Measured and calculated reference data
Measured reference patterns often score better for phase identification. The data source is a real powder diffraction measurement made on a pure, carefully selected and characterized material. This is possible even when the crystal structure is unknown. Some databases contain the original powder scan (reference scan) next to the stick pattern.
Calculated reference patterns always come with crystal structure data, as required for Rietveld refinements or structure analysis. The data source is a single crystal experiment most of the time. The theoretical stick pattern is calculated from the crystal structure, and, sometimes, optimized for search-match.
Commercial and free reference patterns
The reference patterns in the commercial databases have gone through a profound editorial process with automatic checks on data quality and consistency. They have quality marks and cross references attached and they are reedited from time to time. Typical examples are the PDF2 (measured data) and the PDF4+ (measured and calculated data) products from the ICDD.
A free reference pattern is either just a measurement of a pure phase used for search-match purposes; or it is calculated from a known (single) crystal structure. The quality is usually not documented and often lots of additional (physical and chemical) data are absent. Collections of free measured reference patterns are often small and serve dedicated purposes like for instance kidney stone characterization.
Crystal structure only databases
These databases cannot be used directly for phase identification. Typical examples are the commercial ICSD and the free COD crystal structure databases. They are used for standardless phase quantifications with the Rietveld method.
A free-of-charge alternative is based on the COD crystal structures and can be downloaded ready for use, from the COD webpage.
Collections of free calculated patterns can be big. Their size only depends on the number of crystal structures.
Special and user reference databases
These databases are often small and contain measured pure substances or even phase mixtures. The HighScore software supports any number of user-defined reference databases to be used either alone or in any combination. Another example of a commercial database for a specific purpose is Malvern Panalytical's CanDI-X. This search-match database is based on measured controlled substances, which have been characterized with other methods.