Stabilization of over-stoichiometric Mn⁴⁺ in layered Na_2/3MnO₂

R. Stoyanova (1, 2) , Dany Carlier-Larregaray (2) , M. Sendova-Vassileva (3) , M. Yoncheva (1) , E. Zhecheva (1) , D. Nihtianova (1, 4) , Claude Delmas (2)

1 Institute of General and Inorganic Chemistry
2 ICMCB - Institut de Chimie de la Matière Condensée de Bordeaux
3 CL SENES - Central Laboratory of Solar Energy and New Energy Sources
4 Central Laboratory of Mineralogy and Crystallography

Sodium manganates with nominal composition Na_2/3MnO₂ were prepared by solid state reaction between Na₂CO₃ and MnCO₃ at 1000 °C. The composition and structure of Na_xMnO₂ were controlled by the rate of cooling from the temperature of preparation. This is a consequence of the capability of Na_2/3MnO₂ 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 °C, the orthorhombic distorted modification is stabilized. A phase separation into orthorhombic and hexagonal modifications takes place when Na_2/3MnO₂ 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.

Mots clés: Layered oxides; Sodium manganates; Electron paramagnetic resonance spectroscopy; Raman spectroscopy

Domaines: Matériaux

R. Stoyanova, Dany Carlier-Larregaray, M. Sendova-Vassileva, M. Yoncheva, E. Zhecheva, et al.. Stabilization of over-stoichiometric Mn⁴⁺ in layered Na_2/3MnO₂. Journal of Solid State Chemistry, 2010, 183 (6), pp.1372-1379. ⟨10.1016/j.jssc.2010.04.024⟩. ⟨hal-00494476⟩