A high-pressure Raman study of FeTiO3 ilmenite: Fermi resonance as a manifestation of Fe-Ti charge transfer

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 A(g) symmetry peaks (nu(4) and nu(5)) between similar to 10 and similar to 30 GPa: bands that involve major components of Ti-O and Fe-O-related displacements, respectively. The peaks' relative intensities switch at similar to 18 GPa and they also reach their minimum separation at similar to 20 GPa, indicating that their maximum resonance occurs between 18 and 20 GPa. The negative shift of the Ti-O-associated nu(4) vibration under compression is fully consistent with a shift in valence of Ti from 4 + to 3 + under compression. Anomalously small mode shifts of other, more localized vibrations are also consistent with a charge transfer from Fe to Ti under compression. At higher pressures, we have not found definitive evidence for a transition to the perovskite-structure at 300 K, which has been well characterized at high pressures and temperatures. At 40 GPa, we observe an apparent reversible disordering that persists up to our highest pressure. The 300 K mode shifts of the Raman active modes in FeTiO3 under pressure are notably different from those of other ABO(3) compounds (where A = Mg, Mn and B = Ti, Si); in other ilmenite-structured compounds, the peaks shift at a faster rate and there has not been any observation of Fermi resonance. Thus, iron's complex electronic structure, and its charge transfer with titanium, appears to play a primary role in the behavior of phonons in FeTiO3 ilmenite.

Automated summary

In this study, the high-pressure behavior of ilmenite was investigated using Raman spectroscopy up to 54 GPa at 300 K. The results showed a Fermi resonance between certain peaks indicating a charge transfer from Fe to Ti under compression. The behavior of phonons in FeTiO3 ilmenite appears to be primarily influenced by the complex electronic structure of iron and its interaction with titanium.