CPD News Index

AXES - a software toolbox in powder diffraction

Introduction.

Historical background

• call of structure visualization programs (SCHAKAL, ORTEP, ORTEX, MOLDRAW);
• individual and whole pattern fitting (Mändar, et al. 1994);
• compare multiple diffraction patterns and peaks in graphics;
• search/match on the bases of ICDD/PDF-2 database (Vajakas & Mändar, 1994; see Figure 1. and Figure 2.);
• manual background detection;
• a version of long patterns (190000 points);
• online help;
• improvement of 2q & I calibration algorithms and graphical user interface (GUI).

Commercialisation of GUFI in 1994 urged us to continue the project in a separate direction. The new program, after extracting the GUFI specific pattern processing dialogues and algorithms, and porting it to Pascal 7.0 with DPMI support, was called AXES. Since this time the following main tools have been included:

• structure data import/export for POWDERCELL, ICSD, LAZY-PULVERIX, GSAS, MOLDRAW, SCHAKAL;
• set of dialogues for preparation of input data for FULLPROF (Mändar, et al. 1996);
• crystalline structure analysis using Williamson-Hall Plot (Mändar, et al. 1998; see Figure 3.) and Fourier analysis of peak shapes;
• diffraction peak shape simulation using interference functions and column length distribution functions.

The source size of the program has grown more than three times containing now about 8200 lines. Full set of AXES tools is in form of compiled program available in a version A, which is distributed as shareware and requires a licence. A selected set of tools sufficient for diffraction data preprocessing, peak detection, fitting, indexing, cell refinement, crystallite size estimation and preparation of input files for Rietveld analysis is available free of charge in version B for education from ftp://ftp.physic.ut.ee/pub/pc/axes and the program suite CCP14. The current version of AXES 1.9 (including also the external programs and databases) should meet the needs both a professional crystallographer and a beginner in the field of powder diffraction.

Example.

From 27 different import/export formats of diffraction data available in AXES we use the Diffrac 3.x format to read in the pattern. For Rietveld program the pattern is saved in FULLPROF *.DAT format.

Visual evaluation of background line gives often better estimates compared with analytical functions or Fourier filtering methods. Commands in dialogue Background Manual facilitate graphical detection of background curve which thereafter is saved in FULLPROF *.BAC format.

Set of dialogues Pattern Fitting: Peak Parameters, WPPF

Parameters and Fit Manager (see Figure 4.) are used for calculation of initial values for peak shape parameters (u, v, w, m0). For a large number of peaks it is sufficient to fit peaks in three ranges - in the beginning, middle and end of the pattern, including 4-6 peaks in each range. A default fitting first with Gaussian and next with Pearson VII type function gives u=0.015, v=-0.004, w=0.020, m0=9.

Now follows preparation of parameter files for Rietveld analysis. The shape parameters are entered in dialogue Phase Refinable General Parameters (see Figure 5.) of the FULLPROF *.PCR Editor. The other 19 dialogues of this set are used for preparation, inspecting and editing the refinement model of the sample. In case of structures containing many atoms in the unit cell, the user has a facility to import structure data from different sources: ICSD database, GSAS *.EXP file, input files of structure visualization programs (e.g. MolDraw, PowderCell).

The most important facility of these dialogues is that in general, the user does not need to take care of counting, ordering and format of parameters (except the maximum number of refinable parameters which has to be given in Refin. Models dialogue), this is done by AXES. In case of complicated models, where the codewords are fractional, the user has to edit a *.PCR file directly. This can be done by a text editor accessible from inside the main dialogue of the *.PCR editor.

Export and import of current set of parameters is done automatically by starting and finishing of FullProf. Fitting quality can now be assessed in graphics (see Figure 6.) using commands in dialogue Compare. The curves and marked points in the current graphics window can be saved into an ASCII file in x-y (an n column) format which is easily retrieved by any graphics drawing programs.

Perspectives.

References

• Dinnebier, R.E. Entwicklung eines Programmsystems zur Messung und Auswertung von Röntgen-Pulverdaten. Ph.D. Thesis, Mineralogisch-Petrographisches Institut, Universität Heidelberg, 1993.
• Mändar,H., Felsche,J., Mikli,V., Vajakas,T. AXES 1.9: new tools for estimation of crystallite size and shape by Williamson- Hall analysis. J.Appl.Cryst., 1998, May, 24 pp. (submitted).
• Mändar,H., Vajakas,T., Felsche,J., Dinnebier,R.E. AXES1.4 - a program the for preparation of parameter input files for FULLPROF. J. Appl. Cryst., 1996, 29, 304.
• Mändar, H., Vajakas, T., Haav, A., Felsche, J. A new single- profile and whole-powder-pattern fitting method in a XRPD program GUFI. Zeitschrift für Kristallographie. Supplement Vol. 8 (15th ECM. Book of Abstracts), Dresden, 28.Aug.-2.Sept., 1994, 135.
• Vajakas, T., Mändar, H. Phase identification with X-ray powder diffraction data evaluation program GUFI. Abstracts to "International Conference Powder Diffraction and Crystal Chemistry PD-94", Saint Petersburg, Russia, June 20-23, 1994, p.29-30.