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1. Stability of H 3 O at extreme conditions and implications for the magnetic fields of Uranus and Neptune

   https://doi.org/10.1073/pnas.1921811117  

   Huang, Peihao ; Liu, Hanyu ; Lv, Jian ; Li, Quan ; Long, Chunhong ; Wang, Yanchao ; Chen, Changfeng ; Hemley, Russell J. ; Ma, Yanming ( March 2020 , Proceedings of the National Academy of Sciences)

   The anomalous nondipolar and nonaxisymmetric magnetic fields of Uranus and Neptune have long challenged conventional views of planetary dynamos. A thin-shell dynamo conjecture captures the observed phenomena but leaves unexplained the fundamental material basis and underlying mechanism. Here we report extensive quantum-mechanical calculations of polymorphism in the hydrogen–oxygen system at the pressures and temperatures of the deep interiors of these ice giant planets (to >600 GPa and 7,000 K). The results reveal the surprising stability of solid and fluid trihydrogen oxide (H 3 O) at these extreme conditions. Fluid H 3 O is metallic and calculated to be stable near the cores of Uranus and Neptune. As a convecting fluid, the material could give rise to the magnetic field consistent with the thin-shell dynamo model proposed for these planets. H 3 O could also be a major component in both solid and superionic forms in other (e.g., nonconvecting) layers. The results thus provide a materials basis for understanding the enigmatic magnetic-field anomalies and other aspects of the interiors of Uranus and Neptune. These findings have direct implications for the internal structure, composition, and dynamos of related exoplanets. 

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2. Coexistence of vitreous and crystalline phases of H 2 O at ambient temperature

   https://doi.org/10.1073/pnas.2117281119  

   Shargh, Ali K. ; Picard, Aude ; Hrubiak, Rostislav ; Zhang, Dongzhou ; Hemley, Russell J. ; Deemyad, Shanti ; Abdolrahim, Niaz ; Saffarian, Saveez ( July 2022 , Proceedings of the National Academy of Sciences)

   Formation of vitreous ice during rapid compression of water at room temperature is important for biology and the study of biological systems. Here, we show that Raman spectra of rapidly compressed water at greater than 1 GPa at room temperature exhibits the signature of high-density amorphous ice, whereas the X-ray diffraction (XRD) pattern is dominated by crystalline ice VI. To resolve this apparent contradiction, we used molecular dynamics simulations to calculate full vibrational spectra and diffraction patterns of mixtures of vitreous ice and ice VI, including embedded interfaces between the two phases. We show quantitatively that Raman spectra, which probe the local polarizability with respect to atomic displacements, are dominated by the vitreous phase, whereas a small amount of the crystalline component is readily apparent by XRD. The results of our combined experimental and theoretical studies have implications for detecting vitreous phases of water, survival of biological systems under extreme conditions, and biological imaging. The results provide additional insight into the stable and metastable phases of H 2 O as a function of pressure and temperature, as well as of other materials undergoing pressure-induced amorphization and other metastable transitions. 

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3. Equation of State of TiN at High Pressures and Temperatures: A Possible Host for Nitrogen in Planetary Mantles

   https://doi.org/10.1029/2020JB020074  

   Daviau, Kierstin ; Fischer, Rebecca A. ; Brennan, Matthew C. ; Dong, Junjie ; Suer, Terry‐Ann ; Couper, Samantha ; Meng, Yue ; Prakapenka, Vitali B. ( January 2021 , Journal of Geophysical Research: Solid Earth)

   Nitrogen, the most abundant element in Earth's atmosphere, is also a primary component of solid nitride minerals found in meteorites and on Earth's surface. If they remain stable to high pressures and temperatures, these nitrides may also be important reservoirs of nitrogen in planetary interiors. We used synchrotron X‐ray diffraction to measure the thermal equation of state and phase stability of titanium nitride (TiN) in a laser‐heated diamond anvil cell at pressures up to ∼70 GPa and temperatures up to ∼2,500 K. TiN maintains the cubic B1 (NaCl‐type) crystal structure over the entire pressure and temperature range explored. It hasK₀ = 274 (4) GPa,K₀′ = 3.9 (2), andγ₀ = 1.39 (4) for a fixedV₀ = 76.516 (30) ų(based on experimental measurements),q = 1, andθ₀ = 579 K. Additionally, we collected Raman spectra of TiN up to ∼60 GPa, where we find that the transverse acoustic (TA), longitudinal acoustic (LA), and transverse optical phonon modes exhibit mode Grüneisen parameters of 1.66(17), 0.54(15), and 0.93 (4), respectively. Based on our equation of state, TiN has a density of ∼5.6–6.4 g/cm³at Earth's lower mantle conditions, significantly more dense than both the mantle of the Earth and the estimated densities of the mantles of other terrestrial planets, but less dense than planetary cores. We find that TiN remains stable against physical decomposition at the pressures and temperatures found within Earth's mantle, making it a plausible reservoir for deep planetary nitrogen if chemical conditions allow its formation.

    

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4. Large H 2 O solubility in dense silica and its implications for the interiors of water-rich planets

   https://doi.org/10.1073/pnas.1917448117  

   Nisr, Carole ; Chen, Huawei ; Leinenweber, Kurt ; Chizmeshya, Andrew ; Prakapenka, Vitali B. ; Prescher, Clemens ; Tkachev, Sergey N. ; Meng, Yue ; Liu, Zhenxian ; Shim, Sang-Heon ( April 2020 , Proceedings of the National Academy of Sciences)

   Sub-Neptunes are common among the discovered exoplanets. However, lack of knowledge on the state of matter in${\mathrm{H}}_2$O-rich setting at high pressures and temperatures ($P-T$) places important limitations on our understanding of this planet type. We have conducted experiments for reactions between${\mathrm{S}\mathrm{i}\mathrm{O}}_2$and${\mathrm{H}}_2$O as archetypal materials for rock and ice, respectively, at high$P-T$. We found anomalously expanded volumes of dense silica (up to 4%) recovered from hydrothermal synthesis above ∼24 GPa where the${\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{l}}_2$-type (Ct) structure appears at lower pressures than in the anhydrous system. Infrared spectroscopy identified strong OH modes from the dense silica samples. Both previous experiments and our density functional theory calculations support up to 0.48 hydrogen atoms per formula unit of (${\mathrm{S}\mathrm{i}}_{1-x}{\mathrm{H}}_{4x}$)${\mathrm{O}}_2\hspace{0.17em}\left(x=0.12\right)$. At pressures above 60 GPa,${\mathrm{H}}_2$O further changes the structural behavior of silica, stabilizing a niccolite-type structure, which is unquenchable. From unit-cell volume and phase equilibrium considerations, we infer that the niccolite-type phase may contain H with an amount at least comparable with or higher than that of the Ct phase. Our results suggest that the phases containing both hydrogen and lithophile elements could be the dominant materials in the interiors of water-rich planets. Even for fully layered cases, the large mutual solubility could make the boundary between rock and ice layers fuzzy. Therefore, the physical properties of the new phases that we report here would be important for understanding dynamics, geochemical cycle, and dynamo generation in water-rich planets.

    

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5. Reaction between Hydrogen and Ferrous/Ferric Oxides at High Pressures and High Temperatures—Implications for Sub-Neptunes and Super-Earths

   https://doi.org/10.3847/PSJ/acab03  

   Horn, H. W. ; Prakapenka, V. ; Chariton, S. ; Speziale, S. ; Shim, S. -H. ( February 2023 , The Planetary Science Journal)

   Sub-Neptune exoplanets may have thick hydrogen envelopes and therefore develop a high-pressure interface between hydrogen and the underlying silicates/metals. Some sub-Neptunes may convert to super-Earths via massive gas loss. If hydrogen chemically reacts with oxides and metals at high pressures and temperatures (P−T), it could impact the structure and composition of the cores and atmospheres of sub-Neptunes and super-Earths. While H₂gas is a strong reducing agent at low pressures, the behavior of hydrogen is unknown at theP−Texpected for sub-Neptunes’ interiors, where hydrogen is a dense supercritical fluid. Here we report experimental results of reactions between ferrous/ferric oxides and hydrogen at 20–40 GPa and 1000–4000 K utilizing the pulsed laser-heated diamond-anvil cell combined with synchrotron X-ray diffraction. Under these conditions, hydrogen spontaneously strips iron off the oxides, forming Fe-H alloys and releasing oxygen to the hydrogen medium. In a planetary context where this reaction may occur, the Fe-H alloy may sink to the metallic part of the core, while released oxygen may stabilize as water in the silicate layer, providing a mechanism to ingas hydrogen to the deep interiors of sub-Neptunes. Water produced from the redox reaction can also partition to the atmosphere of sub-Neptunes, which has important implications for understanding the composition of their atmospheres. In addition, super-Earths converted from sub-Neptunes may contain a large amount of hydrogen and water in their interiors (at least a few wt% H₂O). This is distinct from smaller rocky planets, which were formed relatively dry (likely a few hundredths wt% H₂O).

    

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