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M. Ulreich, L. Boatner, I. Sokolovic, M. Reticcioli, B. Stoeger, F. Poelzleitner, C. Franchini, M. Schmid, U. Diebold, M. Setvin:
"Defect chemistry of Eu dopants in NaI scintillators studied by atomically resolved force microscopy";
Physical Review Materials, 3 (2019), 0750041 - 0750047.

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.