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5 9 I H I G H L I G H T S 2 0 2 3

PRINCIPAL PUBLICATION AND AUTHORS

Multiscale reciprocal space mapping of magnetite mesocrystals, A. Chumakova (a,b), T. Steegemans (c), A. Mistonov (d), I.A. Baburin (e), I.S. Dubitskiy (c), J. Schlotheuber (c), F. Kirner (c,f), S. Sturm (g,h), A. Lubk (g,h), K. Müller-Caspary (h), I. Wimmer (d), M. Fonin (d), E.V. Sturm (c,f), A. Bosak (a), Adv. Mater. 35, 2207130 (2022); https:/doi.org/10.1002/adma.202207130 (a) ESRF (b) Outstation at Heinz Maier-Leibnitz Zentrum (MLZ), Institute of Crystallography (IfK) RWTH Aachen University, Aachen (Germany) (c) Department of Chemistry, University of Konstanz (Germany) (d) Department of Physics, University of Konstanz (Germany) (e) Department of Chemistry, TU Dresden (Germany) (f) Department of Earth and Environmental Sciences, Section of Crystallography, Ludwig-Maximilians- Universität München, Munich (Germany) (g) Leibniz Institute for Solid State and Materials Research (IFW), Dresden (Germany) (h) Department of Chemistry and Pharmacy-physical Chemistry at the Ludwig-Maximilians-Universität München, Munich (Germany)

REFERENCES

[1] E.V. Sturm et al., Crystals 7, 207 (2017). [2] A. Girard et al., J. Synchrotron Radiat. 26, 272-279 (2019).

In this work, a detailed structural characterisation of magnetite nanoparticles and their mesocrystals of different shapes and sizes (1010-1012 particles, sizes from tens to hundreds of micrometres) was carried out, combining scanning electron microscopy (SEM) with SAXS and WAXS simultaneously measured at beamline ID28 utilising the PILATUS-based diffractometer [2]. The high dynamical range of the detector sheds light on the weak features of scattering, significantly increasing the information content. Supplementary magnetisation measurements were also carried out.

Analysis of the SEM data correlated the mesocrystal habitus to the size and shape of their constituent particles. These shapes, such as oblate rhombohedral, prolate rhombohedral and orthogonal pyramidal (Figure 41, 1st column), in turn can be related to the translational periodicity recovered from SAXS (Figure 41, 3rd column). Notably, up to 10 orders of small-angle diffraction are resolved without substantial angular spread. WAXS data beyond some momentum transfer should be considered as an incoherent sum of individual diffraction patterns of nanoparticles for regularly packed monodisperse particles it gives a rare occasion to recover the particle shape in a way similar to coherent diffraction imaging (Figure 41, 2nd column).

The results obtained on the shape, packing and orientation distribution function of the nanoparticles augments previously available transmission electron microscopy (TEM) data. It was found that with an increasing degree of cube truncation, the packing goes from trigonal to centered tetragonal structure, through surprisingly the region of coexistence of trigonal and monoclinic structures. This enables a semi-quantitative explanation of the packing details based on the analysis of the particle contact distances. Furthermore, the study confirms that the size and shape of nanoparticles and their arrangement significantly impact the magnetic

properties of solid mesocrystalline materials. More particularly, highly truncated particles had higher blocking temperatures, which increased notably when self-assembled into mesocrystals. However, smaller particles with a low degree of truncation showed similar blocking temperatures across all arrangements.

The features revealed in this work provide valuable input to the models of mesocrystal growth and the choice of structural motif and impact on magnetic properties. Though a detailed analysis of the physical factors influencing the magnetic behaviour of ordered assemblies extends beyond this study, it is essential to note that studying mesocrystals at different length scales plays a pivotal role in understanding the structure property relationship of such systems.