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

Journal Article · · Physics and Chemistry of Minerals
ORCiD logo [1]; ORCiD logo [2]
  1. Univ. of Chicago, IL (United States); Univ. of Utah, Salt Lake City, UT (United States)
  2. Univ. of California, Santa Cruz, CA (United States)
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In this work, 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 Ag symmetry peaks (ν4 and ν5) between ~ 10 and ~ 30 GPa: bands that involve major components of Ti–O and Fe–O-related displacements, respectively. The peaks’ relative intensities switch at ~ 18 GPa and they also reach their minimum separation at ~ 20 GPa, indicating that their maximum resonance occurs between 18 and 20 GPa. The negative shift of the Ti–O-associated ν4 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 ABO3 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.

Research Organization:
Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
Sponsoring Organization:
USDOE National Nuclear Security Administration (NNSA); National Science Foundation (NSF)
Grant/Contract Number:
AC52-07NA27344; EAR-1855336; EAR-1620423; EAR-2017294
OSTI ID:
1885118
Report Number(s):
LLNL-JRNL-824462; 1037429
Journal Information:
Physics and Chemistry of Minerals, Vol. 48, Issue 9; ISSN 0342-1791
Publisher:
SpringerCopyright Statement
Country of Publication:
United States
Language:
English

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
ilmenite
high pressure
Raman spectroscopy
Fermi resonance