Stabilization of over-stoichiometric Mn4+ in layered Na2/3MnO2

Sodium manganates with nominal composition Na2/3MnO2 were prepared by solid state reaction between Na2CO3 and MnCO3 at 1000 degrees C. The composition and structure of NaxMnO2 were controlled by the rate of cooling from the temperature of preparation. This is a consequence of the capability of Na2/3MnO2 to accommodate over-stoichiometric Mn4+ ions up to 12 5%. Structural characterization was carried out by XRD powder diffractions, TEM analysis and Raman spectroscopy. The composition and oxidation state of manganese were determined by chemical analysis and magnetic susceptibility measurements The manganese distribution in the layers was analysed using electron paramagnetic resonance (EPR) spectroscopy. By quenching from 1000 degrees C, the orthorhombic distorted modification is stabilized A phase separation Into orthorhombic and hexagonal modifications takes place when Na2/3MnO2 is slow cooled. The structure changes are concomitant with an increase in the oxidation state of Mn The over-stoichiometric Mn4+ ions are accommodated in the hexagonal modification by creation of vacancies in the MnO2-layers. (C) 2010 Elsevier Inc. All rights reserved