Fermi resonance observed in high-pressure ilmenite (FeTiO3)

Abstract:

Ilmenite is a common accessory mineral in igneous and metamorphic rocks on the surface of the Earth, and is a primary component of the kimberlitic MARID xenolith suite. FeTiO₃ ilmenite has been of interest for its weak magnetization and its influence on the magnetization of the crust, including remnant magnetization of deeper layers of the crust. Additionally, ilmenite is an important mineral for understanding the thermo-chemical development of the lunar mantle. We investigated the 300 K high-pressure behavior of ilmenite using Raman spectroscopy to 54 GPa. Upon compression, we observe a Fermi resonance between the lowest frequency E_g symmetry peaks (n₄ and n₅). The peaks’ relative intensities switch at ~18 GPa, and reach their minimum splitting at ~20 GPa. We have not found evidence for a transition to the perovskite-structure, which has been well characterized at high pressures and temperatures. At 40 GPa, we observe an apparent reversible amorphization that persists up to 50 GPa. The 300 K mode shifts of the Raman active modes in FeTiO₃ under pressure are remarkably different from those of other ABO₃ compounds (where A = Mg, Mn and B = Ti,Si); in other ilmenite structure compounds, the peaks shift at a faster rate and there has not been any observation of Fermi resonance. Thus, iron’s complex electronic structure appears to play a primary role in the behavior of phonons in ilmenite structured compounds.