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

mineYAMANAKAFe_3-xTi_xO₄, 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 e_g orbit at the tetrahedral site for Fe² under high-pressure or low-temperature condition prefers electronic state on d_z2 orbit to d_x2-y2, inducing the tetragonal distortion with c/a<1. Difference Fourier maps of Fe₂TiO₄ reveal the electron density of d_z2 state in e_g 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.