Index

Modulated structures dictionary (msCIF) version 3.2.1

Category ATOM_SITE_OCC_SPECIAL_FUNC

Parent category:
MS_GROUP

Definition:

   

      Data items in the ATOM_SITE_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 JANA2006
      suite of programs (Petricek, Dusek & Palatinus, 2014).  Although
      this approach is far from being general,
      it has the advantage that the functions are tightly defined and
      therefore the atomic displacements and occupations can be
      calculated easily. In this dictionary, only the special functions
      available in JANA2006 have been included.
      These are:
    (1) Sawtooth functions for atomic displacive modulation along
       x, y and z.
    (2) Zig-Zag functions for atomic displacive modulation along
        x, y and z.
    (3) Crenel functions for the occupational modulation of atoms
       and rigid groups. Both of these only apply to
       one-dimensional modulated structures.

      References: Petricek, V., Dusek, M. & Palatinus, L. (2014).
              Z. Kristallogr. 229(5), 345-352.  DOI 10.1515/zkri-2014-1737     
              Crystallographic Computing System JANA2006: General features

              Petricek, V., Eigner, V., Dusek, M. & Cejchan, A. (2016). Z. 
              Kristallogr. 231(5), 301-312. DOI 10.1515/zkri-2015-1913 
              Discontinuous modulation functions and their application for 
              analysis of modulated structures with the computing system JANA2006

Category is of type Loop