Yinwei Li

The metallization and superconductivity of dense hydrogen sulfide

Hydrogen sulfide (H2S) is a prototype molecular system and a sister molecule of water (H2O). The phase diagram of solid H2S at high pressures remains largely unexplored arising from the challenges in dealing with the pressure-induced weakening of S-H bond and larger atomic core difference between H and S. Metallization is yet achieved for H2O, but it was observed for H2S above 96 GPa. However, the metallic structure of H2S remains elusive, greatly impeding the understanding of its metallicity and the potential superconductivity. We have performed an extensive structural study on solid H2S at pressure ranges of 10-200 GPa through an unbiased structure prediction method based on particle swarm optimization algorithm. Besides the findings of candidate structures for nonmetallic phases IV and V, we are able to establish stable metallic structures violating an earlier proposal of elemental decomposition into sulfur and hydrogen [R. Rousseau, M. Boero, M. Bernasconi, M. Parrinello, and K. Terakura, Phys. Rev. Lett. 85, 1254 (2000)]. Our study unravels a superconductive potential of metallic H2S with an estimated maximal transition temperature of ∼80 K at 160 GPa, higher than those predicted for most archetypal hydrogen-containing compounds (e.g., SiH4, GeH4, etc.).

Origin of hardness in WB4 and its implications for ReB4, TaB4, MoB4, TcB4, and OsB4

First-principles calculations were performed on the superhard material, WB4 (Vicker hardness exceeding 46GPa), to reveal the origin of its high hardness. Our simulated lattice parameters, bulk modulus, and hardness are in excellent agreement with the experimental data. A three-dimensional B network with a peculiar B2 dimer along the z-axis and a xy planar honeycomb B sublattice is uncovered to be mainly responsible for the high hardness. We further predicted that five other transition metal B compounds (TMB4, TM=Re, Mo, Ta, Os, and Tc) within the WB4 structure are potential superhard materials.

High-Pressure Hydrogen Sulfide from First Principles: A Strongly Anharmonic Phonon-Mediated Superconductor

We use first-principles calculations to study structural, vibrational, and superconducting properties of H_{2}S at pressures P≥200u2009u2009GPa. The inclusion of zero-point energy leads to two different possible dissociations of H2S, namely 3H2S→2H3S+S and 5H2S→3H3S+HS2, where both H3S and HS2 are metallic. For H3S, we perform nonperturbative calculations of anharmonic effects within the self-consistent harmonic approximation and show that the harmonic approximation strongly overestimates the electron-phonon interaction (λ≈2.64 at 200 GPa) and Tc. Anharmonicity hardens H─S bond-stretching modes and softens H─S bond-bending modes. As a result, the electron-phonon coupling is suppressed by 30% (λ≈1.84 at 200 GPa). Moreover, while at the harmonic level Tc decreases with increasing pressure, the inclusion of anharmonicity leads to a Tc that is almost independent of pressure. High-pressure hydrogen sulfide is a strongly anharmonic superconductor.

Superconductivity at ∼100 K in dense SiH4(H2)2 predicted by first principles

Motivated by the potential high-temperature superconductivity in hydrogen-rich materials, the high-pressure structures of SiH4(H2)2 in the pressure range 50–300 GPa were extensively explored by using a genetic algorithm. An intriguing layered orthorhombic (Ccca) structure was revealed to be energetically stable above 248 GPa with the inclusion of zero-point energy. The Ccca structure is metallic and composed of hydrogen shared SiH8 dodecahedra layers intercalated by orientationally ordered molecular H2. Application of the Allen-Dynes modified McMillan equation yields remarkably high superconducting temperatures of 98–107 K at 250 GPa, among the highest values reported so far for phonon-mediated superconductors. Analysis reveals a unique superconducting mechanism that the direct interactions between H2 and SiH4 molecules at high pressure play the major role in the high superconductivity, while the contribution from H2 vibrons is minor.

Quantum hydrogen-bond symmetrization in the superconducting hydrogen sulfide system.

Ion Errea1,2,∗ Matteo Calandra3,† Chris J. Pickard, Joseph Nelson, Richard J. Needs, Yinwei Li, Hanyu Liu, Yunwei Zhang, Yanming Ma, and Francesco Mauri3,9‡ Fisika Aplikatua 1 Saila, EUITI Bilbao, University of the Basque Country (UPV/EHU), Rafael Moreno “Pitxitxi” Pasealekua 3, 48013 Bilbao, Basque Country, Spain Donostia International Physics Center (DIPC), Manuel Lardizabal pasealekua 4, 20018 Donostia/San Sebastián, Basque Country, Spain IMPMC, UMR CNRS 7590, Sorbonne Universités UPMC Univ. Paris 06, MNHN, IRD, 4 Place Jussieu, F-75005 Paris, France 4 Department of Materials Science & Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK Theory of Condensed Matter Group, Cavendish Laboratory, J J Thomson Avenue, Cambridge CB3 0HE, UK School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, People’s Republic of China 7 Geophysical Laboratory, Carnegie Institution of Washington, Washington D.C. 20015, USA State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China and 9 Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale Aldo Moro 5, I-00185 Roma, ItalyThe quantum nature of the proton can crucially affect the structural and physical properties of hydrogen compounds. For example, in the high-pressure phases of H2O, quantum proton fluctuations lead to symmetrization of the hydrogen bond and reduce the boundary between asymmetric and symmetric structures in the phase diagram by 30 gigapascals (ref. 3). Here we show that an analogous quantum symmetrization occurs in the recently discovered sulfur hydride superconductor with a superconducting transition temperature Tc of 203 kelvin at 155 gigapascals—the highest Tc reported for any superconductor so far. Superconductivity occurs via the formation of a compound with chemical formula H3S (sulfur trihydride) with sulfur atoms arranged on a body-centred cubic lattice. If the hydrogen atoms are treated as classical particles, then for pressures greater than about 175 gigapascals they are predicted to sit exactly halfway between two sulfur atoms in a structure with symmetry. At lower pressures, the hydrogen atoms move to an off-centre position, forming a short H–S covalent bond and a longer H···S hydrogen bond in a structure with R3m symmetry. X-ray diffraction experiments confirm the H3S stoichiometry and the sulfur lattice sites, but were unable to discriminate between the two phases. Ab initio density-functional-theory calculations show that quantum nuclear motion lowers the symmetrization pressure by 72 gigapascals for H3S and by 60 gigapascals for D3S. Consequently, we predict that the phase dominates the pressure range within which the high Tc was measured. The observed pressure dependence of Tc is accurately reproduced in our calculations for the phase, but not for the R3m phase. Therefore, the quantum nature of the proton fundamentally changes the superconducting phase diagram of H3S.

Pressure-stabilized superconductive yttrium hydrides

The search for high-temperature superconductors has been focused on compounds containing a large fraction of hydrogen, such as SiH4(H2)2, CaH6 and KH6. Through a systematic investigation of yttrium hydrides at different hydrogen contents using an structure prediction method based on the particle swarm optimization algorithm, we have predicted two new yttrium hydrides (YH4 andYH6), which are stable above 110u2009GPa. Three types of hydrogen species with increased H contents were found, monatomic H in YH3, monatomic H+molecular “H2” in YH4 and hexagonal “H6” unit in YH6. Interestingly, H atoms in YH6 form sodalite-like cage sublattice with centered Y atom. Electron-phonon calculations revealed the superconductive potential of YH4 and YH6 with estimated transition temperatures (Tc) of 84–95u2009K and 251–264u2009K at 120u2009GPa, respectively. These values are higher than the predicted maximal Tc of 40u2009K in YH3.

Dissociation products and structures of solid H 2 S at strong compression

Hydrogen sulfides have recently received a great deal of interest due to the record high-

Dissociation of methane under high pressure

{T}_{c}

Twofold Coordinated Ground-State and Eightfold High-Pressure Phases of Heavy Transition Metal Nitrides MN2 (M = Os, Ir, Ru, and Rh)

of up to 203 K observed on strong compression of H

Visible-light-enhanced gating effect at the LaAlO 3 /SrTiO 3 interface

{}_{2}