ABSTRACT
Atom Location by Channelling Enhanced Microanalysis (ALCHEMI) is a technique employed to investigate site occupancies and site symmetries within a particular crystalline unit cell. It relies on the existence of Bloch waves that result from the solution of the Schrodinger equation for an electron in a periodic lattice potential. For the simple case of a crystal at an exact Bragg orientation, corresponding to diffraction from a particular set of (hkl) atomic planes, two standing waves are set up. One standing wave has its maximum amplitude peaked directly on the atomic planes, while the other has its maximum amplitude peaked mid-way between the atomic planes. As the specimen is tilted through the Bragg condition, the relative intensities of these Bloch waves will vary. This variation in intensity will influence the probability of inner shell ionization, and hence the relative intensity of an ionization edge in an EELS spectrum. The exact nature of this variation will also depend on the location within the unit cell of the atoms being ionized, relative to those atoms which compose the atomic planes undergoing Bragg diffraction. As X-ray emission follows inner shell ionization, these channelling effects will also affect the intensities in the X-ray emission (EDX) spectrum.
