Synthesis of Ilmenite-type ε-Mn₂O₃ and Its Properties

Abstract

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.

The electronic structure of novel high-pressure and high-temperature phases of Mn2O3: exploring perovskite and ilmenite-type structures

“…S2, ESI †), 9 implying a smaller mean oxidation state than Mn 4+ . Taking the high-pressure nature of the ilmenite phase into account, suggesting a higher density, 5 it seems unlikely that the Mn 1 octahedra could be occupied exclusively by tetravalent ions. Hence, they could be filled by both Mn 4+ and Mn 3+ cations.…”

“…We employed a multi-anvil multi-tonne press and followed the same procedures as described in previous papers. [3][4][5] A pressure-temperature phase diagram of Mn 2 O 3 , including stability regions of the ilmenite and perovskite phases, has been accurately determined in an earlier study. 5 We synthesized these high-pressure polymorphs under P-T conditions corresponding to the centres of their stability regions in the diagram, namely, at 16.5 GPa and 1250 1C for the ilmenite phase, and at 20 GPa and 900 1C for the perovskite phase.…”

“…The manganese-ilmenite phase is thereby the first simple transition metal oxide that can be stabilized in the ilmenite-type structure due to charge redistribution, though a significant tetragonal distortion has been observed previously. 5…”

Electronic properties of single-crystalline Fe4O5

“…Microscopic single crystals and millimeter-sized polycrystals of Fe 4 O 5 were synthesized under high-pressure high-temperature (HP-HT) conditions using multi-anvil presses at Bayerisches Geoinstitut. 55,56 The starting samples were prepared under air conditions shortly before their synthesis from magnetite (Aldrich, 99.99% purity) and iron (Aldrich, 99.999% purity) powders, which were weighed and mixed well in an agate mortar to form uniform mixtures with a nominal 'Fe 4 O 5 ' stoichiometry. The samples were synthesized over several hours at pressures of about 14 GPa and temperatures of about 1100-1200 °C (for polycrystals) and 1350-1450 °C (for single crystals).…”

High-pressure A-site manganites: Structures and magnetic properties

“…[67] It is only when using HPHT (12 < P < 17 GPa at 1200 K) that the initially identified as Mn2O3 corundum-type appears (ε-Mn2O3, S.G. R-3c). Later on, this phase was properly identified as the charge ordered ε-Mn 2+ Mn 4+ O3 ilmenite type structure (S.G. R-3) [49,68] . Provided the cation disorder from a strict chemical point of view, this phase is still included in this disordered section.…”