Anisotropic electrical conductivity changes in FeTiO₃ structure transition under high pressure

Electrical resistivity measurements on oriented FeTiO₃ ilmenite using single crystals at high pressures proves that FeTiO₃ ilmenite shows anisotropic electrical resistivity. The resistivity in the direction perpendicular to the c-axis decreased monotonously with increasing pressure. In contrast, the resistivity in the parallel direction to the c-axis initially decreased and slightly increased with increasing pressure above 6 GPa. It then resumed decreasing above 8 GPa. The hallow-shape of the curvature was observed. Neutron and synchrotron X-ray diffraction experiments provided an accurate picture of the pressure-induced changes of the FeTiO₃ ilmenite structure. FeTiO₃ transforms neither into perovskite nor LiNbO₃ phase under pressures up to 28 GPa. However, different compression curves were observed for both FeO₆ and TiO₆ octahedra below 8 GPa. FeO₆ is more compressible and flexible than TiO₆. Among Fe–Fe, Ti–Ti and Fe–Ti interatomic distances, the shortest Fe–Ti distance presents the highest electrical restivity and electron mobility according to Fe²⁺Ti⁴⁺ and Fe³⁺Ti³⁺ by electron super-exchange mechanism, which is enhanced during compression. At high pressure, the electron configuration of Fe²⁺ (3d⁶) is more strongly changed than Ti⁴⁺ (3d⁰) and the former cation is the emphasized by Jahn–Teller effect in the ligand field of C_3v molecular symmetry. The anisotropic electrical resistivity and non-uniform structure change of Fe–Ti interatomic distance can be explained by possible spin transition. The spin transition of FeKβ from high-spin to intermediate-spin state is possible in the electronic state change of FeTiO₃.