High-pressure single crystal structure analysis and phase transition by charge transfer

Abstract

Many of Mn₂O₃ compounds have a corundum structure with space group of R3c. Compounds with r(M)/r(O)>0.87 are characterized by A-type rare earth structure. Those with 0.60< r(M)/r(O)<0.87 are C-type rare earth structure. Bixbyite (Mn,Fe) ₂O₃ has a cubic structure of C-type rare earth type. Pure M₂O₃ has an orthorhombic structure of Pbnm which is a little deformed structure of cubic structure. Jahn-Teler effect du to Mn³⁺ induces the distortion from the cubic site symmetry of ideal octahedron. Prewitt et at., (1969) proposed Mn₂O₃ changed from C-type to corundum structure under compression. Shono et al., (1997) reported a high-pressure phase at about 20GPa. In the present experiment structure analysis of bixbyite has been conducted by single crystal diffraction study through using diamond anvil cell with laboratory X-ray source and SR at BL02B2 Spring-8 and PF BL-10A. Powder X-ray diffraction study was executed at PF BL-18C with increasing pressure up to 40 GPa in order to elucidate the compressibility and pressure-induced phase transition. Diffraction peaks of high-pressure phase were observed above 20GPa. No abrupt change was found in the lattice constants and the lattice constants continuously change. The high-pressure phase was not quenchable, inferring a reversible phase transformation. The isothermal Birch-Murnagham equation of state was applied to P-V-T data. The observed data of Ko=176.5(4.8)GPa and Ko=7.56(0.67) were indicates a little smaller than those of Al₂O₃, V₂O₃, Cr₂O₃ and Fe₂O₃. These data imply Mn₂O₃ is less rigid in comparison of corundum structure. Bixbyite has two different metal sites of Mn1 and Mn2, which have the site symmetry of (8a) and .2. (24d), respectively. The former is an almost ideal octahedron of Mn1O6 but the latter has a largely distorted octahedron. All six Mn1-O bonds are equivalent. But three different Mn2-O bonds,, Mn2-O(1), Mn2-O(2) and Mn2-O(3) show different compression with pressure. Mean Mn2-O distance more decrease than Mn1-O. Both octahedra do not show a remarkable Jahn-Teller effect due to Mn³⁺. A high-pressure phase has monoclinic symmetry rather than rhombohedral such as corundum, ilmenite LiNbO3 type structure. Charge dispropor- tionation such as 2Mn³⁺=Mn²⁺+ Mn⁴⁺ is now tested by measuring the magnetic moment under high pressure.