Robust Diffusive Proton Motions in Phase IV of Solid Hydrogen

Systematic first-principles molecular dynamics (MD) simulations with long simulation times (7–13 ps) for phase IV of solid hydrogen using different supercell sizes of 96, 288, 576, and 768 atoms established that the diffusive proton motions process in the graphene-like layer is an intrinsic property and independent of the simulation cell sizes. The present study highlights an often overlook issue in first-principles calculations that long time MD is essential to achieve ergodicity, which is mandatory for a proper description of dynamics of a system. It is inappropriate to make arguments on the analysis of MD results, which are far from ergodic. Furthermore, we have simulated the vibrational density of states of phase IV based on our proton diffuse model at a pressure range of 225–300 GPa, which is qualitatively in agreement with experimental data

Structure and Electronic Properties of Fe₂SH₃ Compound under High Pressure

Inspired by the diverse properties of hydrogen sulfide, iron sulfide, and iron hydrides, we combine first-principles calculations with structure prediction to find stable structures of Fe–S–H ternary compounds with various Fe_<i>x</i>S_<i>y</i>H_<i>z</i> (<i>x</i> = 1–2; <i>y</i> = 1–2; <i>z</i> = 1–6) compositions under high pressure with the aim of finding novel functional materials. It is found that Fe₂SH₃ composition stabilizes into an orthorhombic structure with <i>Cmc</i>2₁ symmetry, whose remarkable feature is that it contains dumbbell-type Fe with an Fe–Fe distance of 2.435 Å at 100 GPa, and S and H atoms directly bond with the Fe atoms exhibiting ionic bonding. The high density of states at the Fermi level, mainly coming from the contribution of Fe-3d, indicates that it satisfies the Stoner ferromagnetic condition. Notably, its ferromagnetic ordering gradually decreases with increasing pressure, and eventually collapses at a pressure of 173 GPa. As a consequence, magnetic and nonmagnetic transition can be achieved by controlling the pressure. In addition, there is a very weak electron–phonon coupling in <i>Cmc</i>2₁-structured Fe₂SH₃. The different superconducting mechanisms between Fe₂SH₃ and H₃S were compared and analyzed

Crystal Structures and Electronic Properties of Xe–Cl Compounds at High Pressure

Crystal structure prediction techniques coupled with enthalpies obtained at 0 K from density functional theory calculations suggest that pressure can be used to stabilize the chlorides of xenon. In particular, XeCl and XeCl₂ were calculated to become metastable by 10 GPa and thermodynamically stable with respect to the elemental phases by 60 GPa. Whereas at low pressures Cl₂ dimers were found in the stable phases, zigzag Cl chains were present in <i>Cmcm</i> XeCl at 60 GPa and atomistic chlorine comprised <i>P</i>6₃/<i>mmc</i> XeCl and <i>Fd</i>3̅<i>m</i> XeCl₂ at 100 GPa. A XeCl₄ phase that was metastable at 100 GPa contained monomers, dimers, and trimers of chlorine. XeCl, XeCl₂, and XeCl₄ were metallic at 100 GPa, and at this pressure they were predicted to be superconducting below 9.0, 4.3, and 0.3 K, respectively. Spectroscopic properties of the predicted phases are presented to aid in their eventual characterization, should they ever be synthesized

The Exotically Stoichiometric Compounds and Superconductivity of Lithium–Copper Systems under High Pressure

Pressure, as a useful tool, can push elements to new oxidation states by altering the stoichiometry of compounds, leading to materials with exotic physical and chemical properties. Herein, structure searches for Li–Cu systems were carried out under pressure. Three Li-rich Li–Cu compounds with exotic stoichiometries (i.e., Li4Cu, Li5Cu, and Li6Cu) are predicted at high pressure. Remarkably, the Li6Cu consists of a Cu-centered face-sharing icosahedron. Further simulations reveal that the captured electrons from Li atoms prompt Cu atoms to achieve high negative oxidation states beyond −1 and to act as a 4p group element. Moreover, our results unravel the superconductivity of the Li-rich Li–Cu system and the R3̅ phase of Li6Cu with Tc of ∼15 K at 50 GPa. The present results can greatly improve the understanding of the exotic electronic behavior of Li–Cu systems under high pressure

Crystal Structures and Chemical Bonding of Magnesium Carbide at High Pressure

Recent studies of the magnesium carbide (Mg–C) system under pressure were motivated by the successful high-pressure and high-temperature synthesis of Mg₂C and Mg₂C₃. Here, we systematically investigate the high-pressure structures and chemical bonding of the Mg₂C, Mg₂C₃, and MgC₂ system using the swarm optimization technique in combination with first-principles electronic structure methodology. The structural evolution with pressure of the Mg–C systems clearly shows a systematic trend with a progressive increase of electron donation from the Mg to C. To accommodate the electrons, the C valence sp orbitals rebybridized continually and adopted different modes of chemical bonding. We demonstrated that the evolution of the electronic and crystal structures can be explained from a Zintl–Klemen charge-transfer concept. Therefore, at sufficiently high pressure metallic MgC₂ and Mg₂C transformed to semiconductors, while Mg₂C₃ undergoes an insulator–metal transition. The present results established the richness of carbon bonding of different stoichiometries under high pressure

Stable Calcium Nitrides at Ambient and High Pressures

The knowledge of stoichiometries of alkaline-earth metal nitrides, where nitrogen can exist in polynitrogen forms, is of significant interest for understanding nitrogen bonding and its applications in energy storage. For calcium nitrides, there were three known crystalline forms, CaN₂, Ca₂N, and Ca₃N₂, at ambient conditions. In the present study, we demonstrated that there are more stable forms of calcium nitrides than what is already known to exist at ambient and high pressures. Using a global structure searching method, we theoretically explored the phase diagram of CaN_<i>x</i> and discovered a series of new compounds in this family. In particular, we found a new CaN phase that is thermodynamically stable at ambient conditions, which may be synthesized using CaN₂ and Ca₂N. Four other stoichiometries, namely, Ca₂N₃, CaN₃, CaN₄, and CaN₅, were shown to be stable under high pressure. The predicted CaN_<i>x</i> compounds contain a rich variety of polynitrogen forms ranging from small molecules (N₂, N₄, N₅, and N₆) to extended chains (N_∞). Because of the large energy difference between the single and triple nitrogen bonds, dissociation of the CaN_<i>x</i> crystals with polynitrogens is expected to be highly exothermic, making them as potential high-energy-density materials

Crystal Structure and Superconductivity of PH₃ at High Pressures

We have performed a systematic structure search on solid PH₃ at high pressures using the particle swarm optimization method. At 100–200 GPa, the search led to two structures which along with others have P–P bonds. These structures are structurally and chemically distinct from those predicted for the high-pressure superconducting H₂S phase, which has a different topology (i.e., does not contain S–S bonds). Phonon and electron–phonon coupling calculations indicate that both structures are dynamically stable and superconducting. The pressure dependence and critical temperature for the monoclinic (C2/<i>m</i>) phase of 83 K at 200 GPa are in excellent agreement with a recent experimental report

Prediction of Host–Guest Na–Fe Intermetallics at High Pressures

High pressure can fundamentally alter the electronic structure of elemental metals, leading to the unexpected formation of intermetallics with unusual structural features. In the present study, the phase stabilities and structural changes of Na–Fe intermetallics under pressure were studied using unbiased structure searching methods, combined with density functional theory calculations. Two intermetallics with stoichiometries Na₃Fe and Na₄Fe are found to be thermodynamically stable at pressures above 120 and 155 GPa, respectively. An interesting structural feature is that both have form a host–guest-like structure with Na sublattices constructed from small and large polygons similar to the host framework of the self-hosting incommensurate phases observed in Group I and II elements. Apart from the one-dimensional (1D) Fe chains running through the large channels, more interestingly, electrides are found to localize in the small channels between the layers. Electron topological analysis shows secondary bonding interactions between the Fe atoms and the interstitial electrides help to stabilize these structures

Phases of HeN4 and polymeric nitrogen t-N as a function of pressure

Structural (CIF) data for each predicted phase of HeN4, and the output of an NVT-MD simulation of t-N at temperature ≈ 1000 K and pressure ≈ 1atm

Two-Dimensional C₄N Global Minima: Unique Structural Topologies and Nanoelectronic Properties

Atomically thin 2D materials have drawn great attention due to their many potential applications. We herein report two novel structures of 2D C₄N identified by first-principles calculations in combination with a swarm structure search. These two structures (with symmetry of <i>Pm</i> and <i>P</i>2/<i>m</i>) are almost degenerate in energy (with only 4 meV/atom difference) and exhibit quite similar structural topologies, both consisting of alternative arrays of C–N hexagon and arrays of C–N pentagon–octagon–pentagon. The <i>Pm</i> structure is semiconducting with a direct band gap of 90 meV at HSE. In contrast, the <i>P</i>2/m structure is a zero-band-gap semimetal and possesses the distorted Dirac cone, showing the direction-dependent Fermi velocity and electronic properties. Thus the predicted C₄N monolayers are promising for applications in nanoelectronics