Qualitative analysis of powder x-ray diffraction data

Full text: The main task of qualitative analysis is the determination of the presence of major, minor and if practical trace phases in a sample. The attainment of this goal is dependent on the quality of alignment (line position and intensity) as well as quality of sample preparation. Identification is generally on the basis of structure and dependent on the use of reference patterns. There are inherent limitations to the use of reference patterns including lack of reference patterns for the phase of interest (rare), solid solutions or substitutions (likely) and poor quality of reference patterns (unusual). With multiphase samples, there are the added problems of line overlap or interference, determining minor or trace phases in the presence of major phases, the decrease of intensity with concentration and influence of mass absorption on intensities. Within multiphase samples there may be many phases, some of which are in low abundance. It is sometimes possible (and usually desirable) to fractionate the sample by some physical or chemical method (while attempting not to create new phases as a result). It is usually important and often critical to have available supplementary information about the sample - either chemical or physical, or a history of preparation or formation. A central issue is what constitutes a match. A match is an adequate accounting of the distribution of intensity in the diffraction pattern. The match can be thought of in terms of a sum of δ2θ (or δd/d) and/or δI/Iref, and the consistency with known information. The calculated measures give rise to the figures of merit (FOM) of search match programs. It must always be remembered that two compounds of the same space group and similar cell parameters (eg. FCC metals) are going to have very similar diffraction patterns - any search/match technique usually arrives at a stage where there is a list of possibilities that must be decided between. The range search-match techniques span manual based methods of considering significant lines such as with the Hanawalt, Fink and alphabetical index; computer based search-match based on FOM and the more recent graphically based full pattern methods of phase identification. No single approach is foolproof. a range of techniques is recommended if a high level of success is required. Copyright (1999) Australian X-ray Analytical Association Inc