  Recent discovery of superconductive rare earth/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 CaH6 that, however, has long eluded experimental
synthesis, impeding an understanding of pertinent physics. Here, we report
successful synthesis of CaH6 and its measured superconducting critical
temperature Tc of 215 K at 172 GPa, which is evidenced by a sharp drop of
resistivity to zero and a characteristic decrease of Tc 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 CaH6 among the most outstanding
high-Tc superhydrides, marking it as the hitherto only clathrate metal hydride
outside the family of rare earth/actinide hydrides. This exceptional case
raises great prospects of expanding the extraordinary class of high-Tc
superhydrides to a broader variety of compounds that possess more diverse
material features and physics characteristics.
