STANDARDIZATION OF CRYSTAL-STRUCTURE DATA AS AN AID TO THE CLASSIFICATION OF CRYSTAL-STRUCTURE TYPES

Owing to the different ways in which crystal structures may be describ ed, isotypic compounds are often not identified as such. To remedy thi s situation, crystal structure data can be standardized by means of th e STRUCTURE TIDY program. In the standardized data of isotypic structu res, occupied sites have the same Wyckoff representation. This makes i t possible to use the Wyckoff sequence (the letters of occupied Wyckof f sites) to classify crystal structure types. This classification is m uch finer than the previously used classification based on the Pearson code and is of great help if one wants to know whether a particular a tom arrangement is already known. The standardization has enabled us n ot only to demonstrate new cases of isotypism, but also to discover st ructural relationships between different structure types with the same space group, for example substitution, vacancy or filled-in variants.