The high pressure structures, metallization, and superconductivity of recently synthesized H2-containing compounds (H2S)2H2 are elucidated by ab initio calculations. The ordered crystal structure with P1 symmetry is determined, supported by the good agreement between theoretical and experimental X-ray diffraction data, equation of states, and Raman spectra. The Cccm structure is favorable with partial hydrogen bond symmetrization above 37[emsp14]GPa. Upon further compression, H2 molecules disappear and two intriguing metallic structures with R3m and Im-3m symmetries are reconstructive above 111 and 180[emsp14]GPa, respectively. The predicted metallization pressure is 111[emsp14]GPa, which is approximately one-third of the currently suggested metallization pressure of bulk molecular hydrogen. Application of the Allen-Dynes-modified McMillan equation for the Im-3m structure yields high Tc values of 191[emsp14]K to 204[emsp14]K at 200[emsp14]GPa, which is among the highest values reported for H2-rich van der Waals compounds and MH3 type hydride thus far.

Solid hydrogen sulfide is a typical molecular crystal, but its stability under pressure remains controversial. In particular, the recent experimental discovery of high-pressure superconductivity at 190 K in an ${\mathrm{H}}_2\mathrm{S}$ sample (arXiv:1412.0460) inspired efforts to revalidate this controversial issue, the pressure at which ${\mathrm{H}}_2\mathrm{S}$ decomposes and the resultant decomposition products urgent need to be evaluated. In this paper we performed an extensive structural study on different stoichiometries of H_nS with $n>1$ under high pressure using ab initio calculations. Our results show that ${\mathrm{H}}_2\mathrm{S}$ is stable below 43 GPa and at elevated pressure it decomposes into ${\mathrm{H}}_3\mathrm{S}$ and sulfur. ${\mathrm{H}}_3\mathrm{S}$ is stable at least up to 300 GPa, while other H-rich compounds, including ${\mathrm{H}}_4\mathrm{S}, {\mathrm{H}}_5\mathrm{S}$, and ${\mathrm{H}}_6\mathrm{S}$, are unstable in the pressure range of this study.

The discoveries of high-temperature superconductivity in ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{LaH}}_{10}$ have excited the search for superconductivity in compressed hydrides, finally leading to the first discovery of a room-temperature superconductor in a carbonaceous sulfur hydride. In contrast to rapidly expanding theoretical studies, high-pressure experiments on hydride superconductors are expensive and technically challenging. Here, we experimentally discovered superconductivity in two new phases, $Fm\overline{3}m\text{−}{\mathrm{CeH}}_{10}$ (SC-I phase) and $P{6}_3/mmc\text{−}{\mathrm{CeH}}_9$ (SC-II phase) at pressures that are much lower ($<100\mathrm{GPa}$) than those needed to stabilize other polyhydride superconductors. Superconductivity was evidenced by a sharp drop of the electrical resistance to zero and decreased critical temperature in deuterated samples and in external magnetic field. SC-I has $T_c=115\mathrm{K}$ at 95 GPa, showing an expected decrease in further compression due to the decrease of the electron-phonon coupling (EPC) coefficient $\lambda$ (from 2.0 at 100 GPa to 0.8 at 200 GPa). SC-II has $T_c=57\mathrm{K}$ at 88 GPa, rapidly increasing to a maximum $T_c\sim 100\mathrm{K}$ at 130 GPa, and then decreasing in further compression. According to the theoretical calculation, this is due to a maximum of $\lambda$ at the phase transition from $P{6}_3/mmc\text{−}{\mathrm{CeH}}_9$ into a symmetry-broken modification $C2/c\text{−}{\mathrm{CeH}}_9$. The pressure-temperature conditions of synthesis affect the actual hydrogen content and the actual value of $T_c$. Anomalously low pressures of stability of cerium superhydrides make them appealing for studies of superhydrides and for designing new superhydrides with stability at even lower pressures.

Hydrogen-based superconductors provide a route to the long-sought goal of room-temperature superconductivity, but the high pressures required to metallize these materials limit their immediate application. For example, carbonaceous sulfur hydride, the first room-temperature superconductor made in a laboratory, can reach a critical temperature ($T_c$) of 288 K only at the extreme pressure of 267 GPa. The next recognized challenge is the realization of room-temperature superconductivity at significantly lower pressures. Here, we propose a strategy for the rational design of high-temperature superconductors at low pressures by alloying small-radius elements and hydrogen to form ternary H-based superconductors with alloy backbones. We identify a “fluorite-type” backbone in compositions of the form $AX{\mathrm{H}}_8$, which exhibit high-temperature superconductivity at moderate pressures compared with other reported hydrogen-based superconductors. The $Fm\overline{3}m$ phase of ${\mathrm{LaBeH}}_8$, with a fluorite-type H-Be alloy backbone, is predicted to be thermodynamically stable above 98 GPa, and dynamically stable down to 20 GPa with a high $T_c\sim 185\mathrm{K}$. This is substantially lower than the synthesis pressure required by the geometrically similar clathrate hydride ${\mathrm{LaH}}_{10}$ (170 GPa). Our approach paves the way for finding high-$T_c$ ternary H-based superconductors at conditions close to ambient pressures.

Hydrogen atoms can provide high phonon frequencies and strong electron–phonon coupling in hydrogen-rich materials, which are believed to be potential high-temperature superconductors at lower pressure than metallic hydrogen. Especially, recently both of theoretical and experimental reports on sulfur hydrides under pressure exhibiting superconductivity at temperatures as high as 200 K have further stimulated an intense search for room-temperature superconductors in hydrides. This review focuses on crystal structures, stabilities, pressure-induced transformations, metallization, and superconductivity of hydrogen-rich materials at high pressures.

Superhydrides have complex hydrogenic sublattices and are important prototypes for studying metallic hydrogen and high-temperature superconductors. Previous results for LaH₁₀ suggest that the Pr-H system may be especially worth studying because of the magnetism and valence-band f-electrons in the element Pr. Here, we successfully synthesized praseodymium superhydrides (PrH₉) in laser-heated diamond anvil cells. Synchrotron x-ray diffraction analysis demonstrated the presence of previously predicted F 4¯ 3m-PrH₉ and unexpected P6₃/mmc-PrH₉ phases. Experimental studies of electrical resistance in the PrH₉ sample showed the emergence of a possible superconducting transition (T_c) below 9 K and T_c dependent on the applied magnetic field. Theoretical calculations indicate that magnetic order and likely superconductivity coexist in a narrow range of pressures in the PrH₉ sample, which may contribute to its low superconducting temperature. Our results highlight the intimate connections between hydrogenic sublattices, density of states, magnetism, and superconductivity in Pr-based superhydrides.