A PROPOSAL FOR A CRYSTALLOGRAPHIC DATABASE OF MODULATED STRUCTURES

摘要

Although modulated structures have acquired the status of 'normal structures' within the IUCr they have not as yet in their publication a level of distribution comparable to that of periodic crys- tals, Important structural details are only accessible through printed papers or directly from au- thors. The main problem lies on the current inability of including, within the existing crystallo- graphic databases, those features that belong exclusively to modulated structures. Therefore, a specific database for these materials appears to be of capital importance. Basically, the construc-tion of databases using dispersed information needs: 1) a flexible way of sending data in machine readable form, 2) a method to validate, filter and check the data and 3) a program to archive, as compactly as possible, the desired information but permitting fast access and efficient retrieval. The number and rate of growth of published modulated structures are still low enough to permit a solid design of this new database. Crystallographic data of periodic structures are currently trans-mitted through the so-called Crystallographic Information Files (CIFs). A CIF is a comprehensive ASCII file built up using a few simple syntax rules. Validation of any CIF can be done using the CIF dictionary. Public domain software tools have been developed for implementing programs to create, retrieve or convert to machine internal representation crystallographic data. The obvious way to efficiently use CIFs with modulated structures consists in enlarging the CIF dictionary with those items that are characteristic for them. Thus, a specific structural database can be easily cre-ated defining the appropriate mask to filter CIFs. A proposal of the new CIF dictionary entries and a specification of those items that shoud appear in this structual database is presented.