High-Temperature Superconducting Phase in Clathrate Calcium Hydride ${\mathrm{CaH}}_6$ up to 215 K at a Pressure of 172 GPa

The recent discovery of superconductive rare earth and actinide superhydrides has ushered in a new era of superconductivity research at high pressures. This distinct type of clathrate metal hydrides was first proposed for alkaline-earth-metal hydride ${\mathrm{CaH}}_6$ that, however, has long eluded experimental synthesis, impeding an understanding of pertinent physics. Here, we report successful synthesis of ${\mathrm{CaH}}_6$ and its measured superconducting critical temperature $T_c$ of 215 K at 172 GPa, which is evidenced by a sharp drop of resistivity to zero and a characteristic decrease of $T_c$ under a magnetic field up to 9 T. An estimate based on the Werthamer-Helfand-Hohenberg model gives a giant zero-temperature upper critical magnetic field of 203 T. These remarkable benchmark superconducting properties place ${\mathrm{CaH}}_6$ among the most outstanding high-$T_c$ superhydrides, marking it as the hitherto only clathrate metal hydride outside the family of rare earth and actinide hydrides. This exceptional case raises great prospects of expanding the extraordinary class of high-$T_c$ superhydrides to a broader variety of compounds that possess more diverse material features and physics characteristics.

Figure 1

(a) Synchrotron x-ray diffraction pattern of the clathrate calcium hydride from cell_1 obtained following laser heating of Ca and ${\mathrm{BH}}_3{\mathrm{NH}}_3$ at 190 GPa and the Rietveld refinement of the clathrate ${\mathrm{CaH}}_6$ structure. (b) Experimental EOS from the different samples in this Letter in comparison with that of the predicted clathrate ${\mathrm{CaH}}_6$. The EOS data from cell_1, cell_2, and cell_3 were marked with red triangles, blue inverted triangles, and green rhombus, respectively. The superconducting transition with $T_c\sim 210\mathrm{K}$ was observed in the electrical measurement for cell_1 at 190 GPa as shown in the top inset.

Figure 2

(a) Resistance measurements on the synthesized clathrate calcium hydride ${\mathrm{CaH}}_6$. Optical micrograph of a sample at 170 GPa after laser heating is shown in the upper left panel inset. Red curve, sample (cell_4) heated at 181 GPa with $T_c\sim 207\mathrm{K}$; olive curve, sample (cell_5) heated at 170 GPa with $T_c\sim 212\mathrm{K}$; blue curve, sample (cell_6) heated at 170 GPa with $T_c\sim 212\mathrm{K}$; purple curve, sample (cell_7) heated at 172 GPa with $T_c\sim 215\mathrm{K}$. The resistance data with near zero values are shown on a smaller scale in the left bottom inset. (b) The dependence of the critical temperature $T_c$ on pressure (cell_1, cell_2, and cell_4–15); the results from 12 different experiments are marked in different colors. The data with the same color are from the same sample in two figures. The open and solid symbols represent the data obtained on compression and decompression, respectively. The temperature dependence of the resistance at high pressures for the symbols of black square (cell_6), navy triangle (cell_8), and magenta star (cell_9) are plotted in Figs. S4, S5(a), and S5(b) [32], respectively.

Figure 3

(a) Temperature dependence of the electrical resistance under applied magnetic fields of $H=0$, 3, 5, 7, and 9 T at 172 GPa. (b) Upper critical field $H_{c2}$ versus temperature following the criteria of 90% of the resistance in the metallic state at 172 GPa, fitted with the GL and WHH models. Inset: the dependence of the $T_c$ under the applied magnetic field.