Jahn-Teller transition of ulvöspinel ${\text{Fe}}_2{\text{TiO}}_4$ has been investigated by single-crystal x-ray diffraction study under high pressures up to 15 GPa at ambient temperature using synchrotron radiation and under the low-temperature condition down to $-170°\text{C}$ at ambient pressure. The transition from cubic to tetragonal is induced from the tetragonal distortion due to the Jahn-Teller effect of ${\text{Fe}}^{2+}$ at the tetrahedral site. The phase transition takes place at 9 GPa with increasing pressure at $20°\text{C}$ and the transition was found at $-110°\text{C}$ and 1 atm. The $c/a$ ratio is 0.9982(4) at 11.43 GPa. On the other hand, the value is 1.0035(5) at $-170°\text{C}$ and 1 atm. Degeneracy of $e$ orbit of ${{}^{\text{IV}}\text{F}\text{e}}^{2+}$ is different between high-pressure and low-temperature conditions: the former prefers electronic state $d_{x^2\text{−}{y}^2}$ and the latter $d_{z^2}$ orbit in $e$ orbit. Difference Fourier maps on (100) and (001) planes of ${\text{Fe}}_2{\text{TiO}}_4$ reveal the electron density of $e$ electrons of ${{}^{\text{IV}}\text{F}\text{e}}^{2+}$. Orthorhombic structure of ${\text{Fe}}_2{\text{TiO}}_4$ at 30 GPa determined by Rietveld method was ${\text{CaTi}}_2{\text{O}}_4$-type structure by powder-diffraction study. In addition, we found another new higher-pressure polymorph at 48 GPa.

• Received 9 May 2009

DOI:https://doi.org/10.1103/PhysRevB.80.134120