  Covalent and ionic polyhydrides have become the two main camps in searching
for the high-temperature superconductors under pressure. They have been
considered as important platforms for exploring ternary or multiple hydrides in
order to further increase the Tc or decrease the stabilization pressure. In
this work, we successfully synthesized ternary hexagonal La-Ce polyhydrides
stable in the pressure range of 95-130 GPa by laser-heating the La-Ce alloy
(initial ratio La:Ce=2.5-3.5) in ammonia borane. Superconductivity at 176 K was
strikingly preserved to about 100 GPa. The extrapolated upper critical field
Hc2(0) reached 216 T at 100 GPa, the highest value among the synthesized
polyhydrides. We also performed the contrast experiments and stabilized binary
high-temperature superconducting LaHx with Tc-103 K at 78 GPa. In the pressure
range of 95-130 GPa, the ternary hexagonal La-Ce-H system exhibits higher Tc
than the binary La-H system, with the maximum difference of 100 K, and the
compounds of both systems were synthesized at the same pressure and temperature
conditions. These results clearly indicate that the discovered La-Ce-H system
not only enriches the high-temperature superconducting hydrides but also
realizes high-Tc at moderate pressures.
