PHASE TRANSITIONS IN TRANSITION-METAL SESQUIOXIDES

There is substantial interest among the mineral physics and geophysics communities in phase transitions in sesquioxides such as Fe₂O₃ and Al₂O₃.  Although these phases are not considered to be major constituents of Earth's mantle, knowing their high-pressure behavior is considered to be essential for understanding overall mantle character and, in particular, its oxidation state.  In addition, recent developments in x-ray emission spectroscopy that provide a powerful new tool in detecting spin crossover in iron-containing materials at high pressure have created wide interest in phase transitions in Fe₂O₃ and other Fe-containing compositions.  In addition, there is much to be learned from high-pressure experiments on other compositions having the hematite/corundum structure under certain conditions, i.e., Ti₂O₃, V₂O₃, Cr₂O₃, Co₂O₃, Ga₂O₃, In₂O₃, Tl₂O₃, and Rh₂O_3.  

Various investigators have reported that hematite Fe₂O₃ transforms at room temperature to an orthorhombic perovskite structure at about 50-60 GPa.  Other work indicates that this structure is not perovskite and that it most likely has the Rh₂O₃ II structure.  Combined X-ray emission spectroscopy and x-ray diffraction experiments confirm that the transition involves spin crossover, but that this takes place only after the structural change has taken place.  Still another investigation of Fe₂O₃, in which the sample was laser-heated at pressure, showed a transition at about 30 GPa, which was interpreted as hematite to a perovskite-like structure, and yet another transition above 40 GPa was interpreted as to a post-perovskite-type structure.  Even with all the high-pressure experiments that have been reported on Fe₂O₃, none has included a careful Rietveld refinement to provide conclusive evidence for specific high-pressure structures.  Lin et al. (2004) did complete such a refinement in their study of Al₂O₃, which indicated a phase transition from the corundum to the Rh₂O₃ II structure at above 96 GPa with laser heating.  This kind of experiment needs to be extended to other compositions as well as to Fe₂O₃.  

Lin, J-F, Degtyareva, O, Prewitt, CT, Dera, P, Sata, N, Gregoryanz, E, Mao, H-k and Hemley, RJ (2004) Crystal structure of a high-pressure/high-temperature phase of alumina by in situ X-ray diffraction, Nature Materials 3: 389-393