Electron channeling, structure factor phases, polarity and atom site determination in crystals

We relate element characteristic electron energy losses and the accompanying X-ray emission under electron channeling conditions to the phases of structure factors. We discuss examples illustrating that structure factor phase information can be extracted from two-beam channeling experiments. This information has been used by several investigators to determine the absolute orientation of non-centrosymmetric crystals. We further discuss how the success of electron channeling in locating atoms in the crystal unit cell, ALCHEMI, is tied to phase information beyond the conventional one. From the perspective of diffraction-based crystallography, ALCHEMI provides information about the phase with which a particular element scatters into a reflection. Therein lies the strength of the ALCHEMI technique as compared to relying on diffraction intensities for the location of small concentrations of elements and neighbors in the periodic system.