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Register a new userOn stable H-C-N-O compounds at high pressure
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The make-up of the outer planets, and many of their moons, are dominated by matter from the H-C-N-O chemical space, commonly assumed to originate from mixtures of hydrogen and the planetary ices H$_2$O, CH$_4$, and NH$_3$. In their interiors, these ices experience extreme pressure conditions, around 5 Mbar at the Neptune mantle-core boundary, and it is expected that they undergo phase transitions, decompose, and form entirely new compounds. In turn, this determines planets interior structure, thermal history, magnetic field generation, etc. Despite its importance, the H-C-N-O space has not been surveyed systematically. Asked simply: at high-pressure conditions, what compounds emerge within this space, and what governs their stability? Here, we report on results from an unbiased crystal structure search amongst H-C-N-O compounds at 5 Mbar to answer this question.
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Silica, water and hydrogen are known to be the major components of celestial bodies, and have significant influence on the formation and evolution of giant planets, such as Uranus and Neptune. Thus, it is of fundamental importance to investigate their states and possible reactions under the planetary conditions. Here, using advanced crystal structure searches and first-principles calculations in the Si-O-H system, we find that a silica-water compound (SiO2)2(H2O) and a silica-hydrogen compound SiO2H2 can exist under high pressures above 450 and 650 GPa, respectively. Further simulations reveal that, at high pressure and high temperature conditions corresponding to the interiors of Uranus and Neptune, these compounds exhibit superionic behavior, in which protons diffuse freely like liquid while the silicon and oxygen framework is fixed as solid. Therefore, these superionic silica-water and silica-hydrogen compounds could be regarded as important components of the deep mantle or core of giants, which also provides an alternative origin for their anomalous magnetic fields. These unexpected physical and chemical properties of the most common natural materials at high pressure offer key clues to understand some abstruse issues including demixing and erosion of the core in giant planets, and shed light on building reliable models for solar giants and exoplanets.
Rocky planets are thought to comprise compounds of Mg and O as these are among the most abundant elements, but knowledge of their stable phases may be incomplete. MgO is known to be remarkably stable to very high pressure and chemically inert under reduced condition of the Earths lower mantle. However, in icy gas giants as well as in exoplanets oxygen may be a more abundant constituent (Ref. 1,2). Here, using synchrotron x-ray diffraction in laser-heated diamond anvil cells, we show that MgO and oxygen react at pressures above 94 GPa and T = 2150 K with the formation of the theoretically predicted I4/mcm MgO2 (Ref.3). Raman spectroscopy detects the presence of a peroxide ion (O22-) in the synthesized material as well as in the recovered specimen. Likewise, energy-dispersive x-ray spectroscopy confirms that the recovered sample has higher oxygen content than pure MgO. Our finding suggests that MgO2 may substitute MgO in rocky mantles and rocky planetary cores under highly oxidizing conditions.
The synthesis of materials in high-pressure experiments has recently attracted increasing attention, especially since the discovery of record breaking superconducting temperatures in the sulfur-hydrogen and other hydrogen-rich systems. Commonly, the initial precursor in a high pressure experiment contains constituent elements that are known to form compounds at ambient conditions, however the discovery of high-pressure phases in systems immiscible under ambient conditions poses an additional materials design challenge. We performed an extensive multi component $ab,initio$ structural search in the immiscible Fe--Bi system at high pressure and report on the surprising discovery of two stable compounds at pressures above $approx36$ GPa, FeBi$_2$ and FeBi$_3$. According to our predictions, FeBi$_2$ is a metal at the border of magnetism with a conventional electron-phonon mediated superconducting transition temperature of $T_{rm c}=1.3$ K at 40 GPa. In analogy to other iron-based materials, FeBi$_2$ is possibly a non-conventional superconductor with a real $T_{rm c}$ significantly exceeding the values obtained within Bardeen-Cooper-Schrieffer (BCS) theory.
Here we report the synthesis of metallic, ultraincompressible (bulk modulus $K_{0}$ = 428(10) GPa) and very hard (nanoindentation hardness 36.7(8) GPa) rhenium (V) nitride pernitride Re$_{2}$(N$_{2}$)N$_{2}$. While the empirical chemical formula of the compound, ReN$_{2}$, is the same as for other known transition metals pernitrides, e.g. IrN$_{2}$, PtN$_{2}$, PdN$_{2}$ and OsN$_{2}$, its crystal chemistry is unique. The known pernitrides of transition metals consist of a metal in the oxidation state +IV and pernitride anions N$_{2}^{4-}$. ReN$_{2}$ contains both pernitride N$_{2}^{4-}$ and discrete N$^{3-}$ anions, which explains its exceptional properties. Moreover, in the original experimental synthesis of Re$_{2}$(N$_{2}$)N$_{2}$ performed in a laser-heated diamond anvil cell via a direct reaction between rhenium and nitrogen at pressures from 40 to 90 GPa we observed that the material was recoverable at ambient conditions. Consequently, we developed a route to scale up its synthesis through a reaction between rhenium and ammonium azide, NH$_{4}$N$_{3}$, in a large-volume press at 33 GPa. Our work resulted not only in a discovery of a novel material with unusual crystal chemistry and a set of properties attractive for potential applications, but also demonstrated a feasibility of surmounting conceptions common in material sciences.
We have carried out first principles structural relaxation calculations on the hydrous magnesium silicate Phase A (Mg7Si2O8(OH)6) under high pressures. Our results show that phase A does not undergo any phase transition upto ~ 45 GPa. We find that non-bonded H--H distance reaches a limiting value of 1.85 angstrom at about 45 GPa. The H--H repulsive strain releasing mechanism in Phase A is found to be dramatically different from the hydrogen bond bending one that was proposed by Hofmeister et al1 for Phase B. It is based on the reduction of one of the O-H bond distances with compression.
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