|
Import of ICSD data | |
PowderCell enables the import of structure data exported by the ICSD (Inorganic Crystal Structure Database). The ICSD is distributed together with a DOS program (Retrieve) for a comfortable use of the data base. This program is able to export the originary binary records as ASCII data. On the one hand it is possible to export the records into a file with the extension *.cry. Until now this is not yet readable for PowderCell. On the other hand Retrieve is able to "print" into a file. Per default the extension is defined as *.txt. For this data structure a import filter has been programmed and implemented into PowderCell.
In the *.txt-files you will find a lot of keywords analyzed by the program. Because of the variation of data structure enabled by Retrieve also for the export, it is very possible that in rare cases the file is not readable. Please check in such cases the data structure of the exported file.
In contrast to the *.cel-files you can export the atomic coordinates as fractions or decimal numbers.
For the determination of the space-group symmetry the space-group number will be analyzed (the number in brackets behind the Hermann-Mauguin symbol). If more than one setting are defined for this space-group type the Hermann-Mauguin symbol will be analyzed additionally.
| Despite of the variety of settings considered it is very possible that a structure description uses another symmetry arrangement. This can't be processed in PowderCell directly without changes. To give you the chance to edit the unusual structure description it will be imported with some restrictions. If there the setting number can't be assigned correctly using the data file icsdkonv.dat, it will be defined as first setting. However, this has the disadvantage that the monoclinic angle will be defined as b. So it is conceivable that you must enter the correct monoclinic angle after the adaption of the space-group symmetry, manually.
Furthermore we found a lot of records where wrong space-group numbers will be given. As example gypsum can be cited. In former versions of the ICSD e.g. the space group I 1 2/c 1 will be described by the space-group number 13 but in IT this will be defined as 15 (of course PowderCell uses 15, too (setting number is 14)). Therefore, of course also the setting can't be assigned correctly but in the dialog box "structure data" both the space-group and the setting number can be corrected manually without problems. For the assignment of Hermann-Mauguin symbol and setting number used in PowderCell you will find the necessary correlation in the Standard and Non-Conventional Settings. By the way, in the cited ICSD edition 345 structures will be found during the search for the space-group number 13 representing a primitive lattice. However, there will be given approximately 80 (!) structure descriptions using a face or body centred lattice but usually these are defined by space-group number 15. We found out that in the present version this mistake described above has been corrected except of a single structure description. |
However, we detected some problems during the detailed use of exported ICSD data.
Especially in the german Windows version the decimal point is per default defined as comma. Therefore in former days we recommended to change this setting in the Windows control panel. In the meantime we adapted the import filter in this manner that it is able to read both the point and the comma.
The second problem detected is caused by Retrieve itselfs. If the standard deviation is activated, sometimes the output string is to long for the defined column length. That means it will be broken and continued in the next line. This disapproves PowderCell and it will be shown a message, that it isn't possible to import the present file. Therefore we recommend to deactivate the output of the standard deviation, especially because the program can't use it. In principal it is always possible to edit the exported data manually. Please notice, that all data corresponding to one and the same atomic position must be given in a single line!
For structures described by relative huge lattice constants (length of basis vectors) sometimes the atomic coordinates will be given up to five digits. In this case you will find only one space between the coordinates of a position. If there signs orrur PowderCell is not able to split the coordinates, respectively and shows a message again, that it isn't possible to import the present file. Therefore please think for the necessary space between the separate data which used by the program. Again it is possible to edit this data manually using an arbitrary ASCII editor.
Federal Institute for Materials Research and Testing
Unter den Eichen 87, D-12205 Berlin,
Germany