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.

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.

Compression of hydrogen-rich hydrides has been proposed as an alternative way to attain the atomic metallic hydrogen state or high-temperature superconductors. However, it remains a challenge to get access to these states by synthesizing novel polyhydrides with unusually high hydrogen-to-metal ratios. Here we synthesize a series of cerium (Ce) polyhydrides by a direct reaction of Ce and H₂ at high pressures. We discover that cerium polyhydride CeH₉, formed above 100 GPa, presents a three-dimensional hydrogen network composed of clathrate H₂₉ cages. The electron localization function together with band structure calculations elucidate the weak electron localization between H-H atoms and confirm its metallic character. By means of Ce atom doping, metallic hydrogen structure can be realized via the existence of CeH₉. Particularly, Ce atoms play a positive role to stabilize the sublattice of hydrogen cages similar to the recently discovered near-room-temperature lanthanum hydride superconductors.

The soft amplitude (AMP) mode demonstrates pressure-induced commensurate–incommensurate–atomic phase transitions.

Dense hydrous magnesium silicate MgSiO_{4}H_{2} is widely regarded as a primary water carrier into the deep Earth. However, the stability fields of MgSiO_{4}H_{2} based on the prevailing structure model are narrower than experimental results at relevant pressure and temperature (P-T) conditions, casting doubts about this prominent mineral as a water carrier into the great depths of the Earth. Here, we report on an advanced structure search that identifies two new crystal structures, denoted as α- and β-MgSiO_{4}H_{2}, that are stable over unprecedentedly wide P-T conditions of 17-68 GPa and up to 1860 K, covering the entire experimentally determined range. Moreover, we performed x-ray diffraction measurements with backscattering electron image, combined with ab initio simulations, to demonstrate the formation of MgSiO_{4}H_{2} and AlOOH solid solutions that exhibit further enhanced P-T stability fields, making them robust carriers of water into the deepest lower mantle. These findings establish and elucidate the new MgSiO_{4}H_{2} phases as potential primary water carriers into the vast depths of the lower mantle, creating a distinct paradigm for the deep Earth water cycle.