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Spectroscopy and Photochemistry; General Theory
High-Pressure Evolution of Crystal Bonding Structures and Properties of FeOOH Cheng Lu, and Changfeng Chen J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00947 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018
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The Journal of Physical Chemistry Letters
High-Pressure Evolution of Crystal Bonding Structures and Properties of FeOOH Cheng Lu∗ and Changfeng Chen∗ †
Department of Physics and Astronomy, High Pressure Science and Engineering Center, University of Nevada, Las Vegas, Nevada 89154, USA Supporting Information ABSTRACT: Recent conflicting reports on the high-pressure structural evolution of iron oxidehydroxide (FeOOH) offer starkly contrasting scenarios for the hydrogen and oxygen cycles in Earth’s interior. Here we explore the crystal structures of FeOOH using an advanced search algorithm combined with first-principles calculations. Our results indicate a phase transition around 70 GPa from the known ε-FeOOH to a new pyrite-type FeOOH (P-FeOOH) phase, and the two phases remain nearly degenerate in an unusually large pressure range. These findings clarify and explain the experimentally observed structural evolution and extensive phase coexistence. Moreover, our structure search identifies a previously unknown monoclinic (M-FeOOH) phase that is energetically close to P-FeOOH at pressures near the core-mantle boundary. We further reveal that the high-pressure FeOOH phases exhibit remarkably distinct sound-velocity profiles, providing key material properties essential to interpreting seismic data and elucidating FeOOH’s influence on geophysical and geochemical processes in deep Earth.
Hydrous minerals play a prominent role in transporting water into Earth’s interior and thereby influencing key geophysical and geochemical processes. 1–4 Laboratory based high-pressure study of such minerals offers insights into fundamental properties essential to understanding the evolution and dynamics of Earth. 5–7 Iron oxide-hydroxide (FeOOH) is among the most common hydrous minerals on Earth’s surface and in its interior. 8–10 At ambient conditions, the most stable α-FeOOH phase comprises double chains of edge-shared octahedra that form 2×1 channels hosting hydrogen bonds; 9 at pressures above 5 GPa, the structure transforms into the ε-FeOOH phase composed of corner sharing single bands of octahedra, and FeOOH decomposes into Fe2 O3 + H2 O at high temperatures. 10 Further explorations at much higher pressures, however, are required to obtain the structure and property of FeOOH for assessing its influence in the lower mantle and core-mantle boundary. A pair of recent studies 11,12 tackled this formidable challenge using x-ray diffraction (XRD) measurements supported by limited structural and energetic calculations. Surprisingly, they reached starkly contrasting conclusions about the structural evolution of FeOOH at high pressures, leading to drastically different scenarios for hydrogen and oxygen cycles inside Earth. The earlier work 11 proposed a pressure driven dehydrogenation transition of FeOOH to gaseous H2 and a solid pyrite-phase FeO2 , suggesting a highly unorthodox scenario for the separation of hydrogen and oxygen cycles in the deep lower mantle. Under this scenario, there would be an upward migration of hydrogen from the middle of the lower mantle while the oxygen-rich FeO2 further subducts to the base of lower mantle. The later work, 12 on the other hand, provided compelling evidence showing that the observed pyrite structure is in fact a new FeOOH phase (P-FeOOH), which remains stable to pressures at the mantle-core boundary, where P-FeOOH would go through a dehydration transition and dissociate into Fe2 O3 and H2 O. The mobile water would then react with core derived iron and form iron hydride and release oxygen in the lowermost mantle. Resolving these
conflicting results has profound implications for understanding the geophysics and geochemistry inside Earth. The most pressing issues concerning FeOOH center on an accurate determination of the pressure-driven structural evolution of various crystal phases and a reliable assessment of key properties essential to interpreting seismic data. In recent studies, the new high-pressure phase was inferred either from an empirical fitting to XRD data 11 or by an ad hoc assignment; 12 such approaches often lead to large uncertainties for complex structures, especially those containing light elements with negligible x-ray scattering. Consequently, the first work 11 assigned the high-pressure structure as a pyrite-type FeO2 with total dehydrogenation, while the second work 12 assigned the structure as a pyrite-type FeOOH with full hydrogenation. A partially dehydrogenated FeOOH x (x
