Thermal behaviour of gamma manganese dioxide: II. Evolution of crystal defects related to structural water

Electrochemically active manganese dioxides ([gamma]-MnO[sub 2]) contain physical and chemical crystal defects, related to structural water, which plays an important part in the mechanisms of proton insertion and migration in charge/discharge cycle. The loss of the structural water of two industrial samples of [gamma]-MnO[sub 2] (EMD and CMD) during static and dynamic thermal treatments in oxidizing atmosphere is studied using several experimental methods, in order to verify Ruetschi's model. The correlation of FTIR spectra, realized in-situ in temperature, with photoluminescence spectra enables the authors to associate hydroxyl groups with crystal point defects like Mn[sup 4+] vacancies. The residual presence of OH groups and Mn[sup 4+] vacancies at 400 C reveals that MnO[sub 2] is not stoichiometric at this temperature. This conclusion is corroborated by chemical measurements of manganese oxidation degrees of annealed samples, by thermogravimetric analysis and by the analysis of gas evolved during heat-treatment. Such results partially contradict Ruetschi's general formula of [gamma]-MnO[sub 2] and its overall waterloss process.