CONCERNING INORGANIC CRYSTAL STRUCTURE TYPES

G.Bergerhoff, M.Berndt, K.Brandenburg and T.Degen

Institute for Inorganic Chemistry, University of Bonn, Germany

Isotypic structures have similar coordination of their constituents in the same space group and on the same Wyckoff positions (= same isopointal set). They can be found systematically in an appropriate database if the term ´similar´ is defined by an algorithm. For this purpose a new database SICS (Standardized Inorganic Crystal Structures) was derived from the Inorganic Crystal Structure Database (ICSD) in three steps:

1. Standardization following Gelato and Parthé (J.appl.Cryst. 20 (1987) 139-143),
2. Calculation of difference D between each pair of structures within an isopointal set,
3. Reduction of the contents of the ICSD to the ´best´ representative of each phase.

The searchable descriptor D now allows to look for related structures directly or to discuss the synopsis of all D values of an isopointal set, the D matrix. The second way, especially, allows interesting uses of the concept of isotypism. Several examples show:

1. structure types and their boundaries:
   i) D matrices with homogeneously low D values,
   ii) interchanged in such a way that ranges with D matrices where lines and rows can be homogeneously low submatrices. allow splitting into submatricesD values
   iii) which a continuous increase of D values takes place indicating a Structure D matrices in Type distorted more and more.
2. relation between symmetry, chemical elements and their stoichiometry.
3. detection of errors in data and limitations of the present program.

Because of the large amount of structures the full information on relationships present can only be found by means of the new database SICS itself. For subscribers of the ICSD it is available from the authors. A detailed description will be published in Acta Cryst. B.