Synthesis of Yttrium Superhydride Superconductor with a Transition Temperature up to 262 K by Catalytic Hydrogenation at High Pressures

The recent observation of room-temperature superconductivity will undoubtedly lead to a surge in the discovery of new, dense, hydrogen-rich materials. The rare earth metal superhydrides are predicted to have very high-$T_c$ superconductivity that is tunable with changes in stoichiometry or doping. Here we report the synthesis of an yttrium superhydride that exhibits superconductivity at a critical temperature of 262 K at $182\pm 8\mathrm{GPa}$. A palladium thin film assists the synthesis by protecting the sputtered yttrium from oxidation and promoting subsequent hydrogenation. Phonon-mediated superconductivity is established by the observation of zero resistance, an isotope effect and the reduction of $T_c$ under an external magnetic field. The upper critical magnetic field is 103 T at zero temperature.

Figure 1

(a) Schematic of a new experimental approach for the preparation of metal films for the synthesis of metal superhydrides. (b) Microphotographs from transmitted light of yttrium film in a hydrogen environment. (i) and (ii) When initially opaque yttrium film is exposed to ${\mathrm{H}}_2$ gas, it readily absorbs hydrogen and forms metallic stoichiometric dihydride (${\mathrm{YH}}_{2\pm \epsilon }$) and, eventually, nonmetallic, hcp trihydride (${\mathrm{YH}}_{3\pm \delta }$ with $\delta \simeq 0.2$) at low pressure. When the mixture of ${\mathrm{YH}}_{3\pm \delta }$ and ${\mathrm{H}}_2$ is further pressurized (to about 5 GPa), fully transparent and yellowish ${\mathrm{YH}}_3$ is synthesized. However, the transmission shifts toward the red (iii) and (iv) above 15 GPa, and eventually the sample again becomes completely opaque to the visible spectrum (v) above 60 GPa at ambient temperature. (vi) Illustrates the synthesized superhydride sample (${\mathrm{YH}}_x$: $x\ge 6+{\mathrm{H}}_2$) with electrical leads for resistance measurements. (c) Temperature-dependent electrical resistance of yttrium superhydride at high pressures, showing the superconducting transition is as high as 262 K at $182\pm 8\mathrm{GPa}$, the highest pressure measured in this experimental run. The data were obtained during the warming cycle to minimize the electronic and cooling noise. Inset: The pressure dependence of the $T_c$ as determined by a sharp drop in electrical resistance, showing the increase in $T_c$ with pressure and plateau around 175 GPa, approaching a dome shape. The colors represent different experiments.

Figure 2

The substitution of hydrogen with deuterium noticeably affects the value of $T_c$, which shifts to 183 K at 177 GPa. The calculated isotope coefficient at 177 GPa with $T_c=256\mathrm{K}$ for ${\mathrm{YH}}_{9\pm x}$ (orange curve) and $T_c=183\mathrm{K}$ for the ${\mathrm{YD}}_{9\pm x}$ (green curve) sample is 0.48. By comparing the transition temperatures at around 183 GPa, we obtained $\alpha =0.46$. Both values are in very good agreement with the Bardeen-Cooper-Schrieffer value of $\alpha =0.5$ for conventional superconductivity.

Figure 3

Superconducting transition under an external magnetic field. Low-temperature electrical resistance behavior under a magnetic field of $\mathrm{H}=0$, 1, 3, and 6 T (increasing from right to left) at 177 GPa. Pressure was determined by the hydrogen vibron position. Inset top: Upper critical field vs temperature. An extrapolation to the lowest temperature yields 103.2 T for the upper critical magnetic field in the limit of zero temperature. Inset bottom: Superconducting transition width, $\mathrm{\Delta }{T}_c$, as a function of H, showing a linear relationship.

Figure 4

Synthesis of yttrium superhydride at high pressures and high temperatures. (a) Pressure-induced Raman changes (background subtracted) of yttrium superhydride for several indicated pressures at room temperature. Before heating (bottom spectra), the pressure was 171 GPa; after heating, it dropped to 168 GPa, suggesting a volume collapse associated with the transition. The observed characteristic Raman modes of the new material are in good agreement with theoretical calculations for ${\mathrm{YH}}_9$ (Fig. S9). The peak around $2048{\mathrm{cm}}^{-1}$ is broad compared to the low-frequency peaks, and there may be a shoulder at $1890{\mathrm{cm}}^{-1}$. The shaded area corresponds to the strong diamond signal. Inset: Microphotographs of transmitted light of the sample before and after heating. For clarity we have included a line (yellow) around the boundary of the sample. The sample became darker after heating to above 1800 K. (b) Pressure-induced Raman shift of different vibrations of the new yttrium superhydride. The colors indicate different experiments.