Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Preparation and chemistryEdit
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 MnII 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.
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). 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.
^Chandiran, Kalaiselvi; Murugesan, Ramesh Aravind; Balaji, Revathi; Andrews, Nirmala Grace; Pitchaimuthu, Sudhagar; Nagamuthu Raja, Krishna Chandar (2020-07-03). "Long single crystalline α-Mn2O3 nanorods: facile synthesis and photocatalytic application". Materials Research Express. IOP Publishing. 7 (7): 074001. doi:10.1088/2053-1591/ab9fbd. ISSN 2053-1591.
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^Zhong-Yong Yuan; Tie-Zhen Ren; Gaohui Du; Bao-Lian Su (2004). "A facile preparation of single-crystalline α-Mn2O3 nanorods by ammonia-hydrothermal treatment of MnO2". Chemical Physics Letters. 389: 83. doi:10.1016/j.cplett.2004.03.064.
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^Thomas Kohler; Thomas Armbruster; Eugen Libowitzky (1997). "Hydrogen Bonding and Jahn-Teller Distortion in Groutite,α-MnOOH, and Manganite,γ-MnOOH, and Their Relations to the Manganese Dioxides Ramsdellite and Pyrolusite". Journal of Solid State Chemistry. 133 (2): 486–500. doi:10.1006/jssc.1997.7516.
^High Pressure Phase transition in Mn2O3 to the CaIrO3-type Phase Santillan, J.; Shim, S. American Geophysical Union, Fall Meeting 2005, abstract #MR23B-0050
^Geller S. (1971). "Structure of α-Mn2O3, (Mn0.983Fe0.017)2O3 and (Mn0.37Fe0.63)2O3 and relation to magnetic ordering". Acta Crystallogr B. 27 (4): 821. doi:10.1107/S0567740871002966.
^Geller S. (1970). "Magnetic and Crystallographic Transitions in Sc+, Cr+, and Ga+ Substituted Mn2O3". Physical Review B. 1: 3763. doi:10.1103/physrevb.1.3763.
^Kim S. H; Choi B. J; Lee G.H.; Oh S. J.; Kim B.; Choi H. C.; Park J; Chang Y. (2005). "Ferrimagnetism in γ-Manganese Sesquioxide (γ−Mn2O3) Nanoparticles". Journal of the Korean Physical Society. 46 (4): 941.