0.00     0.00     0.00     0.00      0.00
  Fe     FE      0.00000  0.00000  0.00000  0.50000   0.00521   0   0   0    0
                    0.00     0.00     0.00     0.00      0.00
  Si      SI      0.00000  0.00000  0.00000  0.50000   0.00521   0   0   0    0
                    0.00     0.00     0.00     0.00      0.00
(general multiplicity = 96, multiplicity of (0,0,0) is 2, so Occ=0.02083 when 
fully occupied by one atom)

But, as many of you know, FullProf assumes that "THE FIRST GIVEN SITE IS FULLY 
OCCUPIED OR THE FIRST AND SECOND ATOMS ARE IN THE SAME SITE WITH TOTAL FULL 
OCCUPATION (If this is not the case, change the order of atoms to obtain 
correct values for the content of the unit cell)” (quoted from a output file).
Accordingly, a cif file generated after running of a pcr with the atomic 
position above shows unintended values as below.
  loop_
    _atom_site_label
    _atom_site_fract_x
    _atom_site_fract_y
    _atom_site_fract_z
    _atom_site_U_iso_or_equiv
    _atom_site_occupancy
    _atom_site_adp_type              # Not in version 2.0.1
    _atom_site_type_symbol

  Co  0.00000  0.00000  0.00000  0.00633  0.66645 Uiso Co  
  Fe  0.00000  0.00000  0.00000  0.00633  0.33355 Uiso Fe  
  Si  0.00000  0.00000  0.00000  0.00633  0.33355 Uiso Si  
This rule seems tricky if one wants to make an unit cell consisting of sites 
occupied by multiple atoms like A2-type Heusler alloys.

Actually, I came up with an idea to place a “transparent” dummy atom using Nsc 
option at the top of the atomic position list to bypass the rule, and it seems 
to work.
But I’m not sure this is the best way. Does anyone have other or better ideas?
Thanks in advance.

Kotaro

//================//================//
  Kotaro SAITO
  High Energy Accelerator Research Organization
  Institute of Materials Structure Science
  1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan
//================//================//

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