Über die Oxydation von Manganoxidhydroxid

Abstract

The oxidation of γ‐MnOOH (manganite) in oxygen and its disproportionation in HNO₃ lead topotactically to β‐MnO₂. The oxidation of synthetic α‐MnOOH (groutite) in oxygen depends on its cristallite size; finely divided crystals oxidise rapidly to Mn₅O₈ which usually is stable but yields β‐MnO₂ by further oxidation. Larger crystals of disperse synthetic α‐MnOOH are topotactically transformed to γ‐MnO₂. In HNO₃ α‐MnOOH disproportionates into γ‐MnO₂ and Mn²⁺. Though strictly topotactical, the reaction α‐MnOOH → γ‐MnO₂ is not single‐phase as might be expected. The discontinuity in the function: JAHN‐TELLER distortion vs. reaction rate, may simply be interpreted as the crosspoint of two different functions attributed to the crystal species α‐MnOOH and γ‐MnO₂, respectively. This distortion confirms the presence of Mn³⁺ in manganite and nsutite. The wide variety of possible X ray powder patterns of the phase γ‐MnO₂ is explained by the superposition of, (i) cristallite size broadening, (ii) intergrowth structure effects on the profile, and (iii) BRAGG angle shifts due to substitution of part of Mn⁴⁺ by Mn³⁺.