Superconductivity in the recently proposed ground-state structures of atomic metallic hydrogen is calculated over the pressure range 500 GPa to $3.5$ TPa. Near molecular dissociation, the electron-phonon coupling $\lambda$ and renormalized Coulomb repulsion are similar to the molecular phase. A nearly continuous increase in the critical temperature $T_c$ with pressure is thus predicted in this range, to $\sim 356$ K near 500 GPa. As the atomic phase stabilizes with increasing pressure, $\lambda$ increases, causing $T_c$ to approach 481 K near 700 GPa. At the first atomic-atomic structural phase transformation near 1–$1.5$ TPa, a discontinuous jump in $\lambda$ occurs, causing $T_c$ to increase up to 764 K.

Ab initio random structure searching using density functional theory is used to determine the ground-state structures of atomic metallic hydrogen from 500 GPa to 5 TPa. Including proton zero-point motion within the harmonic approximation, we estimate that molecular hydrogen dissociates into a monatomic body-centered tetragonal structure near 500 GPa ($r_s=1.23$) that remains stable to 1 TPa ($r_s=1.11$). At higher pressures, hydrogen stabilizes in an $\dots ABCABC\dots$ planar structure that is similar to the ground state of lithium, but with a different stacking sequence. With increasing pressure, this structure compresses to the face-centered cubic lattice near 3.5 TPa ($r_s=0.92$).

Ab initio random structure searching based on density functional theory is used to determine the ground-state structures of ice at high pressures. Including estimates of lattice zero-point energies, ice is predicted to adopt at least three crystal phases beyond $Pbcm$. The underlying sublattice of O atoms remains similar among them, and the transitions can be characterized by reorganizations of the hydrogen bonds. The symmetric hydrogen bonds of ice X and $Pbcm$ are initially lost as ice transforms to structures with symmetries $Pmc{2}_1$ (800–950 GPa) and $P{2}_1$ ($1.17$ TPa), but they are eventually regained at $5.62$ TPa in a layered structure $C2/m$. The $P{2}_1\to C2/m$ transformation also marks the insulator-to-metal transition in ice, which occurs at a significantly higher pressure than recently predicted.

DOI: http://dx.doi.org/10.1103/PhysRevB.85.219902 ©2012 American Physical Society

Search for stable high-pressure compounds in the Ti-H system reveals the existence of titanium hydrides with new stoichiometries, including $Ibam\text{−}{\mathrm{Ti}}_2{\mathrm{H}}_5$, $I4/m\text{−}{\mathrm{Ti}}_5{\mathrm{H}}_{13}$, $I\overline{4}\text{−}{\mathrm{Ti}}_5{\mathrm{H}}_{14}$, $Fddd\text{−}{\mathrm{TiH}}_4$, $Immm\text{−}{\mathrm{Ti}}_2{\mathrm{H}}_{13}$, $P\overline{1}\text{−}{\mathrm{TiH}}_{12}$, and $C2/m\text{−}{\mathrm{TiH}}_{22}$. Our calculations predict $I4/mmm\to R\overline{3}m$ and $I4/mmm\to$ $Cmma$ transitions in TiH and ${\mathrm{TiH}}_2$, respectively. Phonons and the electron-phonon coupling in all searched titanium hydrides are analyzed at high pressure. It is found that $Immm\text{−}{\mathrm{Ti}}_2{\mathrm{H}}_{13}$, rather than the hydrogen-richest $C2/m\text{−}{\mathrm{TiH}}_{22}$, exhibits the highest superconducting critical temperature $T_c$. The estimated $T_c$ of $Immm\text{−}{\mathrm{Ti}}_2{\mathrm{H}}_{13}$ and $C2/m\text{−}{\mathrm{TiH}}_{22}$ are, respectively, 127.4–149.4 K (${\mu }^{*}=0.1$–0.15) at 350 GPa and 91.3–110.2 K at 250 GPa, as found by numerically solving the Eliashberg equations.