Manganese(III) oxide

Heating MnO₂ in air at below 800 °C produces α-Mn₂O₃ (higher temperatures produce Mn₃O₄).^[5] γ-Mn₂O₃ can be produced by oxidation followed by dehydration of manganese(II) hydroxide.^[5] Many preparations of nano-crystalline Mn₂O₃ have been reported, for example syntheses involving oxidation of Mn^II salts or reduction of MnO₂.^[6][7][8]

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

2 MnO₂ + Zn → Mn₂O₃ + ZnO^{[citation needed]}

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.^[9]

Mn₂O₃ is unlike many other transition metal oxides in that it does not adopt the corundum (Al₂O₃) structure.^[5] Two forms are generally recognized, α-Mn₂O₃ and γ-Mn₂O₃,^[10] although a high pressure form with the CaIrO₃ structure has been reported too.^[11]

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).^[12] α-Mn₂O₃ undergoes antiferromagnetic transition at 80 K. ^[13]

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.^[10] γ-Mn₂O₃ is ferrimagnetic with a Néel temperature of 39 K.^[14]

ε-Mn₂O₃ takes on a rhombohedral ilmenite structure (the first binary compound known to do so), wherein the manganese cations divided equally into oxidation states 2+ and 4+. ε-Mn₂O₃ is antiferromagnetic with a Néel temperature of 210 K.^[15]