M. Niedermaier, P. Dolcet, A. Gheisi, G. Tippelt, W. Lottermoser, S. Gross, J. Bernardi, O. Diwald:
"Stability and Local Environment of Iron in Vapor Phase Grown MgO Nanocrystals";
Journal of Physical Chemistry C, 121 (2017), S. 24292 - 24301.

Kurzfassung englisch:
Metallocene injection into a metal combustion
flame has been used for trapping transition metal ions inside
MgO nanocrystals. Vacuum annealing changes the properties of
resulting nonequilibrium solids toward thermodynamic equilibrium
and provides means to control impurity localization and,
as a result, the nanomaterials´ functional properties. By
combining structure characterization (X-ray diffraction and
transmission electron microscopy) with X-ray absorption
spectroscopy and Mössbauer measurements, we tracked valence
state and local chemical environment changes of Fe3+ ions inside
vapor phase synthesized MgO nanocrystals. At a concentration
of (1.5 ± 0.2) at. % Fe about (1400 ± 200) Fe3+ ions are
effectively diluted within 12 nm sized nanocubes, where they
form complexes between Fe3+ ions and Mg2+ vacancies. Increase
of the iron concentration produces additional effects: enhanced ion diffusion and particle coarsening at elevated temperatures,
clustering of Fe3+−Mg2+ vacancy complexes and, after annealing to T = 1173 K, the nucleation of a magnesioferrite phase that
can be detected by X-ray diffraction for 4 at. % samples. At 3 at. % Fe, corresponding impurity ions induce surface energy changes
that have a substantial impact on particle shape. With regard to the functional properties associated with transition metal ions in
insulating MgO host lattices, the here presented insights underline that annealing-induced reorganization of oxide nanoparticles
provides important parameters to control distribution and localization of impurity ions.

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