Texture analysis preferred orientation

Crystallography is a very broad science, stretching from crystal-structure determination to crystal physics (especially the systematic study and mathematical analysis of anisotropy), crystal chemistry and the geometrical study of phase transitions in the solid state, and stretching to the prediction of crystal structures from first principles this last is very active nowadays and is entirely dependent on recent advances in the electron theory of solids. There is also a flourishing field of applied crystallography, encompassing such skills as the determination of preferred orientations, alias textures, in polycrystalline assemblies. It would be fair to say that... 

Preferred Orientation in Deformed Metals and Rocks An Introduction to Modern Texture Analysis. Orlando, Florida Academic Press. 

Frequently, polycrystalline specimens exhibit a preferred orientation of the crystallites or polycrystalline texture. In addition, many manufacturing processes of technological materials can induce texture. In comparison with specimens having randomly oriented crystallites, the relative intensities of the diffraction lines of textured samples are modified. As a consequence the structural and quantitative phase analysis of polycrystalline samples becomes impossible without proper modeling of the texture. 

A second example of the application of laser ablation was reported by Chmielowska et al. [260]. The authors carried out structural analysis of thin cerium dioxide films doped with copper, which were produced for applications as catalytic gas sensors. The thin films deposited on a silicon substrate had a nanocrystalline structure with a well-developed texture. The morphology, as well as the preferred orientation of the films, changed with the volume fraction of copper. The observed variations were found to affect the catalytic properties of the materials. 

For a physical mixture, the powder diffraction pattern is the sum of the patterns of the individual materials. The diffraction pattern can therefore be used to identify the crystalline phases in a mixture. The concentrations of the crystalline phases can be determined by methods based on comparing the intensities of the diffraction peaks with standards (6-8). If the crystal structures of the phases are known, the concentration of each phase can be detamined by Rietveld analysis (20,21). In the Rietveld method, a theoretical diffraction pattern is computed and the difference between the theoretical and observed patterns is minimized. For quantitative analysis, some care should be taken with specimen preparation if accurate and reliable results are to be obtained. The effects of factors such as preferred orientation, texturing, and particle size broadening must be minimized. 

Coming back to the detailed analysis of diffraction patterns, we note that such efforts can be in practice more complicated for real samples for different reasons. First of all, the crystallites (grains) inside a polycrystalline sample might have a preferred orientation (texture), and accordingly, the Bragg reflexes of all other orientations are extremely suppressed in their intensity compared to those expected from calculated structure factors. Such a behavior can be expected e.g. in the case of epitaxially grown thin films that adopt the structure or at least the orientation of the substrate. This is observed e.g. for the passive films on iron discussed above [4], and in part also for those on Ni [19] but also for electrodeposited metal films. 

Chemical identification of unknowns by XRD relies in the accurate determination of a set of ii-spacings for the various crystallographic orientations. The data are screened against database of reference materials which are typically powder data with no preferred orientation. Accuracy of XRD identification of unknown species depends on the careful preparation of the samples, if powder form is required. On the other hand, if samples are not in powder form, care must be taken to account for missing lines in the XRD pattern and for abnormal intensity ratio in the observed peaks due to preferred orientation (texture). Detection limits in this case are within 0.1-1 wt%, which is worse than the ppm or ppb levels provided by surface analysis methods such as XPS or SIMS (discussed in this book). Chemical determination by XRD is limited to crystalline phases rally, but compounds can be identified down to their polymorphic phases. 

H. Siemes, Ch. Hennig-Michaeli, in H.-R. Wenk (Ed.), Preferred Orientation in Deformed Metals and Rocks An Introduction to Modern Texture Analysis, Academic Press, Orlando, FL, 1985,... 

The analysis of the statistical or preferred orientation of the crystallites in solid polycrystaUine materials is commonly referred to as texture analysis. Again, the diffraction technique allows the definition of the relationship between a microscopic property, i.e. the orientation of the crystallites defined as coherent diffraction domains, and the macroscopic physical properties of the crystal aggregate. Texture studies are of course crucial in the characterization of oriented synthetic materials such as cold-rolled metals or oxide thin films, but they are also of great relevance in the study of the formation processes of mineral assemblages. As an example, the texture features of olivine or pyroxene minerals in meteoritic chondrules yield information on the early condensation sequence... 

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