Liuxiang Yang

Dehydrogenation of goethite in Earth’s deep lower mantle

Significance We found at high pressure–temperature (P-T) that the goethite FeO2H transforms to P-phase FeO2 via a two-step dehydrogenation process. First it releases some hydrogen to form P-phase FeO2Hx, and then it continuously releases the remaining hydrogen through prolonged heating. This work provides an important example that the dehydration reaction changes to dehydrogenation of FeO2H at the lower mantle conditions and the cycles of hydrogen and water become separated. The cycling of hydrogen influences the structure, composition, and stratification of Earth’s interior. Our recent discovery of pyrite-structured iron peroxide (designated as the P phase) and the formation of the P phase from dehydrogenation of goethite FeO2H implies the separation of the oxygen and hydrogen cycles in the deep lower mantle beneath 1,800 km. Here we further characterize the residual hydrogen, x, in the P-phase FeO2Hx. Using a combination of theoretical simulations and high-pressure–temperature experiments, we calibrated the x dependence of molar volume of the P phase. Within the current range of experimental conditions, we observed a compositional range of P phase of 0.39 < x < 0.81, corresponding to 19–61% dehydrogenation. Increasing temperature and heating time will help release hydrogen and lower x, suggesting that dehydrogenation could be approaching completion at the high-temperature conditions of the lower mantle over extended geological time. Our observations indicate a fundamental change in the mode of hydrogen release from dehydration in the upper mantle to dehydrogenation in the deep lower mantle, thus differentiating the deep hydrogen and hydrous cycles.

Synthesis of quenchable amorphous diamond

Diamond owes its unique mechanical, thermal, optical, electrical, chemical, and biocompatible materials properties to its complete sp3-carbon network bonding. Crystallinity is another major controlling factor for materials properties. Although other Group-14 elements silicon and germanium have complementary crystalline and amorphous forms consisting of purely sp3 bonds, purely sp3-bonded tetrahedral amorphous carbon has not yet been obtained. In this letter, we combine high pressure and in situ laser heating techniques to convert glassy carbon into “quenchable amorphous diamond”, and recover it to ambient conditions. Our X-ray diffraction, high-resolution transmission electron microscopy and electron energy-loss spectroscopy experiments on the recovered sample and computer simulations confirm its tetrahedral amorphous structure and complete sp3 bonding. This transparent quenchable amorphous diamond has, to our knowledge, the highest density among amorphous carbon materials, and shows incompressibility comparable to crystalline diamond.Diamond’s properties are dictated by its crystalline, fully tetrahedrally bonded structure. Here authors synthesize a bulk sp3-bonded amorphous form of carbon under high pressure and temperature, show that it has bulk modulus comparable to crystalline diamond and that it can be recovered under ambient conditions.

Oxygen-Rich Lithium Oxide Phases Formed at High Pressure for Potential Lithium–Air Battery Electrode

The lithium–air battery has great potential of achieving specific energy density comparable to that of gasoline. Several lithium oxide phases involved in the charge–discharge process greatly affect the overall performance of lithium–air batteries. One of the key issues is linked to the environmental oxygen‐rich conditions during battery cycling. Here, the theoretical prediction and experimental confirmation of new stable oxygen‐rich lithium oxides under high pressure conditions are reported. Three new high pressure oxide phases that form at high temperature and pressure are identified: Li2O3, LiO2, and LiO4. The LiO2 and LiO4 consist of a lithium layer sandwiched by an oxygen ring structure inherited from high pressure ε‐O8 phase, while Li2O3 inherits the local arrangements from ambient LiO2 and Li2O2 phases. These novel lithium oxides beyond the ambient Li2O, Li2O2, and LiO2 phases show great potential in improving battery design and performance in large battery applications under extreme conditions.

Pressure-induced superconductivity in MoP

Topological semimetal, a novel state of quantum matter hosting exotic emergent quantum phenomena dictated by the non-trivial band topology, has emerged as a new frontier in condensed-matter physics. Very recently, a coexistence of triply degenerate points of band crossing and Weyl points near the Fermi level was theoretically predicted and immediately experimentally verified in single crystalline molybdenum phosphide (MoP). Here we show in this material the high-pressure electronic transport and synchrotron X-ray diffraction (XRD) measurements, combined with density functional theory (DFT) calculations. We report the emergence of pressure-induced superconductivity in MoP with a critical temperature Tc of about 2 K at 27.6 GPa, rising to 3.7 K at the highest pressure of 95.0 GPa studied. No structural phase transitions is detected up to 60.6 GPa from the XRD. Meanwhile, the Weyl points and triply degenerate points topologically protected by the crystal symmetry are retained at high pressure as revealed by our DFT calculations. The coexistence of three-component fermion and superconductivity in heavily pressurized MoP offers an excellent platform to study the interplay between topological phase of matter and superconductivity.

TEM Study of Amorphous Carbon with Fully sp3-Bonded Structure

In 2011, Lin etal found that glassy carbon was converted into a new carbon allotrope with a fully sp3-bonded amorphous structure under high pressure of about 45 GPa [1]. However, the transition was reversible upon releasing pressure. Recently, Zeng etal synthesized quenchable amorphous diamond from glassy carbon with the combination of high pressure and in situ laser heating [2]. To understand the atomic structure and chemical bonding, we studied the recovered carbon materials using aberration corrected TEM and electron energy-loss spectroscopy (EELS). We confirmed that the recovered material is amorphous diamond with completely tetrahedral sp3-bonds.