Synthesis of Ilmenite-type ε-Mn2O3 and Its Properties

In contrast to the corundum-type A(2)X(3) structure, which has only one crystallographic site available for trivalent cations (e.g., in hematite), the closely related ABX(3) 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 (epsilon-Mn2O3) is an n-type semiconductor with an indirect narrow band gap of Eg = 0.55 eV. Comparative investigations of the electronic properties of epsilon-Mn2O3 and previously discovered quadruple perovskite zeta- Mn2O3 phase were performed using X-ray absorption near edge spectroscopy. Magnetic susceptibility measurements reveal an antiferromagnetic ordering in epsilon-Mn2O3 below 210 K. The synthesis of epsilon-Mn2O3 indicates that HP-HT conditions can induce a charge disproportionation in simple transition-metal oxides A(2)O(3), 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.

Automated summary

In this study, a binary ilmenite-type compound ε-Mn2O3 was synthesized under high-pressure high-temperature conditions, revealing it as an n-type semiconductor with a narrow indirect band gap and antiferromagnetic ordering. This suggests that high-pressure high-temperature conditions can induce charge disproportionation in simple transition-metal oxides, leading to the stabilization of various mixed-valence polymorphs with different structures.