Prediction of Above-Room-Temperature Superconductivity in Lanthanide/Actinide Extreme Superhydrides.

Achieving room-temperature superconductivity has been an enduring scientific pursuit driven by broad fundamental interest and enticing potential applications. The recent discovery of high-pressure clathrate superhydride LaH with superconducting critical temperatures () of 250-260 K made it tantalizingly close to realizing this long-sought goal. Here, we report a remarkable finding based on an advanced crystal structure search method of a new class of extremely hydrogen-rich clathrate superhydride MH (M: rare-earth/actinide atom) stoichiometric compounds stabilized at an experimentally accessible pressure of 350 GPa.

Structural dynamics of basaltic melt at mantle conditions with implications for magma oceans and superplumes.

Transport properties like diffusivity and viscosity of melts dictated the evolution of the Earth's early magma oceans. We report the structure, density, diffusivity, electrical conductivity and viscosity of a model basaltic (CaMgAlSiO) melt from first-principles molecular dynamics calculations at temperatures of 2200 K (0 to 82 GPa) and 3000 K (40-70 GPa). A key finding is that, although the density and coordination numbers around Si and Al increase with pressure, the Si-O and Al-O bonds become more ionic and weaker.

Electronic structure of dense solid oxygen from insulator to metal investigated with X-ray Raman scattering.

Electronic structures of dense solid oxygen have been investigated up to 140 GPa with oxygen -edge X-ray Raman scattering spectroscopy with the help of ab initio calculations based on density functional theory with semilocal metageneralized gradient approximation and nonlocal van der Waals density functionals. The present study demonstrates that the transition energies (Pi*, Sigma*, and the continuum) increase with compression, and the slopes of the pressure dependences then change at 94 GPa. The change in the slopes indicates that the electronic structure changes at the metallic transition.

First-principles calculations of the epsilon phase of solid oxygen.

The crystal, electronic and magnetic structures of solid oxygen in the epsilon phase have been investigated using the strongly constrained appropriately normed (SCAN) + rVV10 method and the generalized gradient approximation (GGA) + vdW-D + U method. The spin-polarized SCAN + rVV10 method with an 8-atom primitive unit cell provides lattice parameters consistent with the experimental results over the entire pressure range, including the epsilon-zeta structural phase transition at high pressure, but does not provide accurate values of the intermolecular distances d and d at low pressure. The agreement between the intermolecular distances and the experimental values is greatly improved when a 16-atom conventional unit cell is used.

Diffusion Monte Carlo Study of Para-Diiodobenzene Polymorphism Revisited.

We revisit our investigation of the diffusion Monte Carlo (DMC) simulation of para-diiodobenzene (p-DIB) molecular crystal polymorphism. [See J. Phys.

Hydrogen segregation and its roles in structural stability and metallization: silane under pressure.

We present results from first-principles calculations on silane (SiH4) under pressure. We find that a three dimensional P-3 structure becomes the most stable phase above 241 GPa. A prominent structural feature, which separates the P-3 structure from previously observed/predicted SiH4 structures, is that a fraction of hydrogen leaves the Si-H bonding environment and forms segregated H2 units.

Pressure-induced dissociation of water molecules in ice VII.

The neutron diffraction pattern of D2O ice was recently measured at pressures up to 52 GPa by Guthrie et al., who proposed an octahedral interstitial model for ice at pressures above 13 GPa to account for the deviation of the observed crystal structure from that of ice VII. In this article, the octahedral interstitial model was re-examined in terms of the interstitial occupancy and X-ray Raman spectroscopy (XRS) spectra.

Structural morphologies of high-pressure polymorphs of strontium hydrides.

It is now known that the structure and properties of a material can be significantly altered under extreme compression. In this work, a structural search was performed to investigate the phase stabilities and structures of SrH2n (n = 1-5) in the pressure range of 50-300 GPa. The high-pressure polymorphs reveal a variety of hydrogen structural units ranging from monatomic hydride to linear and bent H3 and spiral polymer chains.

Stabilization of H in the high pressure crystalline structure of H Cl ( = 2-7).

The particle-swarm optimization method has been used to predict the stable high pressure structures up to 300 GPa of hydrogen-rich group 17 chlorine (H Cl, = 2-7) compounds. In comparison to the group 1 and 2 hydrides, the structural modification associated with increasing pressure and hydrogen concentration is much less dramatic. The polymeric HCl chains already present in the low temperature phase under ambient pressure persist in all the high pressure structures.

Electrical conductivity of ice VII.

It was discovered that a peak appears near a pressure of Pc = 10 GPa in the electrical conductivity of ice VII as measured through impedance spectroscopy in a diamond anvil cell (DAC) during the process of compression from 2 GPa to 40 GPa at room temperature. The activation energy for the conductivity measured in the cooling/heating process between 278 K and 303 K reached a minimum near Pc. Theoretical modelling and molecular dynamics simulations suggest that the origin of this unique peak is the transition of the major charge carriers from the rotational defects to the ionic defects.

Metallization and superconductivity of BeH2 under high pressure.

Pressure-induced metallization and potential superconductivity of BeH2 has been a topic of interest. In the present study, we extensively explored the crystal structures of BeH2 in a wide pressure range of 0-300 GPa using an unbiased structure searching method coupled with first-principles density functional calculations. A series of pressure-induced structural transformations are predicted for BeH2, as Ibam (α phase) → P-3m1 (phase II) → R-3m (phase III) → Cmcm (phase IV).

First principles molecular dynamics study of filled ice hydrogen hydrate.

We investigated structural changes, phase diagram, and vibrational properties of hydrogen hydrate in filled-ice phase C(2) by using first principles molecular dynamics simulation. It was found that the experimentally reported "cubic" structure is unstable at low temperature and/or high pressure: The "cubic" structure reflects the symmetry at high (room) temperature where the hydrogen bond network is disordered and the hydrogen molecules are orientationally disordered due to thermal rotation. In this sense, the "cubic" symmetry would definitely be lowered at low temperature where the hydrogen bond network and the hydrogen molecules are expected to be ordered.

Superconductive sodalite-like clathrate calcium hydride at high pressures.

Hydrogen-rich compounds hold promise as high-temperature superconductors under high pressures. Recent theoretical hydride structures on achieving high-pressure superconductivity are composed mainly of H(2) fragments. Through a systematic investigation of Ca hydrides with different hydrogen contents using particle-swam optimization structural search, we show that in the stoichiometry CaH(6) a body-centered cubic structure with hydrogen that forms unusual "sodalite" cages containing enclathrated Ca stabilizes above pressure 150 GPa.

First-principles study of liquid gallium at ambient and high pressure.

The static and dynamic properties of liquid Ga close to the melting line have been studied by first-principles molecular dynamics simulations at ambient and elevated pressure up to 5.8 GPa. Below 2.

First-principles studies of liquid lithium under pressure.

Effects of compression on the structural and electronic properties of liquid lithium are investigated with first-principles molecular dynamics calculations. Within a large pressure range up to 60 GPa, along isotherms from 600 to 1000 K, several structural transformations were found. The liquid structures at high pressure are found to be not sensitive to the temperature within this range.

Pressure-induced intermolecular interactions in crystalline silane-hydrogen.

The structure and dynamics of a recently discovered solid silane-hydrogen complex under high pressure are elucidated with first-principles molecular dynamics calculations. A structure with orientationally disordered silane and hydrogen with their centers of mass arranged in a distinctive manner are found. Natural bond orbital analysis reveals that perturbative donor-acceptor interactions between the two molecular species are enhanced by pressure.

Dissociation of methane under high pressure.

Methane is an extremely important energy source with a great abundance in nature and plays a significant role in planetary physics, being one of the major constituents of giant planets Uranus and Neptune. The stable crystal forms of methane under extreme conditions are of great fundamental interest. Using the ab initio evolutionary algorithm for crystal structure prediction, we found three novel insulating molecular structures with P2(1)2(1)2(1), Pnma, and Cmcm space groups.

First-principles prediction on the high-pressure structures of transition metal diborides (TMB2, TM = Sc, Ti, Y, Zr).

We have extensively explored the high-pressure structures of transition-metal diborides (TMB(2), TM = Sc, Ti, Y, and Zr) stabilized with the AlB(2)-type structure at ambient pressure by using first-principles structural prediction. We find two novel high-pressure structures: (i) a monoclinic structure (C2/m, Z = 4) for ScB(2) and YB(2) stable above 208 and 163 GPa, respectively; and (ii) a tetragonal alpha-ThSi(2)-type phase (I4(1)/amd, Z = 4) for TiB(2) stable above 215 GPa. Our calculations show that the electron transfer from transition-metals TM to B under pressure might be the main cause for the structural phase transitions.

High-pressure crystal structures and superconductivity of Stannane (SnH4).

There is great interest in the exploration of hydrogen-rich compounds upon strong compression where they can become superconductors. Stannane (SnH(4)) has been proposed to be a potential high-temperature superconductor under pressure, but its high-pressure crystal structures, fundamental for the understanding of superconductivity, remain unsolved. Using an ab initio evolutionary algorithm for crystal structure prediction, we propose the existence of two unique high-pressure metallic phases having space groups Ama2 and P6(3)/mmc, which both contain hexagonal layers of Sn atoms and semimolecular (perhydride) H(2) units.

Superconducting high pressure phase of germane.

High-pressure structures of germane (GeH4) are explored through ab initio evolutionary methodology to reveal a metallic monoclinic structure of C2/c (4 molecules/cell). The C2/c structure consists of layerlike motifs containing novel "H2" units. Enthalpy calculations suggest a remarkably wide decomposition (Ge+H2) pressure range of 0-196 GPa, above which C2/c structure is stable.

HgTe: a potential thermoelectric material in the cinnabar phase.

We present the calculations of the electronic structure and transport properties on the zinc-blende (ZB) and cinnabar phases of HgTe using the full-potential linearized augmented plane-wave method and the semiclassical Boltzmann theory. Our results show that n-doped cinnabar HgTe has a significant larger Seebeck coefficient and electrical conductivity along the z axis than those of the n-doped ZB phase. This is mainly attributed to the large structural anisotropy originated from its chainlike bonding characters along the z axis, resulting in the anisotropic energy distribution in the lowest conduction band of cinnabar structure.

Linear scaling calculation of an n-type GaAs quantum dot.

A linear scale method for calculating electronic properties of large and complex systems is introduced within a local density approximation. The method is based on the Chebyshev polynomial expansion and the time-dependent method, which is tested on the calculation of the electronic structure of a model n-type GaAs quantum dot.

First-principles investigation on the geometry and electronic structure of the three-dimensional cuboidal C(60) polymer.

The structural stability and electronic properties of the recently characterized three-dimensional (3D) cuboid-shaped C(60) polymer are studied using periodic ab initio density functional methods. It is shown that the experimentally observed structure is metastable and not fully relaxed from the high pressure state. A second polymorph, which is more stable than the experimental structure, is identified from the calculations.