High Resolution STEM Simulations of Beryl Crystal Impurities Using Multislice Methods (QSTEM)

The development of Scanning Transmission Electron Microscopy (STEM) has enabled measuring and analyzing many kinds of materials with atomic resolution. Not only does high resolution imaging give us the opportunity to intuitively interpret crystal structures, but it also enables us to find local crystal defects and impurities. This is especially useful in the analysis of a beryl crystal Al2Be3[Si6O18], where empty channels exist due to the crystal structure. STEM investigations with a high-angle annular dark field detector (HAADF) of a beryl crystal parallel to the hexagonal c-axis reveal all structural details, even the empty channels between the atom columns (Figure 1a). However, some channels do not appear to be empty. This could mean that we see dopant elements, long known from literature, localized in the channels of naturally grown beryl [1,2]. In order to prove that the contrast detected is not an artefact, a multitude of multislice simulations was performed using various detector and microscope settings in the program QSTEM [3]. Several alkaline ions such as Na, K and Cs were placed inside the empty channels of a crystal model with varying thicknesses, with the atoms being chosen according to EDS and WDS spectral analysis of the beryl performed in a scanning electron microscope.
The simulation results show that the contrast of dopant atoms matches that from STEM HAADF measurements. This work proposes methods for the identification of channel atoms and shows their feasibility as well as a wide range of possibilities for improvement of identification quality. Methods of differentiating between stacks of light atoms and singular heavy atoms are proposed as well.