The Inorganic Crystal Structure Database (ICSD)
The Inorganic Crystal Structure Database (ICSD) is the world’s largest database for completely determined inorganic crystal structures. It is developed and maintained by Fachinformationszentrum Karlsruhe (FIZ), Germany. The database offers an extensive collection of known inorganic crystal structures published since 1913 including their atomic coordinates [1]. A typical entry includes the chemical name, formula, unit cell, space group, complete atomic parameters, atomic displacement parameters, site occupation factors and complete literature citation. In addition to the structural data of inorganic compounds, the database also provides information on useful structural descriptors (e.g. Pearson symbol, ANX formula, Wyckoff sequences, mineral name) and synthesis conditions. The atomic coordinates of all structures contained in ICSD have been fully determined or were derived from the corresponding structure types. The current release (2015/1) contains 177 373 crystal structures [1]. The structural data are extracted by regular scanning of scientific journals, bibliographic databases and internet publications. The database is updated twice a year; each time is added approximately 3 500 new entries. The entries are tested for formal errors, plausibility and logical consistency. The ICSD database is offered with two different retrieval interfaces: the off-line FindIt interface and the stand-alone web portal (ICSD web).
Different degrees of similarity between inorganic structures
The theoretical concepts for definition of a structural type and different degrees of similarities between inorganic structures were defined in the report of the IUCr Commission on Crystallographic Nomenclature [2]. The report defines two most important terms, which define different degree of similarity between inorganic structures – isopontial and isoconfigurational structures. According to the report [2], two structures are isopontial if:
(i) they have the same space group or belong to a pair of enantiomorphic space groups
(ii) the atomic positions, occupied either fully or partially at random, are the same in the two structures, i.e. the complete sequence of the occupied Wyckoff positions is the same for the two structures when the structural data have been standardized
The group of isoconfigurational structures can be viewed as a subgroup of isopontial structures. Two structures are isoconfigurational if [2]:
(i) they are isopontial
(ii) for all corresponding Wyckoff positions, both the crystallographic point configurations and their geometrical interrelationships are similar.
These conditions require the entire configuration of the two isoconfigurational structures to be similar. Consequently, all geometrical properties, such as axial ratios, angles between crystallographic axes, values of corresponding adjustable positional parameters (x, y, z) and coordinations of corresponding atoms are similar.
The introduction of the structure types into the ICSD database
Since 2005, FIZ Karlsruhe began to introduce structure types into the ICSD database [3]. In the ICSD database, two crystal structures are regarded as isostructural (i.e. they belong to the same structure type) if they are isoconfigurational. For zeolite crystal structures, only the framework atoms are taken into account in the determination of isoconfigurational structures. Hence, the determination of the isoconfigurational structures in the ICSD database consists of two steps [3]:
1. Determination of isopontial structures
2. Subdivision of isopontial structures into the different structural types by means of additional structural descriptors.
For this subdivision, the following criteria are used:
(i) crystallographic composition type (ANX formula)
(ii) range of c/a ratio
(iii) beta range
(iv) necessary elements (combined by “and” or “or”)
(v) forbidden elements (combined by “and” or “or”)
(vi) atomic coordinates
The criteria (iv) and (v) take into consideration the crystal chemistry: same elements occur in representative of a given type (e.g. O in oxide structures or F, Cl, Br, I in halides), whereas O in intermetallics is the forbidden element [3]. The criteria i-v are sufficient for separation and assignment of most structure types. Only in a few cases, the atomic coordinates must be checked as and additional criterion (vi). A final criterion that must be fulfilled before a new structure type is introduced into the ICSD database is that it must represent the structures of at least three different compounds with the same given structure [3]. The prototype of the structure type is an arbitrary chosen representative of this structure type, mostly one of the early published structures. The current release of the ICSD database (2015/1) contains 177 373 structures. More than 143 500 ICSD entries have now been assigned to 9 136 distinct structure types. The most frequent are spinel (Al2MgO4), halite (NaCl) and perovskites (CaTiO3 and GdFeO3).