An in situ high pressure-high temperature powder diffraction study of the formation of a precursor phase of bismuth manganite

The synthesis route for single phase BiMnO3 from Bi2O3 and Mn2O3 has been undertaken, in situ, in a diamond anvil cell. The starting powders were mixed in a stoichiometric ratio and loaded into a diamond anvil cell with a gasket aperture of 150 μm. A ruby chip was used to measure the pressure and a circular resistive heater surrounded the central core of the cell. The reaction was studied as a function of pressure and temperature to 6.7 GPa and 480 °C by X-ray diffraction. There was evidence of the onset of a reaction between the starting oxides at 314 °C and 6.7 GPa but there was no evidence of transformation of the monoclinic Bi2O3 to the cubic form before the reaction began. A monoclinic phase related to the structure of multiferroic BiMnO3 was observed to form with space group Cc, a = 10.21(1), b = 5.356(6), c = 10.38(1) Å and β = 116.87(6)°. A good fit for this structure was obtained by modelling a distorted BiMnO2 unit cell with disordered Bi and Mn sites. We did not go sufficiently high in temperature to observe the formation of single phase BiMnO3 but the precursor phase was observed to persist when returned to room temperature and pressure.