Quantitative imaging of single cesium atoms in the channels of a Beryl crystal

Aberration-corrected scanning transmission electron microscopy (STEM) allows the imaging of single atoms in bulk crystals, usually with high angle annular dark field (HAADF) images [1], but in special cases also with STEM spectroscopy methods [2]. However, the detection of single atoms in a bulk crystal is challenging and depends on various experimental parameters such as specimen thickness, crystal orientation, composition of the matrix and the type of dopants. For substitutional dopants, the channeling of the electron waves in the crystal must be considered, whereas for single atoms on interstitial sites such as in porous materials, other difficulties such as the electron probe confinement arise [3].
In this work, we show how single atoms in the channels of a beryl crystal can be detected and quantified. Beryl (Be3Al2Si6O18) has a hexagonal ring structure with crystal channels of about 0.5 nm diameter aligned parallel to the c-axis. Foreign atoms such as alkali ions or H2O molecules can be embedded in these channels. Recently, atoms were detected in the channels using STEM-HAADF and it was assumed that these were Fe2+ ions [4].
Here we study a beryl that had Fe and Cs concentrations of 0.3% and 0.03%, respectively. The ion-milled crystal was investigated with a probe-corrected Titan3 (Thermo Fisher) operated with an X-FEG at a convergence angle of 15 mrad. The angular ranges of the detectors are from 62 to 214 mrad (FEI) and from 20 to 73 mrad (Gatan). The experimental results were validated by density functional theory (DFT) calculations and the images were simulated with the program QSTEM (C. Koch, Berlin).