Wyckoff sequences

According to Parthe et al. (1993), a standardization procedure is necessary in the presentation of the relevant data characteristic of a crystal structure (see also Parthe and Gelato 1984). A convenient description of the structure types is then possible using the Wyckoff sequence (the letters of the occupied Wyckoff sites). This allows a finer classification of structure types and offers suggestions not only for recognizing isotypic structures but also possible structural relationships like substitution, formation of vacancy or filled-in structure variants. 

Search results are displayed in a table that can be adjusted to one s needs, both regarding items to be displayed as well as the order of the entries. The display of individual entries also contains derived data such as unit cell volume, density, Pearson code, and Wyckoff sequence. Additional programs allow further analysis of structures, i.e., calculation of interatomic distances and angles, and quick generation of simple drawings of crystal structures (CVIS, CD-ROM version). CVIS has an automatic molecule generator that draws pictures of molecules or parts of a structure, e.g., phosphate chains. Interactive calculations of distances and angles in the plot are possible. 

In SICS, therefore, only the best determination of any phase has been selected - as defined by the chemical composition, the space group and the sites occupied in the unit cell (given by the Wyckoff letters and combined in the so-called Wyckoff sequence). These data are given in a standardized form. SICS is available as a supplement to ICSD. Aware of the problem of finding the best determination, there is a reference to the relevant entries in ICSD. Another advantage of SICS is described in Section 11.1.2. 

Structural relationships, e.g., by general stoichiometric formulae (AX2 = CaFi). by subunits (all sulfates), by structural characteristics (all ring-forming stnictures. all molecular compounds), by symmetry (same Wyckoff sequences) Coordination, e.g.. four-coordinated copper, ociahedrally coordinated silicon... 

The space group and the Wyckoff sequence (see Section 2.10) reflect the symmetry at the sites of the atoms making up a structure. Statistical investigation, easily done using suitable retrieval programs (see Section 8) highlights the preferred combinations of symmetry elements in certain classes of structure. 

The Wyckoff symbol is a short designation it consists of a numeral followed by a letter, for example 8/. The cipher 8 states the multiplicity, that is, the number of symmetry-equivalent points in the unit cell. The / is an alphabetical label (a, b,c,...) according to the sequence of the listing of the positions a is always the position with the highest site symmetry. 

If the three-dimensional structure of rat RNase is assumed to be identical to that of the cow, it is seen that all of the 41 changes in sequence are sterically permitted. There is a curious tendency for the changes to occur in pairs so that the charge distribution on the surface is maintained. The active site is invariant, however. Wyckoff (72b) has discussed in detail the sequence changes in relation to the structure of bovine RNase-S. 

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