The Li-F-H ternary system at high pressures.

Evolutionary crystal structure prediction searches have been employed to explore the ternary Li-F-H system at 300 GPa. Metastable phases were uncovered within the static lattice approximation, with LiF3H2, LiF2H, Li3F4H, LiF4H4, Li2F3H, and LiF3H lying within 50 meV/atom of the 0 K convex hull. All of these phases contain HnFn+1 - (n = 1, 2) anions and Li+ cations. Other structural motifs such as LiF slabs, H3 + molecules, and Fδ- ions are present in some of the low enthalpy Li-F-H structures. The bonding within the HnFn+1 - molecules, which may be bent or linear, symmetric or asymmetric, is analyzed. The five phases closest to the hull are insulators, while LiF3H is metallic and predicted to have a vanishingly small superconducting critical temperature. Li3F4H is predicted to be stable at zero pressure. This study lays the foundation for future investigations of the role of temperature and anharmonicity on the stability and properties of compounds and alloys in the Li-F-H ternary system.

Benchmarking boron carbide equation of state using computation and experiment.

Boron carbide (B_{4}C) is of both fundamental scientific and practical interest due to its structural complexity and how it changes upon compression, as well as its many industrial uses and potential for use in inertial confinement fusion (ICF) and high-energy density physics experiments. We report the results of a comprehensive computational study of the equation of state (EOS) of B_{4}C in the liquid, warm dense matter, and plasma phases. Our calculations are cross-validated by comparisons with Hugoniot measurements up to 61 megabar from planar shock experiments performed at the National Ignition Facility (NIF). Our computational methods include path integral Monte Carlo, activity expansion, as well as all-electron Green's function Korringa-Kohn-Rostoker and molecular dynamics that are both based on density functional theory. We calculate the pressure-internal energy EOS of B_{4}C over a broad range of temperatures (∼6×10^{3}-5×10^{8} K) and densities (0.025-50 g/cm^{3}). We assess that the largest discrepancies between theoretical predictions are ≲5% near the compression maximum at 1-2×10^{6} K. This is the warm-dense state in which the K shell significantly ionizes and has posed grand challenges to theory and experiment. By comparing with different EOS models, we find a Purgatorio model (LEOS 2122) that agrees with our calculations. The maximum discrepancies in pressure between our first-principles predictions and LEOS 2122 are ∼18% and occur at temperatures between 6×10^{3}-2×10^{5} K, which we believe originate from differences in the ion thermal term and the cold curve that are modeled in LEOS 2122 in comparison with our first-principles calculations. To account for potential differences in the ion thermal term, we have developed three new equation-of-state models that are consistent with theoretical calculations and experiment. We apply these new models to 1D hydrodynamic simulations of a polar direct-drive NIF implosion, demonstrating that these new models are now available for future ICF design studies.

Superconducting Phases of Phosphorus Hydride Under Pressure: Stabilization by Mobile Molecular Hydrogen.

At 80 GPa, phases with the PH2 stoichiometry, which are composed of simple cubic like phosphorus layers capped with hydrogen atoms and layers of H2 molecules, are predicted to be important species contributing to the recently observed superconductivity in compressed phosphine. The electron-phonon coupling in these phases results from the motions of the phosphorus atoms and the hydrogen atoms bound to them. The role of the mobile H2 layers is to decrease the Coulomb repulsion between the negatively charged hydrogen atoms capping the phosphorus layers. An insulating PH5 phase, the structure and bonding of which is reminiscent of diborane, is also predicted to be metastable at this pressure.

Decomposition Products of Phosphine Under Pressure: PH2 Stable and Superconducting?

Evolutionary algorithms (EAs) coupled with density functional theory (DFT) calculations have been used to predict the most stable hydrides of phosphorus (PHn, n = 1-6) at 100, 150, and 200 GPa. At these pressures phosphine is unstable with respect to decomposition into the elemental phases, as well as PH2 and H2. Three metallic PH2 phases were found to be dynamically stable and superconducting between 100 and 200 GPa. One of these contains five formula units in the primitive cell and has C2/m symmetry (5FU-C2/m). It comprises 1D periodic PH3-PH-PH2-PH-PH3 oligomers. Two structurally related phases consisting of phosphorus atoms that are octahedrally coordinated by four phosphorus atoms in the equatorial positions and two hydrogen atoms in the axial positions (I4/mmm and 2FU-C2/m) were the most stable phases between ∼160-200 GPa. Their superconducting critical temperatures (Tc) were computed as 70 and 76 K, respectively, via the Allen-Dynes modified McMillan formula and using a value of 0.1 for the Coulomb pseudopotential, µ*. Our results suggest that the superconductivity recently observed by Drozdov, Eremets, and Troyan when phosphine was subject to pressures of 207 GPa in a diamond anvil cell may result from these, and other, decomposition products of phosphine.

Theoretical predictions of novel potassium chloride phases under pressure.

Evolutionary structure searches predict two hitherto unknown phases of KCl that are the most stable in the pressure regime of 200-600 GPa. I41/amd-KCl, which has the lowest enthalpy between ∼200-350 GPa, can be thought of as being composed of two three-connected nets. This structure can be compared with that of the Cs-IV electride (Cs(+)e(-)): the potassium ions assume the positions of the cesium ions, and the chloride ions are found roughly in the regions of the valence electrons. Above ∼350 GPa a Pnma phase, which is isotypic with phases of CsH and CsI that are stable under pressure, becomes preferred. Just as in Pnma-CsI, the atoms in Pnma-KCl assume an hcp-like lattice; these alkali halides resemble the high-pressure structures of the isoelectronic noble gas solids Xe and Ar, respectively. The equation of state of KCl is extended to 600 GPa, enabling the use of this alkali halide as a pressure guage in ultra-high pressure static compression experiments. KCl is predicted to remain insulating to at least 420 GPa.

Superconducting High-Pressure Phases Composed of Hydrogen and Iodine.

Evolutionary structure searches predict three new phases of iodine polyhydrides stable under pressure. Insulating P1-H5I, consisting of zigzag chains of (HI)δ+ and H2 δ− molecules, is stable between 30 and 90 GPa. Cmcm-H2I and P6/mmm-H4I are found on the 100, 150, and 200 GPa convex hulls. These two phases are good metals, even at 1 atm, because they consist of monatomic lattices of iodine. At 100 GPa the superconducting transition temperature, Tc, of H2I and H4I is estimated to be 7.8 and 17.5 K, respectively. The increase in Tc relative to elemental iodine results from a larger ωlog from the light mass of hydrogen and an enhanced λ from modes containing H/I and H/H vibrations.

Compressed cesium polyhydrides: Cs+ sublattices and H3(-) three-connected nets.

The cesium polyhydrides (CsH(n), n > 1) are predicted to become stable, with respect to decomposition into CsH and H2, at pressures as low as 2 GPa. The CsH3 stoichiometry is found to have the lowest enthalpy of formation from CsH and H2 between 30 and 200 GPa. Evolutionary algorithms predict five distinct, mechanically stable, nearly isoenthalpic CsH3 phases consisting of H3(­) molecules and Cs+ atoms. The H3(­) sublattices in two of these adopt a hexagonal three-connected net; in the other three the net is twisted, like the silicon sublattice in the α-ThSi2 structure. The former emerge as being metallic below 100 GPa in our screened hybrid density functional theory calculations, whereas the latter remain insulating up to pressures greater than 250 GPa. The Cs+ cations in the most-stable I4(1)/amd CsH3 phase adopt the positions of the Cs atoms in Cs-IV, and the H3(­) molecules are found in the (interstitial) regions which display a maximum in the electron density.