Structure change of Mn₂O₃ under high pressure and pressure-induced transition

Abstract

Bixbyite (Mn,Fe)₂O₃ has a C-type rare-earth oxide structure with space group of Ia-3 and different from corundum structure R-3c. Single-crystal structure analyses and powder diffraction experiments were carried out using synchrotron radiation under high pressures up to 41.21 GPa. Lattice constants and bond distances were elucidated as a function of pressure. Single crystal structure analyses under high pressures up to 9.64 GPa have been executed using DAC. There are two octahedral Mn³⁺ sites: M1 (-3) and M2 (-2.). M1O₆ and M2O₆ octahedral volumes and void space of vacant sites show different compression curves. M1O₆ octahedron is less compressive than M2O₆ octahedron. The former is a little deformed from an ideal octahedron with m-3m of MnO₆ but the latter is a largely distorted octahedron. The quadratic elongation is applied in order to comprehend the polyhedral distortion and finite homogeneous strain. Both octahedra do not show a noticeable Jahn-Teller distortion induced from Mn³⁺. Six bond lengths of M1O₆ are equivalent but the octahedron more elongates along the direction of -3 axis with increasing pressure and is more deformed from the regular octahedron. The M2O₆ octahedron has two long bond distances among six bonds, which are most compressive. The octahedral deformations seem to reduce the Jahn-Teller effect due to the compression. The bulk modulus: Ko = 169.1(4.9) GPa and Ko′ = 7.35(0.99), was observed from the volume change with pressure.

Pressure-induced phase transition was confirmed at about 21 GPa with a large hysteresis. The transition is reversible and non-quenchable. Powder indexing of the high-pressure phase was carried out using diffraction pattern taken at 35.06 GPa. It has a monoclinic symmetry and is not a corundum, Rh₂O₃(II) or perovskite structure.