Imaging grain boundary segregation by electron diffractive imaging

The advantage of Z-contrast STEM over high-resolution phase contrast TEM for measuring segregation of (heavy) impurity atoms to grain boundaries is its increased sensitivity to atomic number. However, varying occupancy of segregation sites, insufficient imaging resolution, may make it very difficult to determine the exact position of the segregation site, especially in cases where the resolution does not suffice to resolve atomic columns in the crystal structure of the grains while the boundary is oriented parallel to the electron beam. We will show that, given the proper experimental conditions and due to the absence of lens aberrations in diffraction data, the Fourier transform of a selected-area electron diffraction pattern from the boundary may determine the segregation site with very high accuracy.