Novel Valence Transition in Elemental Metal Europium around 80 GPa.

Valence transition could induce structural, insulator-metal, nonmagnetic-magnetic and superconducting transitions in rare-earth metals and compounds, while the underlying physics remains unclear due to the complex interaction of localized 4f electrons as well as their coupling with itinerant electrons. The valence transition in the elemental metal europium (Eu) still has remained as a matter of debate. Using resonant x-ray emission scattering and x-ray diffraction, we pressurize the states of 4f electrons in Eu and study its valence and structure transitions up to 160 GPa. We provide compelling evidence for a valence transition around 80 GPa, which coincides with a structural transition from a monoclinic (C2/c) to an orthorhombic phase (Pnma). We show that the valence transition occurs when the pressure-dependent energy gap between 4f and 5d electrons approaches the Coulomb interaction. Our discovery is critical for understanding the electrodynamics of Eu, including magnetism and high-pressure superconductivity.

Rare Helium-Bearing Compound FeO_{2}He Stabilized at Deep-Earth Conditions.

There is compelling geochemical evidence for primordial helium trapped in Earth's lower mantle, but the origin and nature of the helium source remain elusive due to scarce knowledge on viable helium-bearing compounds that are extremely rare. Here we explore materials physics underlying this prominent challenge. Our structure searches in conjunction with first-principles energetic and thermodynamic calculations uncover a remarkable helium-bearing compound FeO_{2}He at high pressure-temperature conditions relevant to the core-mantle boundary. Calculated sound velocities consistent with seismic data validate FeO_{2}He as a feasible constituent in ultralow velocity zones at the lowermost mantle. These mutually corroborating findings establish the first and hitherto only helium-bearing compound viable at pertinent geophysical conditions, thus providing vital physics mechanisms and materials insights for elucidating the enigmatic helium reservoir in deep Earth.

Phase transition and superconductivity in ReS2, ReSe2 and ReTe2.

Transition metal dichalcogenides have attracted significant attention due to both fundamental interest and their potential applications. Here, we have systematically explored the crystal structures of ReX2 (X = S, Se, and Te) over the pressure range of 0-300 GPa, employing swarm-intelligence-based structure prediction methodology. Several new structures are found to be stable at high pressures. The calculated enthalpy of formation suggested that all predicted high-pressure structures are stable against decomposition into elemental end-members. Moreover, we found that the simulated X-ray diffraction patterns of ReSe2 are in good agreement with experimental data. Pressure-induced metallization of ReX2 has been revealed from the analysis of its electronic structure. Our electron-phonon coupling calculations indicate ReSe2 and ReTe2 are superconducting phases at high pressures.

Tellurium Hydrides at High Pressures: High-Temperature Superconductors.

Observation of high-temperature superconductivity in compressed sulfur hydrides has generated an irresistible wave of searches for new hydrogen-containing superconductors. We herein report the prediction of high-T_{c} superconductivity in tellurium hydrides stabilized at megabar pressures identified by first-principles calculations in combination with a swarm structure search. Although tellurium is isoelectronic to sulfur or selenium, its heavier atomic mass and weaker electronegativity makes tellurium hydrides fundamentally distinct from sulfur or selenium hydrides in stoichiometries, structures, and chemical bondings. We identify three metallic stoichiometries of H_{4}Te, H_{5}Te_{2}, and HTe_{3}, which are not predicted or known stable structures for sulfur or selenium hydrides. The two hydrogen-rich H_{4}Te and H_{5}Te_{2} phases are primarily ionic and contain exotic quasimolecular H_{2} and linear H_{3} units, respectively. Their high-T_{c} (e.g., 104 K for H_{4}Te at 170 GPa) superconductivity originates from the strong electron-phonon couplings associated with intermediate-frequency H-derived wagging and bending modes, a superconducting mechanism which differs substantially with those in sulfur or selenium hydrides where the high-frequency H-stretching vibrations make considerable contributions.

Catenation of carbon in LaC2 predicted under high pressure.

Carbon has the capability of forming various bonding states that affect the structures and properties of transition metal carbides. In this work, structural search was performed to explore the structural diversity of LaC2 at pressures of 0.0-30.0 GPa. Five stable structures of LaC2 reveal a variety of carbon structural units ranging from a dimer to bent C3, zigzag C4 and armchair polymer chains. A series of pressure-induced structural transformations are predicted, I4/mmm (i.e. experimental α phase) →C2/c→Pnma→Pmma, which involve the catenation of carbon from a dimer to zigzag C4 units and further to armchair polymer chains. The bent C3 unit appears in a novel Immm structure. This structure is the theoretical ground state of LaC2 under ambient conditions, but is kinetically inaccessible from the experimental α phase. LaC2 becomes thermodynamically metastable relative to La2C3 + diamond above 17.1 GPa, and eventually decomposes into constituent elements above 35.6 GPa. The presented results indicate that catenation of carbon can be realized even in simple inorganic compounds under nonambient conditions.

Direct H-He chemical association in superionic FeO2H2He at deep-Earth conditions.

Hydrogen and helium are known to play crucial roles in geological and astrophysical environments; however, they are inert toward each other across wide pressure-temperature (P-T) conditions. Given their prominent presence and influence on the formation and evolution of celestial bodies, it is of fundamental interest to explore the nature of interactions between hydrogen and helium. Using an advanced crystal structure search method, we have identified a quaternary compound FeO2H2He stabilized in a wide range of P-T conditions. Ab initio molecular dynamics simulations further reveal a novel superionic state of FeO2H2He hosting liquid-like diffusive hydrogen in the FeO2He sublattice, creating a conducive environment for H-He chemical association, at P-T conditions corresponding to the Earth's lowest mantle regions. To our surprise, this chemically facilitated coalescence of otherwise immiscible molecular species highlights a promising avenue for exploring this long-sought but hitherto unattainable state of matter. This finding raises strong prospects for exotic H-He mixtures inside Earth and possibly also in other astronomical bodies.

Stable Structures and Superconductivity in a Y-Si System under High Pressure.

Recently, the discovery of superconductivity in compressed electrides offers a promising route toward searching for high superconductivity in a high-pressure community. However, only a few superconducting electrides have been successfully found thus far, thereby limiting the variety of superconducting electride examples. In this work, we performed extensive structure searches on a high-pressure Y-Si system by using CALYPSO structure prediction methodology. Our simulations identified several stable stoichiometries of YSi, YSi2, YSi3, Y5Si3, Y2Si, and Y3Si under high pressure. These structures contain a diversity of structure configurations, including silicon chains, Si3 trilaterals, Si4 quadrilaterals, Si6 hexagons, Si8 rings, a Si4-Si6-Si8 frame, as well as a silicon layer. Remarkably, Y3Si is predicted to be an electride with a superconducting critical temperature (Tc) of ∼11.2 and 14.5 K at 30 and 50 GPa, respectively. These results highlight the role of the electrons at the Fermi surface in determining the superconductivity of predicted structures.

Association between comorbid asthma and prognosis of critically ill patients with severe sepsis: a cohort study.

Basic research suggests some contributing mechanisms underlying asthma might at the same time benefit patients with asthma against sepsis, while the potential protective effect of comorbid asthma on prognosis of sepsis has not been well studied in clinical research. The study aimed to assess the association between comorbid asthma and prognosis in a cohort of patients admitted to intensive care unit (ICU) with severe sepsis. Patients with severe sepsis admitted to ICUs were included from the MIMIC-III Critical Care Database, and categorized as patients without asthma, patients with stable asthma, and patients with acute exacerbation asthma. The primary study outcome was 28-day mortality since ICU admission. Difference in survival distributions among groups were evaluated by Kaplan-Meier estimator. Multivariable Cox regression was employed to examine the association between comorbid asthma and prognosis. A total of 2469 patients with severe sepsis were included, of which 2327 (94.25%) were without asthma, 125 (5.06%) with stable asthma, and 17 (0.69%) with acute exacerbation asthma. Compared with patients without asthma, patients with asthma (either stable or not) had a slightly younger age (66.73 ± 16.32 versus 64.77 ± 14.81 years), a lower proportion of male sex (56.81% versus 40.14%), and a lower median SAPS II score (46 versus 43). Patients with acute exacerbation asthma saw the highest 28-day mortality rate (35.29%), but patients with stable asthma had the lowest 28-day mortality rate (21.60%) when compared to that (34.42%) in patients without asthma. Consistent results were observed in Kaplan-Meier curves with a p-value for log-rank test of 0.016. After adjusting for potential confounding, compared to being without asthma, being with stable asthma was associated with a reduced risk of 28-day mortality (hazard ratio (HR) 0.65, 95% confidence interval (CI) 0.44-0.97, p = 0.0335), but being with acute exacerbation asthma was toward an increased risk of 28-day mortality (HR 1.82, 95% 0.80-4.10, p = 0.1513). E-value analysis suggested robustness to unmeasured confounding. These findings suggest comorbid stable asthma is associated with a better prognosis in critically ill patients with severe sepsis, while acute exacerbation asthma is associated with worse prognosis.

Computational prediction of a +4 oxidation state in Au via compressed AuO2 compound.

Much effort has been devoted to the investigation of the physical and chemical properties of the Au-O system over a range of pressures, owing to the considerable importance of these materials in fundamental and practical applications. To date, however, only Au1+, Au2+, Au3+, and Au5+ oxidation states have been identified in the Au-O system, but tetravalent Au4+ has not been found. Here, we report the results of structure prediction for the Au-O system at high pressure via the effective structure prediction methodology within a first-principles electronic structure framework. We have uncovered an intriguing structure with AuO2 composition and tetravalent Au, stable at high pressures. This phase shows an electronic transition from a metal to a semiconducting phase as a function of pressure. The present results provide fundamental understanding of the structural and physicochemical properties of compressed Au-O compounds.

Phase Diagram and High-Temperature Superconductivity of Compressed Selenium Hydrides.

Recent discovery of high-temperature superconductivity (Tc = 190 K) in sulfur hydrides at megabar pressures breaks the traditional belief on the Tc limit of 40 K for conventional superconductors, and opens up the doors in searching new high-temperature superconductors in compounds made up of light elements. Selenium is a sister and isoelectronic element of sulfur, with a larger atomic core and a weaker electronegativity. Whether selenium hydrides share similar high-temperature superconductivity remains elusive, but it is a subject of considerable interest. First-principles swarm structure predictions are performed in an effort to seek for energetically stable and metallic selenium hydrides at high pressures. We find the phase diagram of selenium hydrides is rather different from its sulfur analogy, which is indicated by the emergence of new phases and the change of relative stabilities. Three stable and metallic species with stoichiometries of HSe2, HSe and H3Se are identified above ~120 GPa and they all exhibit superconductive behaviors, of which the hydrogen-rich HSe and H3Se phases show high Tc in the range of 40-110 K. Our simulations established the high-temperature superconductive nature of selenium hydrides and provided useful route for experimental verification.