Simple Self Calibration

Please note that these run-throughs do not in general go into "best practice" indexing methods but mainly demonstrate how to start interacting with the CRYSFIRE software. The tutorials make use of the self calibration option in CRYSFIRE purely to show its functionality (Self Calibration is to Powder Diffraction as is DIFABS to single crystal methods?). It is far better to make use of a well aligned and maintained Powder X-ray diffractometer and/or use of an appropriate internal position standard such as NBS/NIST Silicon 640(a-c) to correct for any systematic offsets that may have leaked into the experiment. Even minor systematic offset errors in the peak positions can cause all the indexing packages to fail in suggesting the correct result.

If running under Windows, go into a DOS Prompt and create/enter the appropriate subdirectory where you wish to perform the indexing. Run Crysfire, and load in the required CDT Crysfire file using the LO command (or import some raw data using the IM command)

Type SC for self calibration to be given the following menu options. The default is to do a "Zero" correction, but for this laboratory Bragg-Bretano data we will use the T (Specimen-Displacement ratio)

Type T to select the Specimen-Displacement ratio; then go with the defaults until you are given a histogram of specimen-displacement T ratio vs line-pair number. Which in this case spread between 0.0000 and 0.0003

Press [Enter] again to be given a "List of Possible Specimen-Displacement T ratios found by Self-Calibration". Using empirical peak profiling of Bragg Bretano laboratory XRD data, the lowest angle lines would also be the most inaccurate due to geometry abberations. While the higher angle lines would most likely suffer from overlap problems.

However as this data was profiled using Fundamental Parameters peak profiling using XFIT by Bob Cheary and Alan Coelho, the low angle peaks are of equal accuracy to the high angle reflections.

Thus (in this case), when prompted by Select H, L, D, M, S or a line-pair number (default=no change):, type 1 to select the first line pair for the self calibration correction.

Crysfire will then show you the effects on all the peak positions (just type [Enter] to continue).

When prompted to save the changed dataset, press [Enter] to go with the default of Yes, after which Crysfire will automatically append SCT to the filename and the relevant information to the Title. This means that you original data is still available in your original starting file.

Now continue on with the indexing. Perhaps trying the IN command to start indexing with the various avaiable programs.