A systematic structure search in the La–H and Y–H systems under pressure reveals some hydrogen-rich structures with intriguing electronic properties. For example, LaH₁₀ is found to adopt a sodalite-like face-centered cubic (fcc) structure, stable above 200 GPa, and LaH₈ a C2/m space group structure. Phonon calculations indicate both are dynamically stable; electron phonon calculations coupled to Bardeen–Cooper–Schrieffer (BCS) arguments indicate they might be high-T_c superconductors. In particular, the superconducting transition temperature T_c calculated for LaH₁₀ is 274–286 K at 210 GPa. Similar calculations for the Y–H system predict stability of the sodalite-like fcc YH₁₀ and a T_c above room temperature, reaching 305–326 K at 250 GPa. The study suggests that dense hydrides consisting of these and related hydrogen polyhedral networks may represent new classes of potential very high-temperature superconductors.

Hydrogen sulfide (H₂S) is a prototype molecular system and a sister molecule of water (H₂O). The phase diagram of solid H₂S at high pressures remains largely unexplored arising from the challenges in dealing with the pressure-induced weakening of S–H bond and larger atomic core difference between H and S. Metallization is yet achieved for H₂O, but it was observed for H₂S above 96 GPa. However, the metallic structure of H₂S remains elusive, greatly impeding the understanding of its metallicity and the potential superconductivity. We have performed an extensive structural study on solid H₂S at pressure ranges of 10–200 GPa through an unbiased structure prediction method based on particle swarm optimization algorithm. Besides the findings of candidate structures for nonmetallic phases IV and V, we are able to establish stable metallic structures violating an earlier proposal of elemental decomposition into sulfur and hydrogen [R. Rousseau, M. Boero, M. Bernasconi, M. Parrinello, and K. Terakura, Phys. Rev. Lett. 85, 1254 (2000)]. Our study unravels a superconductive potential of metallic H₂S with an estimated maximal transition temperature of ∼80 K at 160 GPa, higher than those predicted for most archetypal hydrogen-containing compounds (e.g., SiH₄, GeH₄, etc.).

We use first-principles calculations to study structural, vibrational, and superconducting properties of ${\mathrm{H}}_2\mathrm{S}$ at pressures $P\ge 200\mathrm{GPa}$. The inclusion of zero-point energy leads to two different possible dissociations of ${\mathrm{H}}_2\mathrm{S}$, namely $3{\mathrm{H}}_2\mathrm{S}\to 2{\mathrm{H}}_3\mathrm{S}+\mathrm{S}$ and $5{\mathrm{H}}_2\mathrm{S}\to 3{\mathrm{H}}_3\mathrm{S}+{\mathrm{HS}}_2$, where both ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{HS}}_2$ are metallic. For ${\mathrm{H}}_3\mathrm{S}$, we perform nonperturbative calculations of anharmonic effects within the self-consistent harmonic approximation and show that the harmonic approximation strongly overestimates the electron-phonon interaction ($\lambda \approx 2.64$ at 200 GPa) and $T_c$. Anharmonicity hardens H─S bond-stretching modes and softens H─S bond-bending modes. As a result, the electron-phonon coupling is suppressed by 30% ($\lambda \approx 1.84$ at 200 GPa). Moreover, while at the harmonic level $T_c$ decreases with increasing pressure, the inclusion of anharmonicity leads to a $T_c$ that is almost independent of pressure. High-pressure hydrogen sulfide is a strongly anharmonic superconductor.

The recent theory-orientated discovery of record high-temperature superconductivity ($T_c\sim 250\mathrm{K}$) in sodalitelike clathrate ${\mathrm{LaH}}_{10}$ is an important advance toward room-temperature superconductors. Here, we identify an alternative clathrate structure in ternary ${\mathrm{Li}}_2{\mathrm{MgH}}_{16}$ with a remarkably high estimated $T_c$ of $\sim 473\mathrm{K}$ at 250 GPa, which may allow us to obtain room-temperature or even higher-temperature superconductivity. The ternary compound mimics a Li- or electron-doped binary hydride of ${\mathrm{MgH}}_{16}$. The parent hydride contains ${\mathrm{H}}_2$ molecules and is not a good superconductor. The extra electrons introduced break up the ${\mathrm{H}}_2$ molecules, increasing the amount of atomic hydrogen compared with the parent hydride, which is necessary for stabilizing the clathrate structure or other high-$T_c$ structures. Our results provide a viable strategy for tuning the superconductivity of hydrogen-rich hydrides by donating electrons to hydrides via metal doping. Our approach may pave the way for finding high-$T_c$ superconductors in a variety of ternary or quaternary hydrides.