Study of the aqueous Mnx+-citrate precursor and its application towards the synthesis of Ca3Mn2O7

Abstract

Manganese is a very versatile element. It can be found in various functional materials, such as catalysts, electrodes in battery materials and magnetoelectric multiferroics. The latter are especially interesting due to their specific properties: ferroelectricity, ferromagnetism and a coupling between both. However, from an application point of view, it is necessary to acquire a complete control over these properties at room temperature, and this is still difficult to obtain. One mechanism that would allow for room temperature operation is Hybrid Improper Ferroelectricity (HIF), proposed by Benedek and Fennie.[1] The synthesis and characterization of Ca3Mn2O7, used as a model system in their first-principles calculations, is therefore very interesting. Even though the aforementioned materials are often synthesized via physical techniques, an aqueous solution-gel based approach offers some advantages, e.g. in cost. This requires the development of a suitable aqueous precursor solution. Therewithal, manganese shows a complex redox chemistry in aqueous solutions. For this reason, and in view of a broad applicability, an understanding of the aqueous chemistry of this metal is necessary. Here, we present the synthesis of a new aqueous Mnx+-citrate precursor, and the study of the Mn2+-Mn3+ redox chemistry hereof using, inter alia, UV-Vis and EPR spectroscopy. Over time, a complex cycle of Mn2+/Mn3+ oxidation and Mn3+/Mn2+ reduction takes place. Oxidation is the net result in the first month, after which reduction, followed by eventual precipitation as MnCO3, takes over. Thereby, the influence of several parameters and reagents, such as concentration, pH, citric acid ratio, atmosphere, temperature, hydrogen peroxide and UV-light is tested. It is found that they have an effect on this cycle, resulting in a shift of reaction speeds and/or stability against precipitation. An example hereof is the temperature effect: increasing the temperature leads to an increase in oxidation speed, but also to a faster precipitation. Overall, the involved complexes vary, proving the precursor solution to be a dynamic entity. This new manganese precursor solution is combined with a Ca2+ solution and used for the synthesis of Ca3Mn2O7. Different thermal treatment steps lead to the successful preparation of Ca3Mn2O7 and Ca4Mn3O10. The crystallinity, morphology and composition of the resulting powders are analyzed via XRD, SEM and Raman. Apparently, the age of the precursor entails an effect on the phase formation, since the aged precursor did not lead to the formation of Ca3Mn2O7. This indicates the importance of understanding the aqueous chemistry. In a next phase, the precursor solution will be used for the preparation of epitaxial Ca3Mn2O7 films.