Pressure induced Fe²⁺ + Ti⁴⁺ → Fe³⁺ + Ti³⁺ intervalence charge transfer and the Fe³⁺/Fe ²⁺ ratio in natural ilmenite (FeTiO³) minerals

The Fe³⁺/Fe²⁺ ratio in natural ilmenite (FeTiO ³) samples shows rapid increase under pressure from ambient up to ∼2 GPa, as deduced from ⁵⁷Fe Mössbauer pressure studies. The ratio, which is initially 0.2 at ambient pressure, saturates at ∼0.65 beyond 2-4 GPa to the highest pressure of 14 GPa in this study. Dramatic changes occur at low pressure where the compressibility of the unit cell is appreciably anisotropic, the c-axis being more compressible than the a-axis, and coincides with the rapid decrease observed in the electrical resistance under pressure. Pressure induced intervalence charge transfer away from the ferrous sites, conceivably across regions of octahedra, to the empty 3d manifold of the Ti ⁴⁺ cation in an adjacent face-sharing layer along the c-axis may account for the change in the Fe³⁺/Fe²⁺ ratio.