Preferred-Orientation Correction and Normalization Procedure for Ab Initio Structure Determination from Powder Data

A preferred-orientation analysis for powder diffraction data is proposed, based upon an extension of the Wilson-plot intensity analysis, in order to correct the integrated intensities before the structure determination is started. The technique is based on an empirical exponential model for preferred orientation proposed earlier in the literature, of which a formal derivation is now given. Prerequisites are disc-shaped behaviour of the crystallites and spinning of the specimen. Furthermore, the indexed powder pattern and the unit-cell contents are required, but neither a structural model nor pole-figure observations are necessary. The influence of preferred orientation on the statistics of the normalized structure factor are assessed. It is shown that preferred orientation diminishes the quality of the triplet-phase-sum statistics. Improvements of the normalization procedure for powder diffraction intensities are proposed. Finally, it is shown by means of simulated powder data under which conditions the (most likely) preferred-orientation direction and corresponding strength can be established. Test results indicate that a reconstruction of the preferred-orientation model imposed is feasible provided the maximum individual intensity enhancement (or decrease) as a result of imposed preferred orientation is at least 70%, when compared with data in which preferred orientation is absent.