Modulated Structures CIF Dictionary version 1.0

_atom_site_occ_special_func_[]

Name

Category: category_overview

Data type: null

Examples:

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    In this example the occupational modulation of the Mn(2) atom was
    modelled using a square-wave crenel function.
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     loop_
         _atom_site_occ_special_func_atom_site_label
         _atom_site_occ_special_func_crenel_c
         _atom_site_occ_special_func_crenel_w
             Mn(2)    0.25    0.623(4)

Example 1 - extracted from Elding-Pont\'en, M., Stenberg, L., Lidin, S., Madariaga, G. & P\'erez-Mato, J.M. [(1997). Acta Cryst. B53, 364-372.]

Definition

    	
   Data items in the ATOM_SITE_DISPLACE_OCC_SPECIAL_FUNC category 
   record details about the occupational modulation of a given atom
   or rigid group in a modulated structure when it is not described
   by Fourier series. Special functions are effective in some cases 
   where the modulations are highly anharmonic since the number of 
   parameters is drastically reduced. However they are in general 
   discontinuous or with discontinuous derivatives and therefore 
   these functions describe an ideal situation that never occurs in 
   a real modulated crystal. Up to now only a few types of special 
   functions have been used and all of them come from the suite of 
   programs JANA. In this dictionary only the special functions 
   available in JANA2000 have been included. Although this approach
   is far from being general it has the advantage that functions are
   tightly defined and therefore the atomic displacements and 
   occupations can be easily calculated.
   Summarising the special functions included in the dictionary are:

     1) Sawtooth functions for atomic displacive modulation along x,
        y and z. Only applies to one-dimensional modulated 
        structures.
     2) Crenel functions for the ocuppational modulation of atoms
        and rigid groups. Only applies to one-dimensional modulated
        structures.