Analytical Atomic‐Resolution Microscopy of Oxygen Deficient Ca2Mn3O8‐d

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Manganese related perovskites (AMnO 3 , 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, Ca 2 Mn 3 O 8 crystallizes in a monoclinic layered structure [3] with sheets of Mn IV in octahedral coordination held together by Ca 2+ cations alternately stacked along a axis (Fig. 1). The total reduction process of this material leads to Ca 2 Mn 3 II O 5 with rock‐salt type structure. By means of partial reduction, different samples have been stabilized in the Ca 2 Mn 3 O 8‐ d system. Among them, Ca 2 Mn 3 O 6.5 , a new layered calcium manganese oxide with only Mn 3+ , has been stabilized. The different samples obtained in the Ca 2 Mn 3 O 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.