Thermal and Athermal Structure Change of Fe3-xTixO4 spinel solid solution.

Earth and Space Science, Osaka University* Center for Quantum Science and Technology, Osaka University**
â—‹Takamitsu Yamanaka* Tetsuro Mine* Sachiko Asogawa* Yuki Nakamoto**

mineYAMANAKAFe3-xTixO4, continuous solid solutions have a spinel structure (Fd3m). Various pressure-induced transforms have been reported into CaMn2O4 (Pbcm), CaTi2O4 (Bbmm), or CaFe2O4 (Pnma) structure. Using synchrotron radiation we performed high-pressure single-crystal structure analyses with diamond anvil cell up to 15GPa and powder diffraction experiments up to 60GPa. The cubic-to-tetragonal transition pressure of decreases with increasing Ti content. The transition was induced from the tetragonal distortion due to the Jahn-Teller effect of Fe2+at the tetrahedral site. But the spinel with less than x=0.6 does not transform. Structure refinements of Fe2TiO4 were executed at various pressures at room temperature and at low-temperatures down to -170C at ambient pressure. All least-squares analyses are converged to R=0.015~0.04 at low temperature and 0.03~0.09 at high pressure. The transition to the low-temperature tetragonal phase takes place at -110C. The c/a ratio is 0.9982(4) at 11.43GPa at room temperature and 0.9967(5) at -170C at ambient pressure. Base on the high-spin state condition, degeneracy of eg orbit at the tetrahedral site for Fe2 under high-pressure or low-temperature condition prefers electronic state on dz2 orbit to dx2-y2, inducing the tetragonal distortion with c/a<1. Difference Fourier maps of Fe2TiO4 reveal the electron density of dz2 state in eg electrons of Fe2+. Another high-pressure phase of orthorhombic structure of Fe2TiO4 was found at 30GPa and the structure was determined to be CaTi2O4-type structure by Reitveld method. In addition, we found a new higher-pressure polymorph for specimens with x=0.8 and 1.0 at 59GPa and 45GPa, respectively. We could not confirm the phase for specimens x=0.7.