Peak-based Search-Match

When using this conventional, proven method for search-match, Match! compares the peak data found in the experimental diffraction pattern to the peak data stored in the reference database. For each reference database pattern, Match! calculates a numerical value describing the quality of the agreement with the pattern of the unknown sample, the so-called "figure-of-merit" (FoM).
Note that the search-match calculation can either be performed for all peaks of the diffraction pattern or for marked peaks only.

Workflow for Peak-based Search-Match

A common strategy for qualitative analysis using the conventional (peak-based) search-match functionality goes like this:

  1. Import your raw (profile) diffraction data
  2. Adjust the background (orange curve) that has been calculated from the profile data automatically at pattern import.
  3. Process your raw diffraction data carefully, until the profile calculated from the experimental peak data is in reasonable agreement with the experiment. It is also essential to eliminate any potential 2θ axis errors (e.g. zero point or specimen displacement) in this step.
    The goal is to obtain a list of peaks (2theta and intensity values) with highest possible precision. It is extremely important that this step is performed as accurate as possible in order to obtain reasonable results in the search-match process later on.
  4. In the periodic table on the upper right-hand side (tab "Restraints / Composition"), mark all elements in red ("to be excluded", e.g. press the "Toggle" button), then mark only those elements in yellow color ("optional", "maybe present") that are in good agreement with your chemical or XRF analysis (or your knowledge about the sample).
    Selecting reasonable elements helps to eliminate "false positives" from the result pretty much!
    Other additional information that might be available for your sample (like chemical class, density, color etc.) can be applied as restraints in this step as well.
  5. Start with a full search-match calculation (menu command "Search/Search-Match", shortcut <Ctrl+M>, toolbar button ) and the selection of a first (typically major) phase. The latter can e.g. be performed by double-clicking on the corresponding line in the candidate list.
  6. When the search-match calculation has finished, the entries are ranked according to their FoM values. The entries with the highest FoM are the ones which are most likely to be present in the sample. The ranked list of entries is then presented to the user.
  7. Analyze the entries at the top of the candidate list and select those which are most likely to be present in the sample ("matching entries/phases").
  8. Afterwards, a common strategy is to focus on experimental peaks that are not covered by identified phases yet. In order to make this easier, you can activate the "Show uncorrelated peaks" option, e.g. by pressing <Ctrl+Shift+K>.
  9. Mark (click on) a peak that cannot be "explained" with the already selected phase(s), i.e. which has not been correlated to an identified phase yet.
  10. Run the menu command "Search/Search-Match (marked peaks only)" (or press the keyboard shortcut <Ctrl+Shift+M> or the toolbar button ). Wait until the update of the candidate list (figure-of-merit values) has been completed.
  11. Investigate the entries/phases at the top of the candidate list as usual, and select one (or more) phases that you assume to be present, based on the results for the marked peak(s).
  12. Match! will automatically run a new full search-match calculation (for all peaks) at this point, in order to update the figure-of-merit values for all reference database entries with regard to the new "situation" (list of selected phases as well as the peak intensities covered by them).
  13. Repeat the steps 9-12 until the profile calculated from the selected phases (and the background) is in reasonable agreement with the experimental diffraction profile, so that it is rather likely that all phases in the sample have been identified.
  14. Always keep the possibility in mind that one or the other phase may be missing in the current reference database.
  15. Once the matching phases have been selected, we recommend to run a Rietveld refinement of these phases against the original raw (profile) data. A successful refinement is generally taken as the proof that the phase analysis is correct and complete.
  16. Finally, you can read and/or print the report in order to document the result of your phase analysis.

If no reasonable phase is found when running a search-match calculation for one (or more) marked peaks, this could be due to one of the following reasons:

Please pay attention to the following hints, too: