The Rh1/Fe3O4(001) “single-atom” catalyst evolves differently upon exposure to O2 and CO, which results in distinct mechanisms of CO2 production.

Activator impurities and their distribution in the host lattice play a key role in scintillation phenomena. Here a combination of cross-sectional noncontact atomic force microscopy (nc-AFM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) was used to study the distribution of Eu 2+ dopants in a NaI scintillator activated by 3% of EuI2. A variety of Eu-based structures was identified in crystals subjected to different post-growth treatments. Transparent crystals with good scintillation properties contained mainly small precipitates with a cubic crystal structure and a size below 4 nm. Upon annealing, Eu segregated towards the surface, resulting in the formation of an ordered hexagonal overlayer with a EuI2 composition and a pronounced, unidirectional moire pattern. Crystals with poor optical transparency showed a significant degree of mosaicity and the presence of precipitates. All investigated crystals contained a very low concentration of Eu dopants present as isolated point defects; most of the europium was incorporated in larger structures. arXiv:1809.01347v2 [cond-mat.mtrl-sci]