New high-entropy oxides in a mullite-type structure Andrea Kirsch 1 , Espen Drath Bøjesen 2 , Niels Lefeld 3 , Rasmus Larsen 2 , Jette Katja Mathiesen 4 , Susanne Linn Skjærvø 1 , Rebecca K. Pittkowski 1 , Denis Sheptyakov 5 , Kirsten M.Ø. Jensen 1 1 University of Copenhagen, Department of Chemistry and Nanoscience Center, Denmark, 2 Aarhus University, Interdisciplinary Nanoscience Center, Denmark, 3 University of Bremen, Institute of Inorganic Chemistry and Crystallography, Germany, 4 Technical University of Denmark, Denmark, 5 Lab. for Neutron Scattering and Imaging, Paul Scherrer Institut, Switzerland Engineering compositional disorder into materials has attracted immense interest especially since the introduction of the high-entropy concept in 2004 [1] . High-entropy materials represent a class of compounds containing a high number of metal cations in ca. equal amounts statistically mixed in a single-phase solid solution. Only in 2015, the first high-entropy oxide (HEO) was synthesized [2] . Until now, it has been assumed that they crystallize in simple crystal structures [3] , and mainly HEOs in a rock-salt, fluorite, spinel, perovskite, and pyrochlore structure have been reported. Yet, with this study, we demonstrate the feasibility to synthesize HEOs possessing complex connectivity of polyhedral units in a mullite-type structure (Figure 1) [4]. The parent compounds Bi 2 M 4 O 9 (M = Al 3+ , Ga 3+ , and Fe 3+ ) and REMn 4 O 10 (RE = rare earth elements, Y and Bi) show a variety of attractive properties including multiferroicity. To map out the synthesizability in this system, we produced five HEOs with compositions of Bi 2 (Al 0.25 Ga 0.25 Fe 0.25 Mn 0.25 ) 4 O 9 , (Eu 0.2 HE) 2 Mn 4 O 10 (HE = Nd 0.2 Sm 0.2 Y0.2Bi 0.2 ), (Er 0.2 HE) 2 Mn 4 O10, (Ce0.2HE)2Mn4O 10 and (Nd 0.2 Sm 0.2 Y 0.2 Er 0.2 Eu 0.2 ) 2 Mn 4 O 10 . We show that the materials represent mixed solid solutions using a combination of neutron and X-ray powder diffraction (XRD) with subsequent Rietveld analysis, X-ray total scattering and Pair Distribution Function (PDF) analysis, transmission electron microscopy, infrared, and Raman spectroscopy. In addition, we follow their formation in situ by XRD and X-ray absorption spectroscopy. We find that all form through a metastable amorphous phase without the formation of any crystalline intermediates. We believe that our synthetic route is excellently suited for synthesizing diverse HEOs and therefore will have a significant impact on their future exploration. References
1. Yeh, J.-W. et al. Adv. Eng. Mater., 6, 299-303 (2004) 2. Rost, C. M. et al. Nat. Commun., 6, 8485 (2015)
3. Oses, C., Toher, C. & Curtarolo, S. Nat. Rev. Mater., 5, 295–309 (2020) 4. Kirsch, A.*et al. Chemistry; preprint (2022) doi:10.26434/chemrxiv-2022-txfgj
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