High-Temperature Superconducting Phases in Cerium Superhydride with a Tc up to 115 K below a Pressure of 1 Megabar

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.