Über die Oxydation von Manganoxidhydroxid

Abstract

Abstract The oxidation of γ‐MnOOH (manganite) in oxygen and its disproportionation in HNO 3 lead topotactically to β‐MnO 2 . The oxidation of synthetic α‐MnOOH (groutite) in oxygen depends on its cristallite size; finely divided crystals oxidise rapidly to Mn 5 O 8 which usually is stable but yields β‐MnO 2 by further oxidation. Larger crystals of disperse synthetic α‐MnOOH are topotactically transformed to γ‐MnO 2 . In HNO 3 α‐MnOOH disproportionates into γ‐MnO 2 and Mn 2+ . Though strictly topotactical, the reaction α‐MnOOH → γ‐MnO 2 is not single‐phase as might be expected. The discontinuity in the function: J AHN ‐T ELLER distortion vs. reaction rate, may simply be interpreted as the crosspoint of two different functions attributed to the crystal species α‐MnOOH and γ‐MnO 2 , respectively. This distortion confirms the presence of Mn 3+ in manganite and nsutite. The wide variety of possible X ray powder patterns of the phase γ‐MnO 2 is explained by the superposition of, (i) cristallite size broadening, (ii) intergrowth structure effects on the profile, and (iii) B RAGG angle shifts due to substitution of part of Mn 4+ by Mn 3+ .