in the toolbar, or run the menu command "Quantify / Degree of crystallinity (DOC)". in the toolbar again to proceed.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:
in the toolbar, or run the menu command "Quantify / Degree of crystallinity (DOC)". in the toolbar again to proceed.In general, the "Degree of crystallinity (DOC) analysis" window 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.
Important facts to keep in mind::
The degree of crystallinity (DOC) calculation is to a large degree "affected" by the background choice. It is essential to understand that the part of the diffraction pattern above the background curve is attributed to the crystalline material, while the part below (the "background") is attributed to the amorphous material.
The latter may contain contributions both from your sample as well as from your diffractometer, so you first need to remove the contribution caused by the diffractometer, the "empty sample holder pattern". Once you have done so, Match! can "compare" the area (counts) of the profile above the background curve with the area (counts) below it, and calculate the degree of crystallinity from the relation of the two.
Match! determines the background automatically when importing raw diffraction data files, however, the automatically determined background may not always correct. For example, it may happen in some cases that Match! does not include the "amorphous humps" in the amorphous background below the background curve, but rather attributes them to the "crystalline material" area above the curve.
In fact, the decision what is "amorphous background" and what "broad peaks caused by very small crystallites" sometimes is not really obvious. For example, there could be broad peaks which might belong to tiny little crystallites. These peaks could be so broad though that it could become a "matter of taste" how to define the background in this area.
Hence, it is rather essential to visually check that the orange background curve really is a good representation of the "background" in your diffraction pattern before running the DOC analysis.
If you believe that the automatically determined background is not "flexible" (does not follow the profile closely) enough, you should increase the flexibility of the background, by pressing <Ctrl+F2> (<Cmd+F2> on the Mac) several times until the orange curve (background) has adapted to the amorphous "humps".
If it looks as if the determined background is too "flexible" (follows the diffraction profile too closely and maybe includes broad peaks), you can decrease the flexibility of the background, by pressing <Ctrl+F3> (<Cmd+F3> on the Mac) one or multiple times.
As an alternative, you could also define the background manually, e.g. by starting with a simple background line (menu "Pattern / Background / Two points" or "Pattern / Background / Set background to zero"), and then manually adding and adjusting background control points as required.