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M. Niedermaier, T. Schwab, P. Dolcet, J. Bernardi, S. Gross, M. Bockstedte, O. Diwald:
"Cobalt and Iron Ions in MgO Nanocrystals: Should They Stay or Should They Go";
Journal of Physical Chemistry C, 123 (2019), 25991 - 26004.

Identification and manipulation of transition metal ion impurities in oxide nanoparticles require
an in-depth understanding of their stability, segregation behavior and, at the same time,
knowledge about their surface reactivity. Powders of magnesium oxide nanoparticles with
admixtures of iron or cobalt ions, as two next neighbors in the periodic table, were synthesized
in the gas phase via injection of metal organic precursors into the magnesium combustion
flame followed by temperature quenching of resulting nanocrystals in argon. In these model
systems of cubic nanocrystals, we explored the distinct stability of these impurities in great
detail. While Co2+ ions keep their divalent valence state and substitute the host ions in the
cationic sublattice, Fe3+ ions emerge due to the energy gain provided by charge compensation
and impurity-vacancy complex formation. The very different behavior of Co and Fe ions in the
MgO host lattice, their changes in the local environment and the different trends in segregation
have been investigated by means of X-ray absorption and photoelectron spectroscopies and
structure characterization techniques. Abundance and energetics of the defects and defect
complexes were determined within the framework of density functional theory and enabled us
to explain consistently the reported experimental observations. Oxidation state and nature of
the defects cluster have a significant impact on particle size and annealing induced morphology
evolution, which determine their materials properties as components in heterogeneous
catalysis and functional ceramics.

http://dx.doi.org/10.1021/acs.jpcc.9b07350