High-temperature superconductivity up to 223 K in the Al stabilized metastable hexagonal lanthanum superhydride

As compressed hydrides constantly refresh the records of superconducting critical temperatures (T) in the vicinity of room temperature, this further reinforces the confidence to find more high-temperature superconducting hydrides. In this process, metastable phases of superhydrides offer enough possibilities to access superior superconducting properties. Here we report a metastable hexagonal lanthanum superhydride (P6₃/mmc-LaH₁₀) stabilized at 146 GPa by introducing an appropriate proportion of Al, which exhibits high-temperature superconductivity with T ∼ 178 K, and this value is enhanced to a maximum T ∼ 223 K at 164 GPa. A huge upper critical magnetic field value H₂(0) reaches 223 T at 146 GPa. The small volume expansion of P6₃/mmc-(La, Al) H₁₀ compared with the binary LaH₁₀ indicates the possible interstitial sites of Al atoms filling into the La-H lattice, instead of forming conventional ternary alloy-based superhydrides. This work provides a new strategy for metastable high-temperature superconductors through the multiple-element system.