Modulated Structures CIF Dictionary version 1.0

_atom_site_displace_special_func_[]

Name

Category: category_overview

Data type: null

Examples:

# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    In this example the displacive modulation of the O(4) atom was modelled
    using a sawtooth-shaped function.
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     loop_
         _atom_site_displace_special_func_atom_site_label
         _atom_site_displace_special_func_sawtooth_ax
         _atom_site_displace_special_func_sawtooth_ay
         _atom_site_displace_special_func_sawtooth_az
         _atom_site_displace_special_func_sawtooth_c
         _atom_site_displace_special_func_sawtooth_w
              O(4)  -1.46(3)  0.12(5)  0.42(5)  0.42(2)  1.07(2)

Example 1 - extracted from Gao, Coppens, Cox & Moodenbaugh [(1993). Acta Cryst. A49, 141-148.]

Definition

    	
   Data items in the ATOM_SITE_DISPLACE_SPECIAL_FUNC category record
   details about the displacive modulation of an atom site 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.