Giant enhancement of superconducting critical temperature in substitutional alloy (La,Ce)H 9 .

A sharp focus of current research on superconducting superhydrides is to raise their critical temperature T c at moderate pressures. Here, we report a discovery of giant enhancement of T c in CeH 9 obtained via random substitution of half Ce by La, leading to equal-atomic (La,Ce)H 9 alloy stabilized by maximum configurational entropy, containing the LaH 9 unit that is unstable in pure compound form. The synthesized (La,Ce)H 9 alloy exhibits T c of 148-178 K in the pressure range of 97-172 GPa, representing up to 80% enhancement of T c compared to pure CeH 9 and showcasing the highest T c at sub-megabar pressure among the known superhydrides. This work demonstrates substitutional alloying as a highly effective enabling tool for substantially enhancing T c via atypical compositional modulation inside suitably selected host crystal. This optimal substitutional alloying approach opens a promising avenue for synthesis of high-entropy multinary superhydrides that may exhibit further increased T c at even lower pressures.