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.