- © 2013 Mineralogical Society of America
The structural phase transition of the titanomagnetite (Fe3–xTixO4) solid solution under pressures up to 60 GPa has been clarified by single-crystal and powder diffraction studies using synchrotron radiation and a diamond-anvil cell. Present Rietveld structure refinements of the solid solution prove that the prefered cation distribution is based on the crystal field preference rather than the magnetic spin ordering in the solid solution.
The Ti-rich phases in 0.734 ≤ x ≤1.0 undergo a phase transformation from the cubic spinel of Fd3̄m to the tetragonal spinel structure of I41/amd with c/a < 1.0. The transition is driven by a Jahn-Teller effect of IVFe2+ (3d6) on the tetrahedral site. The c/a < 1 ratio is induced by lifting of the degeneracy of the e orbitals by raising the dx2-y2 orbital below the energy of the dz2 orbital. The distortion characterized by c/a < 1 is more pronounced with increasing Ti content in the Fe3–xTixO4 solid solutions and with increasing pressure. An X-ray emission experiment of Fe2TiO4 at high pressures confirms the spin transition of FeKβ from high spin to intermediate spin (IS) state. The high spin (HS)-to-low spin (LS) transition starts at 14 GPa and the IS state gradually increases with compression. The VIFe2+ in the octahedral site is more prone for the HS-to-LS transition, compared with Fe2+ in the fourfold- or eightfold-coordinated site.
The transition to the orthorhombic post-spinel structure with space group Cmcm has been confirmed in the whole compositional range of Fe3–xTixO4. The transition pressure decreases from 25 GPa (x = 0.0) to 15 GPa (x = 1.0) with increasing Ti content. There are two cation sites in the orthorhombic phase: M1 and M2 sites of eightfold and sixfold coordination, respectively. Fe2+ and Ti4+ are disordered on the M2 site. This structural change is accelerated at higher pressures due to the spin transition of Fe2+ in the octahedral site. This is because the ionic radius of VIFe2+ becomes 20% shortened by the spin transition. At 53 GPa, the structure transforms to another high-pressure polymorph with Pmma symmetry with the ordered structure of Ti and Fe atoms in the octahedral site. This structure change results from the order-disorder transition.