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We investigate the pressure-temperature ($p$-$T$) phase diagram of elemental lithium (Li) up to multiterapascal (TPa) pressures using ab-initio random structure search (AIRSS) and density functional theory (DFT). At zero temperature, beyond the high-pressure $Fd\bar{3}m$ diamond structure predicted in previous studies, we find eleven solid-state phase transitions to structures of greatly varying complexity, in addition to two structures that we calculate will become stable with sufficient temperature. The full $p$-$T$ dependence of the phase boundaries are computed within the vibrational harmonic approximation, and the solid-liquid melting line is calculated using ab-initio molecular dynamics simulations. Notably, between 39.1 TPa and 55.7 TPa, Li adopts an elaborate monoclinic structure with 46 atoms in the primitive unit cell, and between 71.9 TPa and $103$ TPa, an incommensurate host-guest phase of the Ba-IV type. We find that Li, hitherto predicted to be an electride at TPa pressures, abruptly loses its electride character above 16 TPa, reverting back to normal metallic behaviour with a corresponding rise in the Fermi-level electronic density of states (eDOS) and broadening of the electronic bands.

Through density functional theory and molecular dynamics calculations, we have analysed various metal polyhydrides to understand whether hydrogen is present in its molecular or atomic form - tetrahydrides of Ba,Sr,Ra, Cs and La; Ba$_8$H$_{46}$ and BaH$_{12}$. We show that, in experimentally reported binary barium hydrides (BaH$_x$), molecular H$_2$ and atomic H$^-$ can coexist with the metallic cations. In this thorough study of differences between BaH$_4$, higher barium hydrides, and other binary tetrahydrides we find the number of atomic hydrogens is equal to the formal charge of the cations. The remaining hydrogen forms molecules in proportions yielding, e.g. BaH$_2$(H$_2)_x$, at pressures as high as 200 GPa. At room temperature these are highly dynamic structures with the hydrogens switching between H$^-$ and H$_2$ while retaining the 2:x ratio. We find some qualitative differences between our static DFT calculations and previously reported structural and spectroscopic experimental results. Two factors allow us to resolve such discrepancies: Firstly, in static relaxation H$_2$ must be regarded as a non-spherical object, which breaks symmetry in a way invisible to X-rays; Secondly the required number of molecules $x$ may be incompatible with the experimental space group (e.g. $BaH_2(H_2)_5$). In molecular dynamics, bond-breaking transitions between various structural symmetry configurations happen on a picosecond timescale via an H$_3^-$ intermediate. Rebonding is slow enough to allow a spectroscopic signal but frequent enough to average out over the lengthscale involved in diffraction.

Single-crystal synchrotron X-ray diffraction, Raman spectroscopy, and first principles calculations have been used to identify the structure of the high-pressure (HP) phase of molecular methane above 20 GPa up to 71 GPa. The structure of HP phase is trigonal R3, which can be represented as a distortion of the cubic phase B, previously documented at 7-15 GPa and confirmed here. The positions of hydrogen atoms in HP phase have been 19 obtained from first principles calculations. The molecules occupy four different crystallographic sites in phases B and eleven sites in the HP phase, which result in splitting of molecular stretching modes detected in Raman spectroscopy and assigned here based on 22 a good agreement with the Raman spectra calculated from the first principles., Comment: 29 pages, 7 figures, 3 tables, supplemental material

A two-dimensional carbon allotrope, Stone-Wales graphene, is identified in stochastic group and graph constrained searches and systematically investigated by first-principles calculations. Stone-Wales graphene consists of well-arranged Stone-Wales defects, and it can be constructed through a 90$^\circ$ bond-rotation in a $\sqrt{8}$$\times$$\sqrt{8}$ super-cell of graphene. Its calculated energy relative to graphene, +149 meV/atom, makes it more stable than the most competitive previously suggested graphene allotropes. We find that Stone-Wales graphene based on a $\sqrt{8}$ super-cell is more stable than those based on $\sqrt{9} \times \sqrt{9}$, $\sqrt{12} \times \sqrt{12}$ and $\sqrt{13} \times \sqrt{13}$ super-cells, and is a "magic size" that can be further understood through a simple "energy splitting and inversion" model. The calculated vibrational properties and molecular dynamics of SW-graphene confirm that it is dynamically stable. The electronic structure shows SW-graphene is a semimetal with distorted, strongly anisotropic Dirac cones., Comment: Accepted;5 pages;5 figures;53 references and 8 pages of supplementary file

We investigate a range of possible materials containing the supermolecular form of hydrogen comprising 13 hydrogen molecules arranged in an icosahedral arrangement. This supermolecule consists of freely rotating 12 hydrogen molecules in an icosahedral arrangement, enclosing another freely rotating molecule. To date, this supermolecule has only been observed in a compound with Iodane (HI). The extremely high hydrogen content suggests possible application in hydrogen storage so we examine the possibility of supermolecule formation at ambient pressures. We show that ab initio molecular dynamics calculations give a good description of the known properties of the HI(H2)13 material, and make predictions of the existence of related compounds Xe(H2)1, HBr(H2)13 and HCl(H2)13, including a symmetry-breaking phase transition at low temperature. The icosahedral (H2)13 supermolecule, remains stable in all these compounds. This suggests (H2)13 is a widespread feature in hydrogen compounds, and that appropriately-sized cavities could hold hydrogen supermolecules even at low pressure. The structure of the supermolecule network is shown to be independent of the compound at equivalent density.

We investigate the binary phase diagram of helium and iron using first-principles calculations. We find that helium, which is a noble gas and inert at ambient conditions, forms stable crystalline compounds with iron at terapascal pressures. A FeHe compound becomes stable above 4 TPa, and a FeHe$_2$ compound above 12 TPa. Melting is investigated using molecular dynamics simulations, and a superionic phase with sublattice melting of the helium atoms is predicted. We discuss the implications of our predicted helium-iron phase diagram for interiors of giant (exo)planets and white dwarf stars., Comment: 5 pages, 3 figures, includes Supplemental Material

We introduce two-dimensional borane, a single-layered material of BH stoichiometry, with promising electronic properties. We show that, according to Density Functional Theory calculations, two-dimensional borane is semimetallic, with two symmetry-related Dirac cones meeting right at the Fermi energy $E_f$. The curvature of the cones is lower than in graphene, thus closer to the ideal linear dispersion. Its structure, formed by a puckered trigonal boron network with hydrogen atoms connected to each boron atom, can be understood as distorted, hydrogenated borophene (Science \textbf{350}, 1513 (2015)). Chemical bonding analysis reveals the boron layer in the network being bound by delocalized four-center two-electron ${\sigma}$ bonds. Finally, we suggest high-pressure could be a feasible route to synthesise two-dimensional borane., Comment: 5 pages, 3 figures, 1 table

The stability, structure and properties of carbonate minerals at lower mantle conditions has significant impact on our understanding of the global carbon cycle and the composition of the interior of the Earth. In recent years, there has been significant interest in the behavior of carbonates at lower mantle conditions, specifically in their carbon hybridization, which has relevance for the storage of carbon within the deep mantle. Using high-pressure synchrotron X-ray diffraction in a diamond anvil cell coupled with direct laser heating of CaCO$_{3}$ using a CO$_{2}$ laser, we identify a crystalline phase of the material above 40 GPa $-$ corresponding to a lower mantle depth of around 1,000 km $-$ which has first been predicted by \textit{ab initio} structure predictions. The observed $sp^{2}$ carbon hybridized species at 40 GPa is monoclinic with $P2_{1}/c$ symmetry and is stable up to 50 GPa, above which it transforms into a structure which cannot be indexed by existing known phases. A combination of \textit{ab initio} random structure search (AIRSS) and quasi-harmonic approximation (QHA) calculations are used to re-explore the relative phase stabilities of the rich phase diagram of CaCO$_{3}$. Nudged elastic band (NEB) calculations are used to investigate the reaction mechanisms between relevant crystal phases of CaCO$_{3}$ and we postulate that the mineral is capable of undergoing $sp^{2}$-$sp^{3}$ hybridization change purely in the $P2_{1}/c$ structure $-$ forgoing the accepted post-aragonite $Pmmn$ structure., Comment: 12 pages, 8 figures

Computational searches for stable and metastable structures of water ice and other H:O compositions at TPa pressures have led us to predict that H$_2$O decomposes into H$_2$O$_2$ and a hydrogen-rich phase at pressures of a little over 5 TPa. The hydrogen-rich phase is stable over a wide range of hydrogen contents, and it might play a role in the erosion of the icy component of the cores of gas giants as H$_2$O comes into contact with hydrogen. Metallization of H$_2$O is predicted at a higher pressure of just over 6 TPa, and therefore H$_2$O does not have a thermodynamically stable low-temperature metallic form. We have also found a new and rich mineralogy of complicated water ice phases which are more stable in the pressure range 0.8--2 TPa than any predicted previously., Comment: 5 pages, 4 figures, 1 table

The phase diagram and equation of state of dense nitrogen are of interest in understanding the fundamental physics and chemistry under extreme conditions, including planetary processes, and in discovering new materials. We predict several stable phases of nitrogen at multi-TPa pressures, including a P4/nbm structure consisting of partially charged N2 pairs and N5 tetrahedra, which is stable in the range 2.5-6.8 TPa. This is followed by a modulated layered structure between 6.8 and 12.6 TPa, which also exhibits a significant charge transfer. The P4/nbm metallic nitrogen salt and the modulated structure are stable at high pressures and temperatures, and they exhibit strongly ionic features and charge density distortions, which is unexpected in an element under such extreme conditions and could represent a new class of nitrogen materials. The P-T phase diagram of nitrogen at TPa pressures is investigated using quasiharmonic phonon calculations and ab initio molecular dynamics simulations., Comment: 5 pages, 4 figures. (jiansun@nju.edu.cn; miguel.c.martinez@ucl.ac.uk)

We have studied solid hydrogen up to pressures of 300 GPa and temperatures of 350 K using density functional theory methods and have found "mixed structures" that are more stable than those predicted earlier. Mixed structures consist of alternate layers of strongly bonded molecules and weakly bonded graphene-like sheets. Quasi-harmonic vibrational calculations show that mixed structures are the most stable at room temperature over the pressure range 250-295 GPa. These structures are stabilized with respect to strongly-bonded molecular phases at room temperature by the presence of lower-frequency vibrational modes arising from the graphene-like sheets. Our results for the mixed structures are consistent with the experimental Raman data [M. I. Eremets and I. A. Troyan, Nature Mater. 10 927 (2011) and R. T. Howie et al. Phys. Rev. Lett. 108 125501 (2012)]. We find that mixed phases are reasonable structural models for phase IV of hydrogen., Comment: 9 pages, 10 figures, 1 table

Computational searches for structures of solid oxygen under pressures in the multi TPa range have been carried out using density-functional-theory methods. We find that molecular oxygen persists to about 1.9 TPa at which it transforms into a semiconducting square spiral-like polymeric structure (I41/acd) with a band gap of about 3.0 eV. Solid oxygen forms a metallic zig-zag chain-like structure (Cmcm) at about 3.0 TPa, but the chains in each layer gradually merge as the pressure is increased and a structure of Fmmm symmetry forms at about 9.5 TPa in which each atom has four nearest neighbors. The superconducting properties of molecular oxygen do not vary much with compression, although the structure becomes more symmetric. The electronic properties of oxygen have a complex evolution with pressure, swapping between insulating, semiconducting and metallic., Comment: 5 pages, 4 figures. (jian.sun@theochem.rub.de)