Manganese Dioxides: Structural Model and In-Situ Neutron Powder Diffraction Investigation of Thermal Annealing and Elect
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MANGANESE DIOXIDES: STRUCTURAL MODEL AND IN-SITU NEUTRON POWDER DIFFRACTION INVESTIGATION OF THERMAL ANNEALING AND ELECTROCHEMICAL REDUCTION* M. RIPERT'
2
, J. PANNETIER , Y. CHABRE3 & C. POINSIGNON2
1- Institut Laue-Langevin, 156X, 38042 Grenoble, France 2 - Laboratoire d'Ionique et d'Electrochimie du Solide, INP Grenoble BP 75, 38402 Saint Martin d'H~res, France 3 - Laboratoire de Spectromdtrie Physique, Universitd Joseph Fourier, BP 87, 38402 Saint Martin d'HIres, France
ABSTRACT Starting from the seminal work of De Wolff [1], we have developed a structural description, based on two kinds of defects, which accounts for the scattering function of all yand e-MnO 2 . Using numerical simulation results, we propose simple methods to estimate the parameters which characterize real manganese dioxide samples. Real time neutron powder diffraction has been used to investigate in situ the transformations undergone by y-MnO 2 during thermal annealing and electrochemical reduction in alkaline solutions. We have found that thermally induced transformation from MnO 2 to cc-Mn20 3 can involve up to seven different steps and that electrochemical reduction of y-MnO 2 in KOD electrolyte proceeds through three stages, the final one leading in most cases to a breakdown of the initial crystal lattice.
INTRODUCTION The electrochemically active forms of MnO 2 have been used in Leclanchd primary batteries for almost a century but their structure and the atomic scale mechanism of their reduction in alkaline media are not yet understood. Chemical analysis have revealed both the presence of protonic species (adsorbed H 2 O and structural OH') and Mn3+; this can be accounted for by the defect 4+ model3+ originaly proposed by Ruetschi [2] which suggests a general formula Mn 4 1 xyMn y3 XO2 .4x_y(OH) 4 x+y,nH20 where 0 stands for vacancy in the cation network. However, a structural description allowing to characterize and classify the (apparently) many forms of manganese dioxides is still lacking and we do believe that this is the necessary step to a better understanding of MnO 2 and mastering of its synthesis and properties. In this paper we propose a unique crystal chemistry description of y- and e-MnO 2 and present the most striking results of a neutron diffraction study of their thermal decomposition and electrochemical reduction. The analysis of the time resolved diffraction data in the framework of our structural model unravels the sequence of chemical and structural transformations occurring during thermal treatment and proton intercalation. Details of the experiments and data analysis will be presented elsewhere. * Work supported by the PIRSEM/CNRS (ARC-IPROM)
Mat. Res. Soc. Symp. Proc. Vol. 210. @1991 Materials Research Society
360
STRUCTURAL MODEL OF y- AND e-MnO 2 The thermodynamically most stable variety of manganese dioxide is pyrolusite P-MnO 2 which is isotypic with rutile. The electrochemically active forms, usually termed EMD (Electrochemical) or CMD (Chemical Manganese Dioxide) according to the method of preparation
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