Spectroscopic Evidence for the α-FeOOH-to-ε-FeOOH Phase Transition: Insights from High-Pressure and High-Temperature Raman Spectroscopy

We conducted in situ Raman spectroscopy measurements on goethite (α-FeOOH) under simultaneous high-pressure and high-temperature conditions using an externally heated diamond anvil cell (EHDAC). Our study investigates spectral changes associated with the α-FeOOH-to-ε-FeOOH phase transition up to ~11 GPa and 563 K. The phase transition was identified based on high-temperature Raman spectra collected at 473 K, 523 K, and 563 K. A key indicator of the transition is the disappearance of a characteristic shoulder peak near 410 cm−1 which occurs near 4.7, 6.0, and 6.6 GPa for temperatures of 473 K, 523 K, and 563 K, respectively. From this, we estimate a linear phase boundary where the transition pressure increases with temperature at a rate of 2.3 ± 0.5 GPa per 100 K. Extrapolation to room temperature (300 K) yields a transition pressure of 0.3 ± 3.1 GPa. These findings extend existing high-pressure Raman data from ambient to elevated temperatures up to 563 K, improving our understanding of hydrogen-bearing phases relevant to Earth’s deep interior.