I analyzed a sample contained calcite using XRD. The pattern attached. It was identified as nitratine by the related software searching the database of XRD patterns. As nitratine is isostructural with calcite, how can this identification be interpreted? Is it misidentification? If so, when such a misidentification may happen?
Your example demonstrates that the XRD identification software has no brain. It tends to come up with a list of possibilities, and the one that is first on the list is not always the correct one. It is up to the operator to then decide what the most likely phase is. XRD phase identification software cannot be used as a 'black box', where you put a sample in and you immediately get the correct answer out. You will need to apply your brain to get a good result. A lot of people do not realise this, unfortunately. So do what Lio suggests, and compare your pattern to the known pattern of calcite. Often, the software that did your (mis)identification allows you to view a stored pattern of a known minerals, and compare it to the one you have. Otherwise, go to an on-line database and look up the pattern of calcite / aragonite / vaterite or anything else that your sample might be.
Hi Hamed,
I don't know why the software didn't work as planned. What I suggest doing is going to the powder diffraction database - ICDD / JCPDS, and looking up calcite and seeing if the powder peaks match what you got. If there isn't a good discrepancy, consider other phases that may have been formed instead (aragonite? others?)
Is your sample in powder form?
If the phase is close to a JCPDS card but not quite that, consider the existence of other phases, nano grains, impurity, defects etc, and try cross checking your hypothesis with another characterization technique.
Your example demonstrates that the XRD identification software has no brain. It tends to come up with a list of possibilities, and the one that is first on the list is not always the correct one. It is up to the operator to then decide what the most likely phase is. XRD phase identification software cannot be used as a 'black box', where you put a sample in and you immediately get the correct answer out. You will need to apply your brain to get a good result. A lot of people do not realise this, unfortunately. So do what Lio suggests, and compare your pattern to the known pattern of calcite. Often, the software that did your (mis)identification allows you to view a stored pattern of a known minerals, and compare it to the one you have. Otherwise, go to an on-line database and look up the pattern of calcite / aragonite / vaterite or anything else that your sample might be.
Searching a database for matches of database entries with your scan the program will compare peak positions and intensities of reflections. Nitratine has peak positions similar to those of calcite. If your sample shows broad peaks, additional doping elements or peak shifts due to misalignment of the specimen or of the goniometer the wrong peak positions may be estimated to provide a better match. The peak intensities are different, as Mo Af showed, but the difference is not as severe as to reject the match.
To minimize mismatch also knowledge about the composition of the sample should be given to the program. As nitratine has quite different elements compared with calcite a filter for elements would have avoided the mismatch.
An the other hand, if your filter is too restrictive this may prevent from accepting the correct match. If you "know" Ca, C and O "must" be present, but the sample consists of MgCO3 your search probably will fail.
As already I have written in other similar cases identification by diffraction without some chemical information can be misleading and viceversa. This is particularly true for isostructural phases (diffraction) and isochemical compounds (chemical analysis). Typical example: both diamond and graphite contain only C.
NaNO3 has the following cell parameters: a = 5.07, c = 16.829, space group: R-3c; while CaCO3 (calcite) shows the cell parameters: a = 4.9896, c = 17.06 with the same space group. It ensues that the systematic extintion rules are the same for both the structures and then the number of diffraction peaks will be also the same. On the contrary, the positions of the peaks are no longer the same in the two diffraction patterns, as you can verify by a simple calculation of some dhkl equidistances. Thus, the two XRPD diagrams cannot be confused since this follows from the diffraction theory.
In practice, a XRPD diagram has to be handled with care: first of all, one should decompose every peak in elementary curves, in order to find if some partial overlapping (this may be the case for nitratine and calcite) occurs between contiguous peaks (There are many programs for decomposition). Only after this decomposition , the experimental position of the peaks can be carefully determined and compared with the expected ones (from theory).
In any case, Marlina's way of thinking is very useful...
All the best
This situation is very common when one uses 'search match' procedure to identify unknown pattern. The software will list all available phases based on d-values/2theta matching. One should refine the search by putting chemical data etc. For this sample history, chemical data and probable phases are required. Ultimately identifying an unknown pattern depends on the skill of a competent diffractionist.
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