Degree-of-crystallinity (DOC) analysis

The "degree-of-crystallinity" (DOC) analysis can be used to determine the amount of amorphous compounds in a sample (quantitative phase analysis). It requires that an additional "empty sample holder" diffraction pattern is available that has been recorded using the same experimental conditions as the sample's pattern. This additional pattern is used to subtract the background contribution caused by the instrument from the one of the sample.

The measurement range of both diffraction patterns (anchor pattern and "empty sample holder" pattern) should be large enough to see both the start and end of the amorphous part stack on the background.

The DOC analysis works best if the crystalline and amorphous parts have similar elemental composition. If the sample consists of crystalline fractions and an amorphous fraction of different chemical compositions, the amounts of each of the individual crystalline phases may be estimated using appropriate standard substances; the amorphous fraction is then deduced indirectly by subtraction.

If this prerequisite is met, it is neither necessary to determine the experimental peaks nor to select the matching phases (qualitative analysis) for the DOC analysis. Instead, it is sufficient to subtract the instrumental background from the sample's diffraction pattern, re-determine the background, and then calculate the area below the background curve and compare it to the area of the whole pattern.

Workflow for DOC analysis:

  1. Collect the sample’s diffraction pattern as usual; the 2theta range should be large enough to see both the start and end of the amorphous part.
  2. Collect an "empty sample holder" diffraction pattern with the same experimental conditions.
  3. Import the sample's diffraction pattern into Match! (menu "File/Import/Diffraction data").
  4. Define/check/modify the background so that it clearly separates the diffraction peaks from the background.
  5. Import the "empty sample holder" diffraction pattern as an additional pattern (menu "Pattern / Insert/overlay…")
  6. For a better visual comparison, you could switch over to "Counts" representation on the y-axis (e.g. by clicking on the corresponding y-axis label), so that the two patterns are scaled identically.
  7. Run the menu command "Quantify/Degree of crystallinity (DOC) / New analysis"
  8. In the dialog that opens, make sure that the option "Use additional pattern" is marked as well as the correct additional pattern is selected.
  9. Mark the option "Full pattern" under "Subtract full 'empty sample holder' pattern or background only".
  10. If you like to see the "real" diffraction pattern of the sample after subtraction of the instrument background, mark the option "Add result as a new experimental pattern".
  11. Press "OK". The result of the DOC analysis will now be displayed. It will also be included/displayed in the report.
  12. If required, you can now modify the background displayed in the new "difference" pattern manually (e.g. by shifting the corresponding control points), and then re-run the DOC calculation, by selecting the command "(Re-)calculate DOC" from the corresponding pattern menu.

The "Degree-of-crystallinity analysis" dialog is operated as follows:

Either mark the option Import from file (in order to import the required "empty sample holder" diffraction pattern from a raw data file), or Use additional pattern if the pattern recorded for the "empty sample holder" has already been imported as an additional experimental pattern.

It is either possible to subtract the Full pattern of the empty sample holder from the anchor pattern (if the empty sample holder pattern is clear from any disturbing surplus features), or to subtract only the Background if this does not apply.

If only the background shall be subtracted, Match! will either determine it automatically (if you use the option Import from file mentioned above), or use the background defined for the additional experimental pattern.

If you mark the checkbox Add result as a new experimental pattern, the diffraction pattern calculated by subtracting the "empty sample holder" pattern from the anchor pattern will be displayed as an additional experimental pattern, so that you can visually check it.

Finally, if you mark the Save as defaults checkbox at the bottom, all current selections in the dialog will be restored when you open it for the next time.

The command "(Re-)calculate DOC" in the "Quantify/Degree of crystallinity (DOC)" submenu recalculates the degree of crystallinity from a single (the anchor) pattern only, e.g. after the background has been modified by the user. It assumes that the instrumental contribution to the background radiation has already been subtracted. If this assumption is not met, the result of the calculation cannot be used!
Similar commands to recalculate the degree of crystallinity are also available in the pattern menus of additional experimental patterns.