BRASS 2004, the Bremen Rietveld Analysis and Structure Suite

J. Birkenstock, R.X. Fischer, T. Messner

Fachbereich Geowissenschaften, Universität Bremen, Klagenfurter Straße, D-28359 Bremen, Germany

BRASS consists of a suite of programs for the refinement, presentation, and evaluation of crystal structures. Its Rietveld kernel is based on the PC-Rietveld plus program [1], a derivative of the Wiles & Young code [2]. Latest extensions are the implementation of a grid search analysis as introduced in [3], the introduction of an extended version of the modified Pseudo-Voigt profile function according to [4], the handling of peak broadening due to anisotropic strain [5], and beam-overflow intensity corrections for circular sample surfaces [6].

The user interface, written in Delphi, handles the complete file management and data transfer between all modules in BRASS. Directories and file structures for input and output, and for the individual program modules are set up upon starting a new project. The user is guided through the Rietveld run by consecutive steps from the powder pattern display to the refinement procedure and the display of the results. Single refinement results can be stored before subsequent runs are performed. A report documenting the actions is written to a log file. The refinement progress is monitored displaying the residuals and divergence criteria in online graphics. All individual parameter changes are recorded for retrospective analyses.

Quantitative phase analyses can be performed on up to 15 crystalline phases. Results are given in mole percent and weight percent for the individual phases and are displayed in a pie chart. In addition a sum formula is calculated for the bulk composition of the whole sample..

The numerical part of the Fourier calculations is based on the FORDAP program [7] extended by comprehensive graphics for the 2D and 3D representation of electron densities and grid search results displaying occupancies and residuals in layers of the unit cell.

The structure drawing part of the program package based on the old version of the STRUPLO program [8] is completely rewritten for the online display of polyhedral representations or ball and stick models. All single objects (atoms, bonds, polyhedra) can be deleted by mouse clicks on the respective objects. A comprehensive redo option allows the recovery of all or just individual objects.

Interatomic distances and angles are listed in a table for all atoms, or they can be displayed online for individual atoms selected by mouse clicks in the graphics display. The void space, i.e., the empty space not occupied by atoms, is calculated according to the procedure described in [9]. This is especially useful to determine diffusion paths, e.g., in porous materials like zeolites.

The program package can be downloaded free of charge from www.brass.uni-bremen.de.

Acknowledgements

This work is sponsored by Panalytical, Almelo, The Netherlands.

[1] Fischer, R.X., Lengauer, C., Tillmanns, E., Ensink, R.J., Reiss, C.A., Fantner, E.J.: Materials Science Forum 133-136 (1993) 287-292.

[2] Wiles, D.B., Young, R.A.: J. Appl. Crystallogr. 14 (1981) 149-151.

[3] Baur, W.H., Fischer, R.X.: Adv. X-ray Anal. 29 (1986) 131-142.

[4] Thompson, P., Cox, D.E., Hastings, J.B.: J. Appl. Crystallogr. 20 (1987) 79-83.

[5] Stephens, P.W.: J. Appl. Crystallogr.32 (1999), 281-289.

[6] Krüger, H., Fischer, R.X.: J. Appl. Crystallogr. 37 (2004) 472-476.

[7] Zalkin, A., Lawrence Berkeley Laboratory, Berkeley, CA, modified by J.A. Ibers, Northwestern University, Evanston, IL.

[8] Fischer, R.X.: J. Appl. Crystallogr. 18 (1985) 258-262.

[9] Küppers, H., Liebau, F.: Proceedings of the DGK-Tagung (2004), Oldenbourg Verlag, München, 120.