Manganese(III) oxide is a chemical compound with the formula Mn
2O
3. It occurs in nature as the mineral
bixbyite
Bixbyite is a manganese iron oxide mineral with chemical formula: (Mn,Fe)2O3. The iron/manganese ratio is quite variable and many specimens have almost no iron. It is a metallic dark black with a Mohs hardness of 6.0 - 6.5. It is a somewhat ra ...
(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 MnO
2 in air at below 800 °C produces α-Mn
2O
3 (higher temperatures produce Mn
3O
4).
γ-Mn
2O
3 can be produced by oxidation followed by dehydration of
manganese(II) hydroxide.
Many preparations of nano-crystalline Mn
2O
3 have been reported, for example syntheses involving oxidation of Mn
II salts or reduction of MnO
2.
Manganese(III) oxide is formed by the redox reaction in an alkaline cell:
: 2 MnO
2 + Zn → Mn
2O
3 + ZnO
Manganese(III) oxide Mn
2O
3 must not be confused with MnOOH manganese(III) oxyhydroxide. Contrary to Mn
2O
3, MnOOH is a compound that decomposes at about 300 °C to form MnO
2.
Structure
Mn
2O
3 is unlike many other transition metal oxides in that it does not adopt the
corundum
Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the pr ...
(
Al2O3) structure.
Two forms are generally recognized, α-Mn
2O
3 and γ-Mn
2O
3,
[Wells A.F. (1984) ''Structural Inorganic Chemistry'' 5th edition Oxford Science Publications ] although a high pressure form with the CaIrO
3 structure has been reported too.
α-Mn
2O
3 has the cubic
bixbyite
Bixbyite is a manganese iron oxide mineral with chemical formula: (Mn,Fe)2O3. The iron/manganese ratio is quite variable and many specimens have almost no iron. It is a metallic dark black with a Mohs hardness of 6.0 - 6.5. It is a somewhat ra ...
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 Fe
3+, pure Mn
2O
3 has an orthorhombic structure (
Pearson symbol oP24, space group Pbca, #61). α-Mn
2O
3 undergoes
antiferromagnetic transition at 80 K.
γ-Mn
2O
3 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-Mn
2O
3 is
ferrimagnetic with a
Néel temperature of 39 K.
ε-Mn
2O
3 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-Mn
2O
3 is antiferromagnetic with a Néel temperature of 210 K.
References
{{oxides
Manganese(III) minerals
Sesquioxides
Transition metal oxides