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Description
The crystallization of molecular materials on isotropic substrates typically results in a so-called fiber or uniplanar texture which comprises crystallites that share a common fiber axis perpendicular to the substrate surface, but which are azimuthally randomly oriented. The crystallographic characterization of such films is performed by grazing-incidence X-ray diffraction (GIXD). Two-dimensional reciprocal space maps are obtained, which incorporate the in-plane component qxy and the out-of-plane component qz for each diffraction peak. The exact position of each diffraction peak depends on the crystallographic lattice and on the orientation of the unit cell relative to the substrate surface. The unit cell orientation can be characterized either by two rotation angles or by the Miller indices of the crystallographic plane parallel to the substrate surface. Equations are derived that allow the calculation of the orientation parameters and describe the relations between them. Dependent on the underlying unit cell, a manifoldness of possible orientations exists. Examples based on molecular crystals of pentacenequinone, diindenoperylene and binaphthalene are given which are illustrative examples comprising triclinic, monoclinic and tetragonal unit cells which have two, four and 16 possible crystal orientations, respectively.