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Breakthroughs in understanding elastic grain interaction and whisker formation made possible by advances in X-ray powder diffraction

Recent work on laboratory parallel-beam diffraction is summarized. Special attention is paid to instrumental aberrations and their corrections. Two types of primary beam optics are considered: X-ray lenses and X-ray multilayer mirrors. A high photon flux, high brilliance and minimal divergence of the incident beam make a recently pioneered setup, based on a combination of a microfocus rotating anode X-ray source with a single-reflection mirror, particularly suitable for grazing-incidence diffraction, and thus for the investigation of very thin films (yielding low diffracted intensities) and of stress and the anisotropy of the microstructure of materials (such as the crystallographic texture, requiring the acquisition of large measured data sets, corresponding to the variation of the orientation of the diffraction vector with respect to the specimen frame of reference). A comparative discussion of primary optics which can be used to realize parallel-beam geometry demonstrates the range of possible applications of parallel-beam diffractometers. Such advances in laboratory parallel-beam diffraction instrumentation enable highly accurate diffraction investigations of thin films and, in particular, diffraction measurement at constant, tuneable penetration depth. These novel possibilities thereby provide a pathway for fundamental investigations of thin films and nanocrystalline materials. In particular, breakthroughs have been realized in the fields of elastic grain interaction in ultrathin films and the understanding of whisker formation on Sn coatings.