Scale factor and profile parameters

Initial residuals (row one in Table 7.3), calculated using profile parameters determined from the full pattern decomposition employing Le Bail s algorithm and the default value of the scale factor (AT = 0.01), are quite 

Under normal circumstances, the scale factor is one of the few least squares parameters, which is always kept as a free variable in a Rietveld refinement. Since in this particular example, all profile and lattice parameters have been refined using Le Bail s approach, the next reasonable step is to release all associated variables before proceeding with individual atomic parameters (the scale factor remains a least squares free variable). As expected, the resultant change in the figures of merit, listed in third row in Table 7.3, is small because the background, peak shape and lattice parameters are already quite accurate. 

In rare cases, e.g. when all sites in the crystal structure are partially occupied, the scale factor may become strongly correlated with the population parameters. This requires special attention and detailed consideration of all possibilities exceeds the scope of this text. One of the options is to use the known scale factor as a fixed parameter while refining population parameters of all crystallographic sites. This option is, however, seldom available because relative and not absolute intensities are customarily measured in a powder diffraction experiment. 

In this structure, only individual displacement and population parameters can be refined. 

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