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Atomic Site And Species Determinations Using Channeling And Related Effects In Analytical Electron Microscopy
Published 1988 · Chemistry
Abstract The formulation, development and applications of a novel crystallographic technique for specific site occupation determinations using the channeling or Borrmann effect in electron diffraction is reviewed. This technique is based on the effect of incident beam orientations on the intensities of either the characteristic X-ray emission or the characteristic energy-loss edges. The formulation of the technique under planar-channeling conditions for simple layered structures (ALCHEMI — atom location by channeling-enhanced microanalysis) and a general formulation for non-layered structures, along with the relevant theory, are reviewed in detail. In the case of characteristic energy-loss edges, in addition to being a function of the diffraction of the incident beam (channeling), the intensities are also a function of the diffraction of the outgoing inelastically scattered fast electron (blocking) and the scattering angle. A judicious choice of these parameters can provide additional information, such as the specific site valence of a particular atomic species. In general, this technique can distinguish neighbors in the periodic table; involves no adjustable parameters, external standards or special specimen preparations; is applicable to trace element concentrations (0.2 – 0.3 wt% or 10 25 atoms/m 3 ); is very accurate (3 – 10% error in site occupancy determinations, depending on the formulation used); and can be routinely applied at very high spatial resolutions (10 – 40 nm). Two crucial assumptions are made: the inelastic scattering events are assumed to be highly localized and the impurities/additions are assumed to be distributed uniformly with depth in the specimen. These and other assumptions, limitations and possible improvements or extensions of the technique are discussed, but throughout the review, the emphasis is directed to practical considerations.