![]() ![]() This phenomenon is common for organic crystals. When crystallites are plate or needle shaped, however, they will pack in an orientation that places the long axis of the crystallite (i.e., short axis of the unit cell) parallel to the packed surface and out of the illumination of incumbent x-rays (3). In an ideal world, the crystallites in a powder pack together in a random manner, with non-directional orientation, which will allow the incumbent x-rays to illuminate all facets of the crystallites. ![]() Of the many systematic errors that affect peak positions and intensities, the most egregious for pharmaceutical compounds is that of preferred orientation (2), which is affected by the shape of the crystallites. The Bragg peak positions and, to a lesser extent, the peak intensities are of key interest in pharmaceutical powder-diffraction analysis (1), and it is important to report them as accurately as possible. The specimen-mount procedure is one of the many actions an investigator employs that could dramatically affect Bragg peak positions and intensities in an x-ray powder-diffraction experiment.
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