Manganese related perovskites (AMnO₃, A=alkaline earth) present a wide range of fascinating functional properties due to the ability of Mn to adopt several oxidation states and different coordination polyhedra. Regarding their catalytic behaviour, CaMnO_3-δ selectively oxidizes, at least on a laboratory scale, propene to benzene and 2-methyl propene ^[1]. Moreover, the Ca-Mn-O system presents a great variety of oxides with different Ca/Mn ratio and crystalline structure and a particular behaviour: their reduction process leads to a rock-salt type structure which, in most cases, can be again oxidized to the starting material ^[2].

Here we show the combination of oxygen engineering performed under adequate controlled atmosphere with local characterization techniques like atomic resolution electron microscopy associated to Energy Electron Loss Spectroscopy (EELS) to provide a complete characterization of other member of the Ca-Mn-O system. In particular, Ca₂Mn₃O₈ crystallizes in a monoclinic layered structure ^[3] with sheets of Mn^IV in octahedral coordination held together by Ca²⁺ cations alternately stacked along a axis (Fig. 1). The total reduction process of this material leads to Ca₂Mn₃^IIO₅ with rock-salt type structure.

By means of partial reduction, different samples have been stabilized in the Ca₂Mn₃O_8-d system. Among them, Ca₂Mn₃O_6.5, a new layered calcium manganese oxide with only Mn³⁺, has been stabilized. The different samples obtained in the Ca₂Mn₃O_8-δ system have been characterized by using High Angle Annular Dark Field (HAADF) and Annular Bright Field (ABF) imaging associated to EELS and Energy Dispersive Spectroscopic (EDS) in an aberration-corrected JEOL JEMARM200cF electron microscope. The structural evolution and the local variation of the Mn oxidation state in different phases with different anionic composition will be discussed.

[1] A. Reller et al, Proc. R. Soc. Lond. A, 394 (1984), 223-241.

[2] A. Varela et al, J. Am. Chem. Soc., 131 (2009), 8660-8668.

[3] G. B. Ansell et al, Acta Crystallogr. B, 38 (1982) 1795-1797.

Figures:

Figure 1. (a) HAADF image of Ca2Mn3O8 acquired simultaneously with the EELS signal. (b) Spectrum obtained from summing the EELS signal from the region of interest. (c) Mn-L2,3 ELNES of the sample (d) Ca-L2,3, and (e) Mn-L2,3 atomic resolution chemical maps. (f) Experimental Ca (red) and Mn (green) composite map. Structural model is shown in the inset.

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EMC Abstracts - https://emc-proceedings.com/abstract/analytical-atomic-resolution-microscopy-of-oxygen-deficient-ca2mn3o8-d/