Structural Stability and Properties of Marokite-Type γ-MnO.

We synthesized single crystals of marokite (CaMnO)-type orthorhombic manganese (II,III) oxide, γ-MnO, in a multianvil apparatus at pressures of 10-24 GPa. The magnetic, electronic, and optical properties of the crystals were investigated at ambient pressure. It was found that γ-MnO is a semiconductor with an indirect band gap of 0.96 eV and two antiferromagnetic transitions () at ∼200 and ∼55 K. The phase stability of the γ-MnO crystals was examined in the pressure range of 0-60 GPa using single-crystal X-ray diffraction and Raman spectroscopy. A bulk modulus of γ-MnO was determined to be = 235.3(2) GPa with ' = 2.6(6). The γ-MnO phase persisted over the whole pressure range studied and did not transform or decompose upon laser heating of the sample to ∼3500 K at 60 GPa. This result seems surprising, given the high-pressure structural diversity of iron oxides with similar stoichiometries. With an increase in pressure, the degree of distortion of MnO polyhedra decreased. Furthermore, there are signs indicating a limited charge transfer between the Mn ions in the octahedra and the Mn ions in the trigonal prisms. Our results demonstrate that the high-pressure behavior of the structural, electronic, and chemical properties of manganese oxides strongly differs from that of iron oxides with similar stoichiometries.