Manganese(III) oxide

Manganese(III) oxide is a chemical compound with the formula Mn₂O₃. It occurs in nature as the mineral bixbyite (recently changed to bixbyite-(Mn)IMA 21-H: Redefinition of bixbyite and definition of bixbyite-(Fe) and bixbyite-(Mn). CNMNC Newsletter, 64, 2021; Mineralogical Magazine, 85, 2021).) and is used in the production of ferrites and thermistors.

Preparation and chemistry

Heating MnO₂ in air at below 800 °C produces α-Mn₂O₃ (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:

: 2 MnO₂ + Zn → Mn₂O₃ + ZnO

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,Wells A.F. (1984) ''Structural Inorganic Chemistry'' 5th edition Oxford Science Publications 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 Ia, #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). α-Mn₂O₃ undergoes antiferromagnetic transition at 80 K.

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.

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.

References

{{oxides
Manganese(III) minerals
Sesquioxides
Transition metal oxides