STRUCTURE CHANGE OF MN2O3 UNDER HIGH PRESSURE DUE TO CHARGE DISPROPORTION

High-pressure single crystal structure analysis up to 11GPa

Prewitt et at., (1969) proposed Mn₂O₃ changed from Rare Earth C-type to corundum structure under compression. Shono et al., (1997) reported it transforms to high-pressure phase at about 20GPa. But the structure has not been analyzed. In spite that Mn belongs to the first transition element group, Mn₂O₃ has a different structure from the other transition metal oxides of A₂O₃. The present investigation aims the structure change under high-pressure condition. Structure analysis as a function of pressure up to 11GPa has been conducted by single crystal diffraction study using diamond anvil cell (DAC) with synchrotron radiation at BL-10A at KEK. Two different metal sites of Mn1 and Mn2, 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 show different compression with pressure. Mean Mn2-O distance more decrease thanMn1-O. Both octahedra do not show a remarkable Jahn-Teller effect induced from Mn³⁺.

Phase transition confirmed by SR powder diffraction study

Ab initio Rietveld structure analysis using powder X-ray diffraction data from at BL-18C at KEK was executed with increasing pressure up to 40GPa. New diffraction peaks of high-pressure phase were observed above 20GPa. But the C-type rare earth phase was detected up to 32GPa because of kinetics of phase transition or deviative stress in DAC. 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. A high-pressure phase has distorted orthorhombic symmetry rather than rhombohedral such as corundum, ilmenite or LiNbO₃ type structure. Charge disproportion such as 2Mn³⁺=Mn²⁺+ Mn⁴⁺ is tested by measuring the magnetic moment under high pressure.

Compression of lattice constants and bulk modulus

The isothermal Birch-Murnagham equation of state was applied to the P-V-T data. The observed Ko=176.5(4.8)GPa and KoÕ=7.56(0.67) were indicates a little smaller than the reported data of Al₂O₃, V₂O₃, Cr₂O₃ and Fe₂O₃.