The Reference Pattern Database
Match! identifies phases in a sample by comparing its powder diffraction pattern to
reference patterns of known phases. Hence, it needs a so-called "reference database"
in which these reference patterns are provided. The reference databases are managed in the
so-called "Reference Database Library" dialog.
There are several options for obtaining a reference database:
You cannot only use one or the other of the above-mentioned reference databases but also combinations
of them. For example, you can combine diffraction patterns of your own compounds with the COD database
that is installed with Match!.
- We recommend to use an ICDD PDF database as reference database (if you have it available), either stand-alone or in combination with the COD database that is installed along with Match!. The background why we recommend the PDF compared to the COD is that we have found that at least in case of natural samples (e.g. minerals, rocks etc.), the PDF sometimes gives better qualitative analysis results than the COD. This seems to be caused by the fact that certain phases show a certain variation of peak positions and intensities, e.g. caused by the insertion or replacement of atoms (leading to different unit cell parameters and hence peak positions) or preferred-orientation effects (causing the intensities of certain peaks to be different from the ones calculated just from the given crystal structure).
In contrast to the COD (which only contains calculated powder diffraction patterns), the PDF also contains a large variety of experimental patterns with very different origin and quality, some of which may also show the "variations" just mentioned. As a result, the probability that the PDF contains a reference pattern that is similar to the one of your sample maybe higher than when using the COD.
- When selecting your reference database, you should always keep in mind
that there may be unforeseen phases in your samples, in which case it is extremely
useful to have a large pattern database available in which even diffraction
patterns of "strange" phases are present.
- If you are using the
normal Match! demoversion, only data from the "Inorganics" subset of the COD are installed.
You can e.g. download
and install the full COD reference database from the Match! download page,
or from a Crystal Impact DVD distributed at various meetings, e.g. the ECM- or IUCr-conferences.