Structure types in the Inorganic Crystal Structure database ICSD

F. Laufek

Czech Geological Survey, Geologická 6, 152 00, Praha 5

frantisek.laufek@geology.cz

Inorganic Crystal Structure Database (ICSD), developed and maintained by Fachinformationszentrum Karlsruhe, represents a basic structural database of inorganic compounds. The ICSD [1] comprises such a comprehensive collection of more than 100,000 determined crystal structures of inorganic materials (including intermetallic compounds and oxalates). It contains all available crystallographic data of those crystal structures that have been published since 1913. The editorial team continuously extracts and abstracts the original data from over 80 leading scientific journals and additional 1,900 scientific journals. The ICSD is updated twice a year, each time adding approximately 3,000 new records. The entries are tested for formal errors, plausibility and logical consistency. The data are stored as published; they are not being standardized.

Since 2005, FIZ Karlsruhe began to introduce structure types into ICSD database [2]. The whole idea of structure types is based on non-modular categories of similarity between inorganic crystal structures defined in the IUCr report [3]. The two most important of them – isopontial and isoconfigurational structures – proved to be sufficient to serve as theoretical concepts in introduction of structure types into the ICSD database. The main problem of the assignment of structural types is the subdivision of isopontial structures (i.e. structures with equal space groups and Wyckoff-sequences describing occupied atomic positions) in several, well-defined structure types. For this subdivision the following criteria are used: axial ratio c/a, ß range, crystallographic composition (ANX formulae), necessary and forbidden elements. For assignment of most structure types are these criteria sufficient. Only in a few cases also the atomic coordinates must be inspected as an additional criterion [4]. A final criterion that must be fulfilled before a new structure type is introduced into the ICSD is that it must represent the structures of at least three different compounds with the same given structure [2].

In 2008, 59,000 structures out of 100,000 structures in the ICSD were classified into 2500 structure types. The most frequent are spinel (Al₂MgO₄), halite (NaCl) and perovskites (CaTiO₃ and GdFeO₃). Because about 1/3 of the structures in ICSD have no isotypic counterparts, the assignment of structures to certain structure types will be nearly finished [4]. Problems connected with standardization of crystal structure data and other special structural relationships will be briefly discussed, too.

1. http://www.fiz-karlsruhe.de/icsd.html

2. R. Allman & R. Hinek, Acta Cryst., A63, (2007), 412.

3. J. Lima-de-Faria, E. Hellner, F. Libeau, E. Makovicky, E. Parthé, Acta Cryst., A46, (1990), 1.

4. R. Allman, Abstract of 16. Jahrestatung der DGK, (2008), V48.