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 Earth's 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.
Magnesium-graphene interphase boundaries created by high-pressure torsion enhance hydrogen storage kinetics:Mechanisms and significance of activation energy and frequency factor
WPI, International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University（九州大学碳中和能源研究国际研究所（WPI-I2CNER））；Department of Automotive Science, Kyushu University（九州大学汽车科学系）；Faculdade de Engenharia Mecânica (FEM), Universidade Estadual de Campinas (UNICAMP)（坎皮纳斯州立大学机械工程学院）；Université de Lorraine, Laboratoire d’Etude des Microstructures et de Mécanique des Matériaux (LEM3 UMR 7239)（洛林大学微观结构与材料力学实验室（LEM3））；Université de Lorraine, Laboratory of Excellence on Design of Alloy Metals for low-mass Structures (DAMAS)（洛林大学低质量结构合金设计卓越实验室（DAMAS））
Pressure-stabilized binary compounds of magnesium and silicon
由镁和硅组成的二元化合物家族相当丰富。除了公知的硅化镁Mg $_2$ Si之外，在沉淀的Al-Mg-Si固溶体中还鉴定和/或提出了其它化合物，包括MgSi $_2$、Mg $_4$ Si $_7$、Mg $_5$ Si $_6$、MgSi和Mg $_9$ Si $_5$。然而，计算研究表明，只有Mg $_2$ Si在环境条件下是热稳定的，而某些非零的静水压力可以稳定Mg $_9$ Si $_5$，使得它可以与Mg $_2$ Si共存。我们进行了一个全面的搜索可行的二元化合物镁$_x$硅$_{1-x}$（$1/3\leq x \leq 2/3$），发现了许多新的结构的所有化合物。一方面，我们发现MgSi $_2$、MgSi和Mg $_9$ Si $_5$可能是压力稳定材料，而另一方面，支持先前的研究，对Mg $_5$ Si $_6$的存在提出质疑，并声称Mg $_4$ Si $_7$的存在仍然是一个悬而未决的问题。因此，我们建议在通过计算讨论这些固体（可能还有其他固体）的稳定性时，应明确考虑（流体静力学和/或非流体静力学）压力。我们还发现，MgSi $_2$在很宽的压力范围内都具有潜在的超导行为，其临界温度可达$7$ K。
Being a lithophile element at ambient pressure, magnesium is long believed to be immiscible with iron. A recent study by Gao et al. [1] showed that pressure turns magnesium into a siderophile element and can produce unconventional Fe-Mg compounds. Here, we extend the investigation to exoplanetary pressure conditions using an adaptive genetic algorithm-based variable-composition structural prediction approach. We identify several Fe-Mg phases up to 3 TPa. Our cluster alignment analysis reveals that most of the predicted Fe-Mg compounds prefer a BCC packing motif at terapascal pressures. This st…
Based on ab initio molecular dynamics simulations, we determined the melting curve of magnesium (Mg) up to ~460 GPa using the solid-liquid coexistence method. Between ~30 and 100 GPa, our melting curve is noticeably lower than those from static experiments, but is in good agreement with recent shock experiments. Up to ~450 GPa, our melting curve is generally consistent with the melting points from first-principles calculations using the small-cell coexistence method. We found that, at high pressures of a few hundred GPa, due to the strong softening of interatomic interactions in the liquid pha…
The origin of water on the Earth is a long-standing mystery, requiring a comprehensive search for hydrous compounds, stable at conditions of the deep Earth and made of Earth-abundant elements. Previous studies usually focused on the current range of pressure-temperature conditions in the Earth's mantle and ignored a possible difference in the past, such as the stage of the core-mantle separation. Here, using ab initio evolutionary structure prediction, we find that only two magnesium hydrosilicate phases are stable at megabar pressures, $α$-Mg$_2$SiO$_5$H$_2$ and $β$-Mg$_2$SiO$_5$H$_2$, stable…
Nanocrystalline structure and strain in magnesium under extreme dynamic compression
M.N. Miheev Institute of Metal Physics, Ural Branch, Russian Academy of Sciences（俄罗斯科学院乌拉尔分校 M.N. 米赫耶夫金属物理研究所）；NANOTECH Centre, Ural Federal University（乌拉尔联邦大学 NANOTECH 中心）
Magnesium and its alloys are the most investigated materials for solid-state hydrogen storage in the form of metal hydrides, but there are still unresolved problems with the kinetics and thermodynamics of hydrogenation and dehydrogenation of this group of materials. Severe plastic deformation (SPD) methods, such as equal-channel angular pressing (ECAP), high-pressure torsion (HPT), intensive rolling and fast forging, have been widely used to enhance the activation, air resistance, and hydrogenation/dehydrogenation kinetics of Mg-based hydrogen storage materials by introducing ultrafine/nanosca…
Higher borides and oxygen-enriched Mg-B-O inclusions as possible pinning centers in nanostructural magnesium diboride and the influence of additives on their formation
High-pressure can transform the structures and compositions of materials either by changing the relative strengths of bonds or by altering the oxidation states of atoms. Both effects cause unconventional compositions in novel compounds that have been synthesized or predicted in large numbers in the past decade. What naturally follows is a question: what if pressure imposes strong effects on both chemical bonds and atomic orbitals in the same material. A systematic DFT and crystal structure search study of Cs polyoxides under high pressure shows a striking transition of chemistry due to the act…
Mg2GeO4 is an analogue for the ultra-high pressure behavior of Mg2SiO4, so we have investigated magnesium germanate to 275 GPa and over 2000 K using a laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction and density functional theory (DFT) computations. The experimental results are consistent with a novel phase with disordered Mg and Ge, in which germanium adopts eight-fold coordination with oxygen: the cubic Th3P4- type structure. Simulations using the special quasirandom structure (SQS) method suggest partial order in the tetragonal I-42d structure, indistingui…
