  The discovery of clathrate superhydrides has approached the long-standing
dream of room-temperature superconductivity and thus inspired their prosperous
research under high pressure. However, how to experimentally optimize these
compelling superhydrides is still a formidable challenge. Here, we find that
half of the Ce atoms in the recently discovered hexagonal close packed (hcp)
CeH9 structure can be randomly replaced by adjacent La, resulting in the
formation of LaH9 unit that is impossible in a binary system. Our experiments
show that hcp (La, Ce)H9 can be synthesized at ~110 GPa and possesses a maximum
Tc of 178 K at higher pressure, which is evidenced by in-situ X-ray diffraction
and electronic transport measurement where a sharp drop of resistivity to zero
and a characteristic decrease of Tc under a magnetic field up to 9 T. More
importantly, the Tc of (La, Ce)H9 is significantly increased by ~50-80 K
compared to CeH9, showing the hitherto highest Tc at megabar pressure. Our
experimental results not only verify the feasibility of improving the
superconductivity of hydrides by introducing other suitable metals, but also
provide important inspiration for finding high-Tc superconductors in various
multinary superhydrides.
