Manganese(III) oxide

Preparation and chemistry

Heating MnO₂ in air at below 800 °C α-Mn₂O₃ is produced (higher temperatures produce Mn₃O₄). γ-Mn₂O₃ can be produced by oxidation followed by dehydration of manganese(II) hydroxide. Many preparations of nano-crystalline Mn₂O₃ have been reported, for example syntheses involving oxidation of Mn^II salts or reduction of MnO₂.

Manganese (III) oxide is formed by the redox reaction in an alkaline cell:

Manganese (III) oxide Mn₂O₃ must not be confused with MnOOH manganese (III) oxyhydroxide. Contrary to Mn₂O₃, MnOOH is a compound that decomposes at about 300 °C to form MnO₂.

Structure

Mn₂O₃ is unlike many other transition metal oxides in that it does not adopt the corundum (Al₂O₃) structure. Two forms are generally recognized, α-Mn₂O₃ and γ-Mn₂O₃, although a high pressure form with the CaIrO₃ structure has been reported too.

α-Mn₂O₃ has the cubic bixbyite structure, which is an example of a C-type rare earth sesquioxide (Pearson symbol cI80, space group Ia3, #206). The bixbyite structure has been found to be stabilised by the presence of small amounts of Fe³⁺, pure Mn₂O₃ has an orthorhombic structure (Pearson symbol oP24,space group Pbca, #61).

γ-Mn₂O₃ has a structure related to the spinel structure of Mn₃O₄ where the oxide ions are cubic close packed. This is similar to the relationship between γ-Fe₂O₃ and Fe₃O₄. γ-Mn₂O₃ is ferrimagnetic with a Néel temperature of 39 K.