There are two ways that GSAS-II can fit a diffraction pattern. Most of this book is concerned with Rietveld analysis, where peak positions are generated from the structure factors generated from a space group, lattice parameters and atom positions/ADPs for one or more crystallographic phases. In the Pawley and Le Bail fitting options (see §20.4), the reflection intensities are optimized directly, rather than being determined from the atom positions/ADPs. The second mechanism uses discrete peaks that are placed at the locations that you choose, independent from any lattice constraints. The position, intensity and, optionally, peak widths for each peak are varied independently. We call this “peak fitting” to distinguish it from the full-profile Rietveld fitting process.
There are a number of reasons why peak fitting may be used. The capability was initially developed as part of the indexing process, where one assigns a lattice to a set of peaks. Use of peak fitting provides much more accurate positions than other methods and can uncover where two peaks are closely spaced – the inclusion of a single peak at a position intermediate between the two actual peaks can derail the fitting process. However, there are other reasons why peak fitting can be useful. It can be used to determine the intensity of peaks, where that is the only quantity needed to be determined perhaps for phase abundance or some ad hoc property is being characterized, possibly where lattice parameters are not known, or where only a narrow data range has been measured. It can be useful for identification of an unknown phase (see 17.5) or for learning about peak widths in the absence of the restrictions of a model.
It should be noted that peak fitting uses the same background fitting tools described in Chapter 11. Background parameters will be optimized in the peak fitting process when their refinement flags are on. The instrumental profile terms described in Chapter 14 may also be used, depending on the peak width mode selected (discussed further in §17.3) and the instrumental terms (located in the histogram’s Instrumental Parameters data tree child entry) can optionally be fit as well.
To perform peak fitting, select the Peak List data tree child item for the histogram of interest. This will present a table of peaks, that will initially be empty. Each peaks is described with four parameters and each of those four parameters has a refinement flag to determine if that parameter will be optimized when peaks are refined.