In situ total scattering studies: Formation mechanisms of ternary multiferroic bismuth ferrates
Solid State and Surface Chemistry, Material Synthesis
Mineralogy, Petrology and Geochemistry
Final Report Abstract
Within this project, we used in situ total scattering experiments and Pair Distribution Function (PDF) analysis to study the formation of mullite-type materials. Precursors were synthesized in a sol-gel approach using the respective metal nitrates and sugar alcohols as the complexing agent. These were subsequently heated while continuously collecting diffraction patterns with a secondscale time resolution to follow the phases qualitatively appearing during crystallization as well as their transition and growth kinetics. We could show that a change in the synthesis parameters, in particular the pH and metal nitrate to complexing agent ratio, leads to different crystallization pathways of the target compounds. More specifically, for Bi2Fe4O9 we observe multiple crystallization pathways including A) the initial formation of rhombohedral BiFeO3 and subsequent transition into orthorhombic Bi2Fe4O9, B) co-crystallization of BiFeO3 and Bi2Fe4O9, or C) the direct formation of Bi2Fe4O9 from the precursor. The crystallization process seems to be predetermined very early in the synthesis and mainly governed by the gel structures formed during evaporation of the solvent and organic components, as suggested by ex situ PDF analysis. Furthermore, it was found that Bi2Fe4O9 exhibits an unusual behavior during crystal growth, where lattice parameter b and c decrease significantly, although Bi2Fe4O9 is known to exhibit positive thermal expansion. To compare the crystallization behavior of isostructural Bi2Fe4O9, Bi2Al4O9, Bi2Ga4O9, and closely related Bi2Mn4O10 we synthesized a precursor under acidic, neutral, and alkaline conditions for each composition. Regardless of the conditions used Bi2Al4O9 and Bi2Ga4O9 show only the formation of the mullite-type structure, however with strong variations in their growth kinetics. For instance, Bi2Al4O9 slowly crystallizes under neutral conditions, whereas Bi2Ga4O9 crystallizes very slowly under acidic conditions. In case of Bi2Fe4O9, crystallization of BiFeO3 can be observed in all media, however with an increased stability regime towards the alkaline medium. Bi2Mn4O10 shows the greatest variety in terms of crystallization behavior, e.g., 4 intermediates in acidic medium, which might be expected since Mn ions possess the largest flexibility in oxidation states within the compounds under study. As a highlight of this project, we synthesized for the first time high-entropy (HE) versions of Bi2M4O9 (M = Al, Fe, Ga, Mn) as well as A2Mn4O10 (A = Nd, Sm, Y, Er, Eu, Ce, Bi) compositions. HE materials represent a new class of compounds with at least 5 different elements in equiatomic amounts in a solid solution. Their compositional diversity make them promising platforms to develop the next generation of functional materials. We show that HE materials can form even in the complex mullite-type structure. This contradicts previous assumptions that HE materials only form in simple crystal structures. Here, we show that the materials are statistically mixed solid solutions using a combination of neutron and X-ray powder diffraction, X-ray total scattering, scanning transmission electron microscopy, infrared and Raman spectroscopy. In addition, we follow their formation in situ by X-ray diffraction and X-ray absorption spectroscopy and find that the HE oxides form directly from an amorphous precursor.
Publications
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Controlling crystallization pathways and kinetics in multiferroic Bi2Fe4O9. Acta Cryst. 2021, A77, C370
Kirsch, A.; Lefeld, N.; Gogolin, M.; Banerjee, S.; Jensen, K.M.Ø.
