Synthesis of Ilmenite-type ε-Mn₂O₃ and Its Properties

In contrast to the corundum-type A₂X₃ structure, which has only one crystallographic site available for trivalent cations (e.g., in hematite), the closely related ABX₃ ilmenite-type structure comprises two different octahedrally coordinated positions that are usually filled with differently charged ions (e.g., in Fe²⁺Ti⁴⁺O₃ ilmenite). Here, we report a synthesis of the first binary ilmenite-type compound fabricated from a simple transition-metal oxide (Mn₂O₃) at high-pressure high-temperature (HP-HT) conditions. We experimentally established that, at normal conditions, the ilmenite-type Mn²⁺Mn⁴⁺O₃ (ε-Mn₂O₃) is an n-type semiconductor with an indirect narrow band gap of E_g = 0.55 eV. Comparative investigations of the electronic properties of ε-Mn₂O₃ and previously discovered quadruple perovskite ζ-Mn₂O₃ phase were performed using X-ray absorption near edge spectroscopy. Magnetic susceptibility measurements reveal an antiferromagnetic ordering in ε-Mn₂O₃ below 210 K. The synthesis of ε-Mn₂O₃ indicates that HP-HT conditions can induce a charge disproportionation in simple transition-metal oxides A₂O₃, and potentially various mixed-valence polymorphs of these oxides, for example, with ilmenite-type, LiNbO₃-type, perovskite-type, and other structures, could be stabilized at HP-HT conditions.