Water ice becomes a superionic phase under the high pressure and temperature conditions of deep planetary interiors of ice planets such as Neptune and Uranus, which affects interior structures and generates magnetic fields. The solid Earth, however, contains only hydrous minerals with a negligible amount of ice. Here we combine high pressure and temperature electrical conductivity experiments, Raman spectroscopy and first-principles simulations to investigate the state of hydrogen in the pyrite-type FeO2Hx (x ≤ 1), which is a potential H-bearing phase near the core–mantle boundary. We find that when the pressure increases beyond 73 GPa at room temperature, symmetric hydroxyl bonds are softened and the H+ (or proton) becomes diffusive within the vicinity of its crystallographic site. Increasing temperature under pressure, the diffusivity of hydrogen is extended beyond the individual unit cell to cover the entire solid, and the electrical conductivity soars, indicating a transition to the superionic state, which is characterized by freely moving protons and a solid FeO2 lattice. The highly diffusive hydrogen provides fresh transport mechanisms for charge and mass, which dictate the geophysical behaviours of electrical conductivity and magnetism, as well as geochemical processes of redox, hydrogen circulation and hydrogen isotopic mixing in Earth’s deep mantle.
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Y.H. is also supported by HPSTAR. We acknowledge X. Du, N. Li, Y. Zhao, W. Yang and E. Greenburg for assistance in conducting laser heating and EC measurements; J. Cheng and B. Li for conducting Raman; and Y. Sun for FPMD technical support. This work is supported by the National Key Research and Development Program of China (2019YFA0708502), the National Natural Science Foundation of China (grant no. U1530402, U1930401, 41774101 and 11774015) and the Youth Innovation Promotion Association of CAS (2020394). XRD patterns were collected at High Pressure Collaborative Access Team (16ID-B) and GeoSoilEnviroCARS (13ID-D), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA’s Office of Experimental Sciences. APS is supported by DOE-BES, under contract no. DE-AC02-06CH11357. GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR - 1634415) and Department of Energy—Geosciences (DE-FG02-94ER14466). M.H. is supported by NSF grant EAR-1847707. Y.Z. is supported by China Postdoctoral Science Foundation grant 18NZ021-0213-216308. J.C. acknowledges the support of NSF grant EAR-1723185. L.D. is supportd by the Strategic Priority Research Program of CAS under grant XDB18000000. J.H.S. is supported by the Brain Pool Program through the National Research Foundation of Korea funded by the Ministry of Science and ICT under grant NRF-2020H1D3A2A02111022. C.J.P. is supported by a Royal Society Wolfson Research Merit Award. R.J.N. is supported by the Engineering and Physical Sciences Research Council under the grant EP/P034616/1. H.-K.M. is supported by NSF grants EAR-1722515 and EAR-1447438. Q.H. is supported by a Tencent XPLORER Prize.
The authors declare no competing interests.
Peer review information Nature Geoscience thanks Jean-Alexis Hernandez and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Rebecca Neely.
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Open circles are experimental volumes in literature. Measurement uncertainties from XRD are generally less than the symbol size. Dash curves are volumes of FeO2Hx with x=0, 0.25, 0.5, 0.75 and 1.0 from theoretical calculation (see Methods). The stability and stoichiometry of FeO2-FeO2H in pyrite structure. In eight different experiments converting FeO2H to the Py-phase at 86.0–133.5 GPa and 1500–2500 K, Hu et al. observed a wide variation of the unit-cell volume, that was attributed to the variation of x from 0.43 to 0.81 in Py-FeO2Hx due to the difference in the P–T of their synthesis. The set of curves for the volume dependence on x and P as shown in Fig. S1. Nishi. et al. conducted a set of experiments with all P–V data correspond to the maximum of Hu’s volume. Subsequent investigations by Yuan et al. from Tohoku University synthesized a Py-FeO2Hx with unit-cell volumes 2% larger than Nishi. et al. approaching the full hydrogenation of x = 1. Many additional publications in 2017–2019 further showed variation of volume and thus H. Most importantly, all experiments of four different groups consistently demonstrated that Py-FeO2Hx (x≤1) is stable under DLM pressures and is at least thermodynamically stable up to 2500–2600 K at 110 GPa. While the exact stoichiometry of the H and its relation to P, T, and hydrogen fugacity remain interesting, it is not the subject of the present work which focuses on the superionization behavior in hydrogen in the Py phase regardless of its stoichiometry. Therefore, our FPMD simulation chose x = 0.5 and 1.0 because they mainly cover the range of x obtained from XRD experiments.
a, The evolution of RO-H as a function of O-O distance RO-O. Magenta triangles: α-FeOOH; green squares: ε-FeOOH; red circles: pyrite FeO2H; orange inverted triangles: δ-AlOOH (Tsuchiya, Geophys. Res. Lett. 2002); cyan stars: Phase D (MgSi2O2H, Tsuchiya, Am. Mineral. 2005); grey diamonds: ice (VII to X) at 100 K simulated by M. Benoit et al. (Benoit, Nature 1998); purple solid line: line with symmetric O-H-O bonds (RO-H = RO-O/2). The lower panel is the electron localization function (ELF) distributions viewed along  direction for ε-FeOOH and  direction for Py-FeO2H. Yellow, white, and red spheres represent Fe, H and O atoms, respectively. Once synthesized at high pressure, the Py-FeO2Hx already features symmetric O-H-O bonding (Gleason Earth Planet. Sci. Lett. 2013). The change of RO-H/RO-O pairs indicated the delocalization of H between the two O atoms in both ε-FeOOH and Py-FeO2H when pressure raised above 50 GPa. b, Distribution of Electron localization function. H in the symmetric bonding partially loss its electrons and form a stronger ionic bond with O atoms. The stiffen of O-H-O bonds results in the blueshifts of hydroxyl Raman peaks (Fig. 2). c, Calculated VDOS of FeO2H at 300 K and pressures of 68.9 (black curve), 93.1 (red curve), and 114.8 GPa (blue curve). The movement of vibration modes is consistent with experiment.
Black, red and blue curves represent MSDs of H, Fe and O ions in FeO2H0.5. a, Ordinary solid FeO2Hx. b, The skyrocketing MSD of hydrogen indicate the onset of superionic state in Fig. 1. Uncertainties of pressure and temperature are sampled from the trajectories. c, Calculated melting temperature using Z method. The open circles show the pressures and temperatures of our simulations. The connection of these circles presents a shape like letter Z. The transition from the solid superheating state to liquid state is pointed out with arrows.
a, Black, red, blue and green symbols represent proton diffusion coefficients of FeO2H at 80 GPa, FeO2H at 130 GPa, FeO2H0.5 at 80 GPa and FeO2H0.5 at 130 GPa, respectively. b, The localized superionic phase with protons distributed between two nearby O ions. c, The delocalized superionic phase with protons delocalizing from O-H-O triplet. Small grey and pink spheres denote the trajectories of the H and O ions, respectively. The intermediate phase was previously studied by Hernandez and Caracas in ice as ice phase VII”. Protons in ice VII” delocalize from the O-H-O triplet and become even more diffusive. The intermediate phase may explain the further raising of the EC in Py-FeO2Hx above ~2000 K and 121 GPa. We have been inspired to conducted FPMD simulations and have also observed the delocalized phase in Py-FeO2H at temperatures above 3000 K. We compared the trajectories of H and O ions at 2000 and 3000 K in FeO2H. At 2000 K, the protons are mostly distributed between two nearby O ions, and they diffuse in the lattice by hopping between two symmetrized hydrogen sites. At 3300 K, movement of protons no longer binds to the nearest O atoms but travels to any possible interstitial sites in the entire lattice. The greater movement of protons can be described by the delocalized superionic phase. Similar to ice VII”, this intermediate phase helps to raise the ionic conductivity from the order of 101-102 to above 103 S m−1 and becomes visible in EC experiments.
Pressure is measured using the equation of state of Pt. The sample completely transformed into the Py-FeO2Hx phase. The top panel is an integrated pattern from the 2D unrolled x-ray diffraction pattern at the bottom. The wavelength of the x-rays is 0.4066 Å. The strong diffraction peaks from the Py-phase confirmed that the majority of the sample is Py-FeO2Hx. The lattice parameter of the Py-phase is a = 4.392 Å, standing for x = 0.87(8) from ref. 16. The remaining diffraction peaks could be indexed to the pressure medium (LiF) and very tiny amounts of ε-FeOOH. After heating, the pressure was equilibrated at 113 GPa and the electrical leads were checked by immersing the DAC into a liquid nitrogen cryostat. The slope of the temperature versus EC plot was positive, clearly showing four electrodes were safely connected to the non-metallic sample without a short circuit (Extended Data Fig. 9).
In the left panel, the pattern taken at the center of heating is still dominated by the Py-phase with lattice parameter a = 4.379 Å and corresponding x = 0.86(8) (ref. 16). The minor peaks in the pattern may associate with dhcp-FeHx and/or Cmcm-type Fe2O3. The reference lattices parameters are dhcp-FeHx (a = 2.458 Å, c = 8.046 Å); Cmcm-type Fe2O3 (a = 2.559 Å, a = 8.257 Å, c = 6.164 Å). Pressure is measured by the equation of state of Pt. The wavelength of the x-ray is 0.3344 Å. The right panel is an XRD mapping contour around the heating area. We first divided the heated portion of sample into (21 × 21 μm2) a 7 × 7 matrix. We collect one XRD pattern on each grid point with the same 10-seconds’ exposure time. Colors on the color bar are proportional to the intensity of Py-phase 111 peak. The contour is then constructed by interpolating those intensities.
The calculated proton conductivities of superionic ice under the P–T conditions shown in the previous figure are compared with previous experimental (Yakushenv J. Exp. Theor., 2000 and Chau J. Chem. Phys., 2001) and simulations (French, Phys. Rev. Lett. 2011).
The sample was sandwiched by two LiF layers acting as the pressure medium and thermal isolation. Cubic-BN was used as the electrical isolator. In left panel, the shaded area corresponds to the actual sample, whose geometry was later used to determine electric conductivity. The mapping image of the x-ray absorption showed that the samples were individually connected by four Pt electrodes. The inset is the corresponding microscopic image of the sample assembly. The right panel shows the x-ray absorption along the radial side of diamond anvil cell (perpendicular to the compression axis) to access the thickness of the sample. We took this figure in a separate experiment with LiF as the pressure medium and cubic-BN isolation. Unlike the EC experiment, we used a Be gasket such that it is transparent to x-ray. The image was taken at 110 GPa after laser heating at 2000 K for 45 minutes. The directly measured thickness of 4.8 μm is consistent with the one we calibrated in EC run (4.5 μm, 113 GPa). Here, we estimate 7% of uncertainty derived from the thickness difference in these two approaches.
The negative T-R slope indicates that the sample is an insulator. Data points were collected every 1 second. The process of warming up from 80 K to room temperature took around 60 minutes. Systematic error for resistance is less than 10−3 Ω. We estimated 5% error in temperature, due to the temperature gradient from the thermal couple to the sample. We conducted cryogenic experiments and measured the electric conductivity of FeO2Hx synthesized at 113 GPa. The purpose of cryogenic experiments is two-folded. First, we attempted to clarify whether FeO2Hx is an insulator or a conductor at room temperature. Secondly, we check the connection of 4-electrode system. It is also known that compression shrinks the available space in the sample chamber. In the worst-case scenario, it might squeeze the electrodes and shorts the circuit. This would lead to a fake metallized loop. In our cryogenic experiments, the insulating nature of the sample eliminates the possibility of electrode contact contamination. These observations secured our measurements taken for the sample instead of touching the platinum electrodes. The appearance of insulating FeO2Hx would guarantee that the van der Pauw four probe system was working. Compared with laser-heated measurements, data points collected in cryogenic experiments were controlled by a cryogenic cooling system. Two thermal couples were attached to the exterior of the diamond anvil cell and the cell was immersed in a liquid nitrogen box. The temperature was completely equilibrated around 80 K, before we removed the liquid nitrogen. The cell slowly warmed up while we measured the resistance (R) from the sample at every second. According to lattice thermal expansion of the sample, sample pressure in the cryogenic cycle is 113 ± 5 GPa.
Excel file layout of P–T conditions of solid or superionic FeO2Hx.
Raman spectra in Fig. 2a.
Excel file layout of electrical conductivities from experiment and simulation.
Excel file layout of calculated volumes of FeO2Hx with x = 0, 0.25, 0.5, 0.75 and 1.
Excel file layout of the evolution of O–H bond length and vibrational density of states in the figure.
Excel file layout of mean-squared displacement and P–T relation from FPMD simulation.
Excel file layout of diffusion coefficients at various pressures and stoichiometries.
X-y column data for XRD spectra at 113 GPa at initial synthesis.
X-y column data for XRD spectra at 121 GPa after EC run.
Excel file layout of calculated EC of superionic ice at 140 GPa.
Temperature versus resistance in a cryogenic tank.
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Hou, M., He, Y., Jang, B.G. et al. Superionic iron oxide–hydroxide in Earth’s deep mantle. Nat. Geosci. 14, 174–178 (2021). https://doi.org/10.1038/s41561-021-00696-2
Nature Geoscience (2021)