And Something More

And Something More

Course Material Index

Section Index

And Something More

The last example illustrates an important point: By using a priori knowledge of the sample, in this case that it was probably a pure metal, the bounds of the search could, at least in the first instance, be reduced from around 100000 to just a dozen or so. Clearly it is important that such information be exploited in search methods be they manual or computer-aided. For the former case a number of manual sub-directories of the PDF have been compiled at various times, such as:

inorganic phases
organic and organometallic phases
minerals
forensics
metals and alloys

Another useful sub-directory is the Alphabetic Index in which phases can be searched by chemical name or by mineral or other known description. This might not at first sight seem so useful as the above, however many practitioners (including this author) find it to be the most useful sub-directory of all since they often have a hunch concerning a potential candidate (which they will therefore want to seek out rather by its name) and also since it is simply a very useful starting point for general information on any crystalline material.

The earliest computer-aided search/match strategies were developed in times (pre-seventies) when typical powder diffraction data (both the unknown and the PDF database entries) suffered from relatively large errors in both d spacings and intensities. Thus large error windows had to be placed on the d-I values and the search was invariably exhaustive, even across the whole database (which then was smaller of course). With improving diffraction techniques, the accuracy of powder diffraction data has improved enormously but the database has grown and the complexity of samples has increased. More intelligent computer search/match strategies have been devised, such as the use of:

Hanawalt-type approach;
Boolean search methods;
Use of a FOM (Figure of Merit) to assess the goodness of match;
Hierarchical searches, such as starting with the 300 most commonly encountered phases in the PDF, then the 2500 most encountered and so on;
Special methods for multi-phases (see later);
Incorporation of chemical composition data.

The last item is particularly useful: Often the case might be that one will know something about the chemical composition from the sample history (e.g. how it was made, where it was found) and simply being able to say that it contains calcium, sulphur, or whatever, is invaluable information towards cutting down the bounds of the search.

Another index that should be briefly mentioned, since it has been well used in some quarters over the years, is the Fink index which is named after a chairman of the earlier JCPDS committee. This method is particularly appropriate for those cases where it is inadvisable to place too much reliance on the intensities, for example when

there is significant preferred orientation in the sample;
the powder pattern has been obtained by electron diffraction.

To counter these difficulties the Fink index has been designed to place greater reliance on the actual values of the d spacings by using the 8 (rather than 3) strongest lines. However the number of permutations with 8 lines is too high so various compromises have been made concerning how multiple entries of each Fink combination are entered into the Fink Search Manual. The figure below illustrates a section taken from a typical Fink index in which the 4 strongest lines appear in bold and each have a turn at being first.

Course Material Index

Section Index

Author(s): Paul Barnes
Martin Vickers