).
Institut für Angewandte Physik,
TU Wien, 1040 Wien, Austria
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, U.S.A.
Faculty of Physics and Center for Computational Materials Science, University of Vienna, 1090 Wien, Austria
X-ray Center, TU Wien, Getreidemarkt 9, 1060 Wien, Austria
Dipartimento di Fisica e Astronomia, Università di Bologna, 40127 Bologna, Italy
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, x-ray photoelectron spectroscopy, and density-functional theory were used to study the distribution of Eu2+ dopants in a NaI scintillator activated by 3% EuI2. A variety of Eu-based structures were identified in crystals subjected to different postgrowth 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 toward the surface, resulting in the formation of an ordered hexagonal overlayer with a EuI2 composition and a pronounced, unidirectional moiré 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.
Corresponding author: Martin Setvin (setvin).
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