Non-random orientation distribution of crystallites in powder specimens or compact polycrystalline samples can influence almost all accurate diffraction measurements. This regards mainly quantitative phase analysis and structure determination because both these methods are based on intensities of reflections from a randomly oriented assembly of particles. However, a strong preferred orientation (texture) causes problems also in phase identification, stress measurements and profile analysis, especially in thin films. The effect of preferred orientation in powder specimens can be reduced by special techniques of their preparation [1]. Such approach cannot be used in the case of compact specimens, and an inhomogeneity of the packing of crystallites in loosely pressed powder specimens will cause an uncontrollable variation in the intensity distribution making accurate measurements impossible [2].
The present accurate measurements are thus based on methods of correction of integrated intensities for the texture effects. A fast spinning of the sample during the intensity measurement introduces axial symmetry which greatly simplifies the problem. In any case one has to use some assumption regarding the shape of the orientation distribution function. There are two main approaches: (i) symmetrized harmonic functions [3], or empirical one- or two-parameter functions [4]. This paper is mainly focused on the second choice.
The direct usage of simple empirical functions is limited to the cases when the texture axis is perpendicular to the sample surface and diffraction vector is parallel to the texture axis for all measured reflections (the latter condition is automatically satisfied in symmetrical Bragg-Brentano scans). A "disk-shape behaviour" of crystallites is assumed in all these methods. Recently the method was generalized also for cases of "inclined textures", "inclined samples" and asymmetric scans used, e.g., in parallel beam or Guinier diffraction geometries [5]. In such cases one cannot use the simple functions directly and a numerical integration over certain trajectory on the surface of the orientation distribution function is needed to refine its free parameters. The generalization to the "inclined samples" makes possible to do a joint texture refinement on samples with a strong texture measured in symmetrical scans at several sample inclinations on an Eurelian cradle [6]. Examples will be given to illustrate abilities and accuracy of these methods.
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