THE [[gamma]]-[[alpha]] Fe₂O₃ TRANSITION. J. S. Olsen, L. Gerward, J. Z. Jiang, S. M[[omicron]]rup, T. Peun, Niels Bohr Institute, Ørsted Laboratory, Universitetsparken 5, DK2100 Copenhagen, Denmark, Physics Department, Technical University of Denmark, DK-2800 Lyngby, Denmark, GeoForschungszentrum, Potsdam, D-14473 Potsdam, Germany

The particle size of a solid material affects many of its physical properties. For example, the crystal structure of small particles may not be the same as that of large particles, and phase transition pressures and temperatures may be different. By use of synchrotron radiation and energy dispersive x-ray diffraction, the equation of state of [[gamma]]-Fe₂O₃ has been measured for pressures up to 50 GPa at ambient temperature in a diamond anvil cell, and at the constant pressure 7 GPa for temperatures up to 1000deg.C in a multi-anvil cell MAX80. Experiments were performed with [[gamma]] Fe₂O₃ powder of large particle size (1 um), and small particle size (7 nm). A comparison of the particle size effect on transition pressures and temperatures of the [[gamma]]-[[alpha]] phase transformation is presented. From the equation of state we calculate the bulk modulus B_o and its pressure derivative B_o^' and compare with available theoretical and experimental data.